{-# LANGUAGE DeriveTraversable   #-}
{-# LANGUAGE DerivingStrategies  #-}
{-# LANGUAGE LambdaCase          #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TupleSections       #-}
{-# LANGUAGE RecursiveDo         #-}
{-# LANGUAGE MultiWayIf          #-}
{-# LANGUAGE RecordWildCards     #-}
{-# LANGUAGE TypeApplications    #-}


{-
(c) The University of Glasgow 2006
(c) The GRASP/AQUA Project, Glasgow University, 1992-1998
-}

-- | Type subsumption and unification
module GHC.Tc.Utils.Unify (
  -- Full-blown subsumption
  tcWrapResult, tcWrapResultO, tcWrapResultMono,
  tcSubType, tcSubTypeSigma, tcSubTypePat, tcSubTypeDS,
  tcSubTypeAmbiguity, tcSubMult,
  checkConstraints, checkTvConstraints,
  buildImplicationFor, buildTvImplication, emitResidualTvConstraint,

  -- Skolemisation
  DeepSubsumptionFlag(..), getDeepSubsumptionFlag, isRhoTyDS,
  tcSkolemise, tcSkolemiseCompleteSig, tcSkolemiseExpectedType,

  -- Various unifications
  unifyType, unifyKind, unifyInvisibleType, unifyExpectedType,
  unifyExprType, unifyTypeAndEmit, promoteTcType,
  swapOverTyVars, touchabilityAndShapeTest, checkTopShape, lhsPriority,
  UnifyEnv(..), updUEnvLoc, setUEnvRole,
  uType,
  makeTypeConcrete,

  --------------------------------
  -- Holes
  tcInfer,
  matchExpectedListTy,
  matchExpectedTyConApp,
  matchExpectedAppTy,
  matchExpectedFunTys,
  matchExpectedFunKind,
  matchActualFunTy, matchActualFunTys,

  checkTyEqRhs, recurseIntoTyConApp,
  PuResult(..), failCheckWith, okCheckRefl, mapCheck,
  TyEqFlags(..), TEFTask(..),
  notUnifying_TEFTask, unifyingLHSMetaTyVar_TEFTask,
  LevelCheck(..), TyEqFamApp(..), FamAppBreaker,
  famAppArgFlags,  checkPromoteFreeVars,

  simpleUnifyCheck, UnifyCheckCaller(..),

  fillInferResult,
  ) where

import GHC.Prelude

import GHC.Hs

import GHC.Tc.Errors.Types ( ErrCtxtMsg(..) )
import GHC.Tc.Errors.Ppr   ( pprErrCtxtMsg )
import GHC.Tc.Utils.Concrete
import GHC.Tc.Utils.Env
import GHC.Tc.Utils.Instantiate
import GHC.Tc.Utils.Monad
import GHC.Tc.Utils.TcMType
import GHC.Tc.Utils.TcType
import GHC.Tc.Types.Evidence
import GHC.Tc.Types.Constraint
import GHC.Tc.Types.CtLoc( CtLoc, mkKindEqLoc, adjustCtLoc )
import GHC.Tc.Types.Origin
import GHC.Tc.Zonk.TcType

import GHC.Core.Type
import GHC.Core.TyCo.Rep
import GHC.Core.TyCo.FVs( isInjectiveInType )
import GHC.Core.TyCo.Ppr( debugPprType {- pprTyVar -} )
import GHC.Core.TyCon
import GHC.Core.Coercion
import GHC.Core.Predicate( mkEqPredRole )
import GHC.Core.Multiplicity
import GHC.Core.Reduction

import qualified GHC.LanguageExtensions as LangExt

import GHC.Builtin.Types
import GHC.Types.Name
import GHC.Types.Id( idType )
import GHC.Types.Var as Var
import GHC.Types.Var.Set
import GHC.Types.Var.Env
import GHC.Types.Basic
import GHC.Types.Unique.Set (nonDetEltsUniqSet)

import GHC.Utils.Error
import GHC.Utils.Misc
import GHC.Utils.Outputable as Outputable
import GHC.Utils.Panic

import GHC.Driver.DynFlags
import GHC.Data.Bag
import GHC.Data.FastString

import Control.Monad
import Data.Maybe (maybeToList, isJust)
import Data.Monoid as DM ( Any(..) )
import qualified Data.Semigroup as S ( (<>) )
import Data.Traversable (for)

{- *********************************************************************
*                                                                      *
              matchActualFunTys
*                                                                      *
********************************************************************* -}

-- | 'matchActualFunTy' looks for just one function arrow,
-- returning an uninstantiated sigma-type.
--
-- Invariant: the returned argument type has a syntactically fixed
-- RuntimeRep in the sense of Note [Fixed RuntimeRep]
-- in GHC.Tc.Utils.Concrete.
--
-- See Note [Return arguments with a fixed RuntimeRep].
matchActualFunTy
  :: ExpectedFunTyOrigin
      -- ^ See Note [Herald for matchExpectedFunTys]
  -> Maybe TypedThing
      -- ^ The thing with type TcSigmaType
  -> (Arity, TcType)
      -- ^ Total number of value args in the call, and
      --   the original function type
      -- (Both are used only for error messages)
  -> TcRhoType
      -- ^ Type to analyse: a TcRhoType
  -> TcM (HsWrapper, Scaled TcSigmaTypeFRR, TcSigmaType)
-- This function takes in a type to analyse (a RhoType) and returns
-- an argument type and a result type (splitting apart a function arrow).
-- The returned argument type is a SigmaType with a fixed RuntimeRep;
-- as explained in Note [Return arguments with a fixed RuntimeRep].
--
-- See Note [matchActualFunTy error handling] for the first three arguments

-- If   (wrap, arg_ty, res_ty) = matchActualFunTy ... fun_ty
-- then wrap :: fun_ty ~> (arg_ty -> res_ty)
-- and NB: res_ty is an (uninstantiated) SigmaType

matchActualFunTy :: ExpectedFunTyOrigin
-> Maybe TypedThing
-> (Int, TcType)
-> TcType
-> TcM (HsWrapper, Scaled TcType, TcType)
matchActualFunTy ExpectedFunTyOrigin
herald Maybe TypedThing
mb_thing (Int, TcType)
err_info TcType
fun_ty
  = Bool
-> SDoc
-> TcM (HsWrapper, Scaled TcType, TcType)
-> TcM (HsWrapper, Scaled TcType, TcType)
forall a. HasCallStack => Bool -> SDoc -> a -> a
assertPpr (TcType -> Bool
isRhoTy TcType
fun_ty) (TcType -> SDoc
forall a. Outputable a => a -> SDoc
ppr TcType
fun_ty) (TcM (HsWrapper, Scaled TcType, TcType)
 -> TcM (HsWrapper, Scaled TcType, TcType))
-> TcM (HsWrapper, Scaled TcType, TcType)
-> TcM (HsWrapper, Scaled TcType, TcType)
forall a b. (a -> b) -> a -> b
$
    TcType -> TcM (HsWrapper, Scaled TcType, TcType)
go TcType
fun_ty
  where
    -- Does not allocate unnecessary meta variables: if the input already is
    -- a function, we just take it apart.  Not only is this efficient,
    -- it's important for higher rank: the argument might be of form
    --              (forall a. ty) -> other
    -- If allocated (fresh-meta-var1 -> fresh-meta-var2) and unified, we'd
    -- hide the forall inside a meta-variable
    go :: TcRhoType   -- The type we're processing, perhaps after
                      -- expanding type synonyms
       -> TcM (HsWrapper, Scaled TcSigmaTypeFRR, TcSigmaType)
    go :: TcType -> TcM (HsWrapper, Scaled TcType, TcType)
go TcType
ty | Just TcType
ty' <- TcType -> Maybe TcType
coreView TcType
ty = TcType -> TcM (HsWrapper, Scaled TcType, TcType)
go TcType
ty'

    go (FunTy { ft_af :: TcType -> FunTyFlag
ft_af = FunTyFlag
af, ft_mult :: TcType -> TcType
ft_mult = TcType
w, ft_arg :: TcType -> TcType
ft_arg = TcType
arg_ty, ft_res :: TcType -> TcType
ft_res = TcType
res_ty })
      = Bool
-> TcM (HsWrapper, Scaled TcType, TcType)
-> TcM (HsWrapper, Scaled TcType, TcType)
forall a. HasCallStack => Bool -> a -> a
assert (FunTyFlag -> Bool
isVisibleFunArg FunTyFlag
af) (TcM (HsWrapper, Scaled TcType, TcType)
 -> TcM (HsWrapper, Scaled TcType, TcType))
-> TcM (HsWrapper, Scaled TcType, TcType)
-> TcM (HsWrapper, Scaled TcType, TcType)
forall a b. (a -> b) -> a -> b
$
      do { HasDebugCallStack => FixedRuntimeRepContext -> TcType -> TcM ()
FixedRuntimeRepContext -> TcType -> TcM ()
hasFixedRuntimeRep_syntactic (ExpectedFunTyOrigin -> Int -> FixedRuntimeRepContext
FRRExpectedFunTy ExpectedFunTyOrigin
herald Int
1) TcType
arg_ty
         ; (HsWrapper, Scaled TcType, TcType)
-> TcM (HsWrapper, Scaled TcType, TcType)
forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
forall (m :: * -> *) a. Monad m => a -> m a
return (HsWrapper
idHsWrapper, TcType -> TcType -> Scaled TcType
forall a. TcType -> a -> Scaled a
Scaled TcType
w TcType
arg_ty, TcType
res_ty) }

    go ty :: TcType
ty@(TyVarTy TcTyVar
tv)
      | TcTyVar -> Bool
isMetaTyVar TcTyVar
tv
      = do { cts <- TcTyVar -> IOEnv (Env TcGblEnv TcLclEnv) MetaDetails
forall (m :: * -> *). MonadIO m => TcTyVar -> m MetaDetails
readMetaTyVar TcTyVar
tv
           ; case cts of
               Indirect TcType
ty' -> TcType -> TcM (HsWrapper, Scaled TcType, TcType)
go TcType
ty'
               MetaDetails
Flexi        -> TcType -> TcM (HsWrapper, Scaled TcType, TcType)
defer TcType
ty }

       -- In all other cases we bale out into ordinary unification
       -- However unlike the meta-tyvar case, we are sure that the
       -- number of arguments doesn't match arity of the original
       -- type, so we can add a bit more context to the error message
       -- (cf #7869).
       --
       -- It is not always an error, because specialized type may have
       -- different arity, for example:
       --
       -- > f1 = f2 'a'
       -- > f2 :: Monad m => m Bool
       -- > f2 = undefined
       --
       -- But in that case we add specialized type into error context
       -- anyway, because it may be useful. See also #9605.
    go TcType
ty = (TidyEnv -> ZonkM (TidyEnv, ErrCtxtMsg))
-> TcM (HsWrapper, Scaled TcType, TcType)
-> TcM (HsWrapper, Scaled TcType, TcType)
forall a.
(TidyEnv -> ZonkM (TidyEnv, ErrCtxtMsg)) -> TcM a -> TcM a
addErrCtxtM (TcType -> TidyEnv -> ZonkM (TidyEnv, ErrCtxtMsg)
mk_ctxt TcType
ty) (TcType -> TcM (HsWrapper, Scaled TcType, TcType)
defer TcType
ty)

    ------------
    defer :: TcType -> TcM (HsWrapper, Scaled TcType, TcType)
defer TcType
fun_ty
      = do { arg_ty <- ExpectedFunTyOrigin -> Int -> TcM (Scaled TcType)
new_check_arg_ty ExpectedFunTyOrigin
herald Int
1
           ; res_ty <- newOpenFlexiTyVarTy
           ; let unif_fun_ty = [Scaled TcType] -> TcType -> TcType
HasDebugCallStack => [Scaled TcType] -> TcType -> TcType
mkScaledFunTys [Scaled TcType
arg_ty] TcType
res_ty
           ; co <- unifyType mb_thing fun_ty unif_fun_ty
           ; return (mkWpCastN co, arg_ty, res_ty) }

    ------------
    mk_ctxt :: TcType -> TidyEnv -> ZonkM (TidyEnv, ErrCtxtMsg)
    mk_ctxt :: TcType -> TidyEnv -> ZonkM (TidyEnv, ErrCtxtMsg)
mk_ctxt TcType
_res_ty = ExpectedFunTyOrigin
-> (Int, TcType) -> TidyEnv -> ZonkM (TidyEnv, ErrCtxtMsg)
mkFunTysMsg ExpectedFunTyOrigin
herald (Int, TcType)
err_info

{- Note [matchActualFunTy error handling]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
matchActualFunTy is made much more complicated by the
desire to produce good error messages. Consider the application
    f @Int x y
In GHC.Tc.Gen.Head.tcInstFun we instantiate the function type, one
argument at a time.  It must instantiate any type/dictionary args,
before looking for an arrow type.

But if it doesn't find an arrow type, it wants to generate a message
like "f is applied to two arguments but its type only has one".
To do that, it needs to know about the args that tcArgs has already
munched up -- hence passing in n_val_args_in_call and arg_tys_so_far;
and hence also the accumulating so_far arg to 'go'.

This allows us (in mk_ctxt) to construct f's /instantiated/ type,
with just the values-arg arrows, which is what we really want
in the error message.

Ugh!
-}

-- | Like 'matchExpectedFunTys', but used when you have an "actual" type,
-- for example in function application.
--
-- INVARIANT: the returned argument types all have a syntactically fixed RuntimeRep
-- in the sense of Note [Fixed RuntimeRep] in GHC.Tc.Utils.Concrete.
-- See Note [Return arguments with a fixed RuntimeRep].
matchActualFunTys :: ExpectedFunTyOrigin -- ^ See Note [Herald for matchExpectedFunTys]
                  -> CtOrigin
                  -> Arity
                  -> TcSigmaType
                  -> TcM (HsWrapper, [Scaled TcSigmaTypeFRR], TcRhoType)
-- If    matchActualFunTys n ty = (wrap, [t1,..,tn], res_ty)
-- then  wrap : ty ~> (t1 -> ... -> tn -> res_ty)
--       and res_ty is a RhoType
-- NB: the returned type is top-instantiated; it's a RhoType
matchActualFunTys :: ExpectedFunTyOrigin
-> CtOrigin
-> Int
-> TcType
-> TcM (HsWrapper, [Scaled TcType], TcType)
matchActualFunTys ExpectedFunTyOrigin
herald CtOrigin
ct_orig Int
n_val_args_wanted TcType
top_ty
  = Int
-> [Scaled TcType]
-> TcType
-> TcM (HsWrapper, [Scaled TcType], TcType)
go Int
n_val_args_wanted [] TcType
top_ty
  where
    go :: Int
-> [Scaled TcType]
-> TcType
-> TcM (HsWrapper, [Scaled TcType], TcType)
go Int
n [Scaled TcType]
so_far TcType
fun_ty
      | Bool -> Bool
not (TcType -> Bool
isRhoTy TcType
fun_ty)
      = do { (wrap1, rho) <- CtOrigin -> TcType -> TcM (HsWrapper, TcType)
topInstantiate CtOrigin
ct_orig TcType
fun_ty
           ; (wrap2, arg_tys, res_ty) <- go n so_far rho
           ; return (wrap2 <.> wrap1, arg_tys, res_ty) }

    go Int
0 [Scaled TcType]
_ TcType
fun_ty = (HsWrapper, [Scaled TcType], TcType)
-> TcM (HsWrapper, [Scaled TcType], TcType)
forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
forall (m :: * -> *) a. Monad m => a -> m a
return (HsWrapper
idHsWrapper, [], TcType
fun_ty)

    go Int
n [Scaled TcType]
so_far TcType
fun_ty
      = do { (wrap_fun1, arg_ty1, res_ty1) <- ExpectedFunTyOrigin
-> Maybe TypedThing
-> (Int, TcType)
-> TcType
-> TcM (HsWrapper, Scaled TcType, TcType)
matchActualFunTy
                                                 ExpectedFunTyOrigin
herald Maybe TypedThing
forall a. Maybe a
Nothing
                                                 (Int
n_val_args_wanted, TcType
top_ty)
                                                 TcType
fun_ty
           ; (wrap_res, arg_tys, res_ty)   <- go (n-1) (arg_ty1:so_far) res_ty1
           ; let wrap_fun2 = HsWrapper -> HsWrapper -> Scaled TcType -> TcType -> HsWrapper
mkWpFun HsWrapper
idHsWrapper HsWrapper
wrap_res Scaled TcType
arg_ty1 TcType
res_ty
           -- NB: arg_ty1 comes from matchActualFunTy, so it has
           -- a syntactically fixed RuntimeRep as needed to call mkWpFun.
           ; return (wrap_fun2 <.> wrap_fun1, arg_ty1:arg_tys, res_ty) }

{-
************************************************************************
*                                                                      *
          Skolemisation and matchExpectedFunTys
*                                                                      *
************************************************************************

Note [Skolemisation overview]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Suppose f :: (forall a. a->a) -> blah, and we have the application (f e)
Then we want to typecheck `e` pushing in the type `forall a. a->a`. But we
need to be careful:

* Roughly speaking, in (tcPolyExpr e (forall a b. rho)), we skolemise `a` and `b`,
  and then call (tcExpr e rho)

* But not quite!  We must be careful if `e` is a type lambda (\ @p @q -> blah).
  Then we want to line up the skolemised variables `a`,`b`
  with `p`,`q`, so we can't just call (tcExpr (\ @p @q -> blah) rho)

* A very similar situation arises with
     (\ @p @q -> blah) :: forall a b. rho
  Again, we must line up `p`, `q` with the skolemised `a` and `b`.

* Another similar situation arises with
    g :: forall a b. rho
    g @p @q x y = ....
  Here again when skolemising `a` and `b` we must be careful to match them up
  with `p` and `q`.

OK, so how exactly do we check @p binders in lambdas?  First note that we only
we only attempt to deal with @p binders when /checking/. We don't do inference for
(\ @a -> blah), not yet anyway.

For checking, there are two cases to consider:
  * Function LHS, where the function has a type signature
                  f :: forall a. a -> forall b. [b] -> blah
                  f @p x @q y = ...

  * Lambda        \ @p x @q y -> ...
                  \cases { @p x @q y -> ... }
    (\case p behaves like \cases { p -> ... }, and p is always a term pattern.)

Both ultimately handled by matchExpectedFunTys.

* Function LHS case is handled by `GHC.Tc.Gen.Bind.tcPolyCheck`:
  * It calls `tcSkolemiseCompleteSig`
  * Passes the skolemised variables into `tcFunBindMatches`
  * Which uses `matchExpectedFunTys` to decompose the function type to
    match the arguments
  * And then passes the (skolemised-variables ++ arg tys) on to `tcMatches`

* For the Lambda case there are two sub-cases:
   * An expression with a type signature: (\ @a x y -> blah) :: hs_ty
     This is handled by `GHC.Tc.Gen.Head.tcExprWithSig`, which kind-checks
     the signature and hands off to `tcExprPolyCheck` via `tcPolyLExprSig`.
     Note that the foralls at the top of hs_ty scope over the expression.

   * A higher order call: h e, where h :: poly_ty -> blah
     This is handlded by `GHC.Tc.Gen.Expr.tcPolyExpr`, which (in the
     checking case) again hands off to `tcExprPolyCheck`.  Here there is
     no type-variable scoping to worry about.

  So both sub-cases end up in `GHC.Tc.Gen.Expr.tcPolyExprCheck`
  * This skolemises the /top-level/ invisible binders, but remembers
    the binders as [ExpPatType]
  * Then it looks for a lambda, and if so, calls `tcLambdaMatches` passing in
    the skolemised binders so they can be matched up with the lambda binders.
  * Otherwise it does deep-skolemisation if DeepSubsumption is on,
    and then calls tcExpr to typecheck `e`

  The outer skolemisation in tcPolyExprCheck is done using
    * tcSkolemiseCompleteSig when there is a user-written signature
    * tcSkolemiseGeneral when the polytype just comes from the context e.g. (f e)
  The former just calls the latter, so the two cases differ only slightly:
    * Both do shallow skolemisation
    * Both go via checkConstraints, which uses implicationNeeded to decide whether
      to build an implication constraint even if there /are/ no skolems.
      See Note [When to build an implication] below.

  The difference between the two cases is that `tcSkolemiseCompleteSig`
  also brings the outer type variables into scope.  It would do no
  harm to do so in both cases, but I found that (to my surprise) doing
  so caused a non-trivial (1%-ish) perf hit on the compiler.

* `tcFunBindMatches` and `tcLambdaMatches` both use `matchExpectedFunTys`, which
  ensures that any trailing invisible binders are skolemised; and does so deeply
  if DeepSubsumption is on.

  This corresponds to the plan: "skolemise at the '=' of a function binding or
  at the '->' of a lambda binding".  (See #17594 and "Plan B2".)

Some wrinkles

(SK1) tcSkolemiseGeneral and tcSkolemiseCompleteSig make fresh type variables
      See Note [Instantiate sig with fresh variables]

(SK2) All skolemisation (even without DeepSubsumption) builds just one implication
      constraint for a nested forall like:
          forall a. Eq a => forall b. Ord b => blah
      The implication constraint will look like
          forall a b. (Eq a, Ord b) => <constraints>
      See the loop in GHC.Tc.Utils.Instantiate.topSkolemise.
      and Note [Skolemisation en-bloc] in that module


Some examples:

*     f :: forall a b. blah
      f @p x = rhs
  `tcPolyCheck` calls `tcSkolemiseCompleteSig` to skolemise the signature, and
  then calls `tcFunBindMatches` passing in [a_sk, b_sk], the skolemsed
  variables. The latter ultimately calls `tcMatches`, and thence `tcMatchPats`.
  The latter matches up the `a_sk` with `@p`, and discards the `b_sk`.

*     f :: forall (a::Type) (b::a). blah
      f @(p::b) x = rhs
  `tcSkolemiseCompleteSig` brings `a` and `b` into scope, bound to `a_sk` and `b_sk` resp.
  When `tcMatchPats` typechecks the pattern `@(p::b)` it'll find that `b` is in
  scope (as a result of tcSkolemiseCompleteSig) which is a bit strange.  But
  it'll then unify the kinds `Type ~ b`, which will fail as it should.

*     f :: Int -> forall (a::Type) (b::a). blah
      f x  @p = rhs
  `matchExpectedFunTys` does shallow skolemisation eagerly, so we'll skolemise the
  forall a b.  Then `tcMatchPats` will bind [p :-> a_sk], and discard `b_sk`.
  Discarding the `b_sk` means that
      f x @p = \ @q -> blah
  or  f x @p = let .. in \ @q -> blah
  will both be rejected: this is Plan B2: skolemise at the "=".

* Suppose DeepSubsumption is on
    f :: forall a. a -> forall b. b -> b -> forall z. z
    f @p x @q y = rhs
  The `tcSkolemiseCompleteSig` uses shallow skolemisation, so it only skolemises
  and brings into scope [a :-> a_sk]. Then `matchExpectedFunTys` skolemises the
  forall b, because it needs to expose two value arguments.  Finally
  `matchExpectedFunTys` concludes with deeply skolemising the remaining type.

  So we end up with `[p :-> a_sk, q :-> b_sk]`.  Notice that we must not
  deeply-skolemise /first/ or we'd get the tyvars [a_sk, b_sk, c_sk] which would
  not line up with the patterns [@p, x, @q, y]
-}

tcSkolemiseGeneral
  :: DeepSubsumptionFlag
  -> UserTypeCtxt
  -> TcType -> TcType   -- top_ty and expected_ty
        -- Here, top_ty      is the type we started to skolemise; used only in SigSkol
        -- -     expected_ty is the type we are actually skolemising
        -- matchExpectedFunTys walks down the type, skolemising as it goes,
        -- keeping the same top_ty, but successively smaller expected_tys
  -> ([(Name, TcInvisTVBinder)] -> TcType -> TcM result)
  -> TcM (HsWrapper, result)
tcSkolemiseGeneral :: forall result.
DeepSubsumptionFlag
-> UserTypeCtxt
-> TcType
-> TcType
-> ([(Name, TcInvisTVBinder)] -> TcType -> TcM result)
-> TcM (HsWrapper, result)
tcSkolemiseGeneral DeepSubsumptionFlag
ds_flag UserTypeCtxt
ctxt TcType
top_ty TcType
expected_ty [(Name, TcInvisTVBinder)] -> TcType -> TcM result
thing_inside
  | DeepSubsumptionFlag -> TcType -> Bool
isRhoTyDS DeepSubsumptionFlag
ds_flag TcType
expected_ty
    -- Fast path for a very very common case: no skolemisation to do
    -- But still call checkConstraints in case we need an implication regardless
  = do { let sig_skol :: SkolemInfoAnon
sig_skol = UserTypeCtxt -> TcType -> [(Name, TcTyVar)] -> SkolemInfoAnon
SigSkol UserTypeCtxt
ctxt TcType
top_ty []
       ; (ev_binds, result) <- SkolemInfoAnon
-> [TcTyVar] -> [TcTyVar] -> TcM result -> TcM (TcEvBinds, result)
forall result.
SkolemInfoAnon
-> [TcTyVar] -> [TcTyVar] -> TcM result -> TcM (TcEvBinds, result)
checkConstraints SkolemInfoAnon
sig_skol [] [] (TcM result -> TcM (TcEvBinds, result))
-> TcM result -> TcM (TcEvBinds, result)
forall a b. (a -> b) -> a -> b
$
                               [(Name, TcInvisTVBinder)] -> TcType -> TcM result
thing_inside [] TcType
expected_ty
       ; return (mkWpLet ev_binds, result) }

  | Bool
otherwise
  = do { -- rec {..}: see Note [Keeping SkolemInfo inside a SkolemTv]
         --           in GHC.Tc.Utils.TcType
       ; rec { (wrap, tv_prs, given, rho_ty) <- case ds_flag of
                    DeepSubsumptionFlag
Deep    -> SkolemInfo
-> TcType
-> IOEnv
     (Env TcGblEnv TcLclEnv)
     (HsWrapper, [(Name, TcInvisTVBinder)], [TcTyVar], TcType)
deeplySkolemise SkolemInfo
skol_info TcType
expected_ty
                    DeepSubsumptionFlag
Shallow -> SkolemInfo
-> TcType
-> IOEnv
     (Env TcGblEnv TcLclEnv)
     (HsWrapper, [(Name, TcInvisTVBinder)], [TcTyVar], TcType)
topSkolemise SkolemInfo
skol_info TcType
expected_ty
             ; let sig_skol = UserTypeCtxt -> TcType -> [(Name, TcTyVar)] -> SkolemInfoAnon
SigSkol UserTypeCtxt
ctxt TcType
top_ty (((Name, TcInvisTVBinder) -> (Name, TcTyVar))
-> [(Name, TcInvisTVBinder)] -> [(Name, TcTyVar)]
forall a b. (a -> b) -> [a] -> [b]
map ((TcInvisTVBinder -> TcTyVar)
-> (Name, TcInvisTVBinder) -> (Name, TcTyVar)
forall a b. (a -> b) -> (Name, a) -> (Name, b)
forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
fmap TcInvisTVBinder -> TcTyVar
forall tv argf. VarBndr tv argf -> tv
binderVar) [(Name, TcInvisTVBinder)]
tv_prs)
             ; skol_info <- mkSkolemInfo sig_skol }

       ; let skol_tvs = ((Name, TcInvisTVBinder) -> TcTyVar)
-> [(Name, TcInvisTVBinder)] -> [TcTyVar]
forall a b. (a -> b) -> [a] -> [b]
map (TcInvisTVBinder -> TcTyVar
forall tv argf. VarBndr tv argf -> tv
binderVar (TcInvisTVBinder -> TcTyVar)
-> ((Name, TcInvisTVBinder) -> TcInvisTVBinder)
-> (Name, TcInvisTVBinder)
-> TcTyVar
forall b c a. (b -> c) -> (a -> b) -> a -> c
. (Name, TcInvisTVBinder) -> TcInvisTVBinder
forall a b. (a, b) -> b
snd) [(Name, TcInvisTVBinder)]
tv_prs
       ; traceTc "tcSkolemiseGeneral" (pprUserTypeCtxt ctxt <+> ppr skol_tvs <+> ppr given)
       ; (ev_binds, result) <- checkConstraints sig_skol skol_tvs given $
                               thing_inside tv_prs rho_ty

       ; return (wrap <.> mkWpLet ev_binds, result) }
         -- The ev_binds returned by checkConstraints is very
         -- often empty, in which case mkWpLet is a no-op

tcSkolemiseCompleteSig :: TcCompleteSig
                       -> ([ExpPatType] -> TcRhoType -> TcM result)
                       -> TcM (HsWrapper, result)
-- ^ The wrapper has type: spec_ty ~> expected_ty
-- See Note [Skolemisation] for the differences between
-- tcSkolemiseCompleteSig and tcTopSkolemise

tcSkolemiseCompleteSig :: forall result.
TcCompleteSig
-> ([ExpPatType] -> TcType -> TcM result)
-> TcM (HsWrapper, result)
tcSkolemiseCompleteSig (CSig { sig_bndr :: TcCompleteSig -> TcTyVar
sig_bndr = TcTyVar
poly_id, sig_ctxt :: TcCompleteSig -> UserTypeCtxt
sig_ctxt = UserTypeCtxt
ctxt, sig_loc :: TcCompleteSig -> SrcSpan
sig_loc = SrcSpan
loc })
                       [ExpPatType] -> TcType -> TcM result
thing_inside
  = do { cur_loc <- TcRn SrcSpan
getSrcSpanM
       ; let poly_ty = TcTyVar -> TcType
idType TcTyVar
poly_id
       ; setSrcSpan loc $   -- Sets the location for the implication constraint
         tcSkolemiseGeneral Shallow ctxt poly_ty poly_ty $ \[(Name, TcInvisTVBinder)]
tv_prs TcType
rho_ty ->
         SrcSpan -> TcM result -> TcM result
forall a. SrcSpan -> TcRn a -> TcRn a
setSrcSpan SrcSpan
cur_loc (TcM result -> TcM result) -> TcM result -> TcM result
forall a b. (a -> b) -> a -> b
$ -- Revert to the original location
         [(Name, TcTyVar)] -> TcM result -> TcM result
forall r. [(Name, TcTyVar)] -> TcM r -> TcM r
tcExtendNameTyVarEnv (((Name, TcInvisTVBinder) -> (Name, TcTyVar))
-> [(Name, TcInvisTVBinder)] -> [(Name, TcTyVar)]
forall a b. (a -> b) -> [a] -> [b]
map ((TcInvisTVBinder -> TcTyVar)
-> (Name, TcInvisTVBinder) -> (Name, TcTyVar)
forall a b. (a -> b) -> (Name, a) -> (Name, b)
forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
fmap TcInvisTVBinder -> TcTyVar
forall tv argf. VarBndr tv argf -> tv
binderVar) [(Name, TcInvisTVBinder)]
tv_prs) (TcM result -> TcM result) -> TcM result -> TcM result
forall a b. (a -> b) -> a -> b
$
         [ExpPatType] -> TcType -> TcM result
thing_inside (((Name, TcInvisTVBinder) -> ExpPatType)
-> [(Name, TcInvisTVBinder)] -> [ExpPatType]
forall a b. (a -> b) -> [a] -> [b]
map (TcInvisTVBinder -> ExpPatType
mkInvisExpPatType (TcInvisTVBinder -> ExpPatType)
-> ((Name, TcInvisTVBinder) -> TcInvisTVBinder)
-> (Name, TcInvisTVBinder)
-> ExpPatType
forall b c a. (b -> c) -> (a -> b) -> a -> c
. (Name, TcInvisTVBinder) -> TcInvisTVBinder
forall a b. (a, b) -> b
snd) [(Name, TcInvisTVBinder)]
tv_prs) TcType
rho_ty }

tcSkolemiseExpectedType :: TcSigmaType
                        -> ([ExpPatType] -> TcRhoType -> TcM result)
                        -> TcM (HsWrapper, result)
-- Just like tcSkolemiseCompleteSig, except that we don't have a user-written
-- type signature, we only have a type comimg from the context.
-- Eg. f :: (forall a. blah) -> blah
--     In the call (f e) we will call tcSkolemiseExpectedType on (forall a.blah)
--     before typececking `e`
tcSkolemiseExpectedType :: forall result.
TcType
-> ([ExpPatType] -> TcType -> TcM result)
-> TcM (HsWrapper, result)
tcSkolemiseExpectedType TcType
exp_ty [ExpPatType] -> TcType -> TcM result
thing_inside
  = DeepSubsumptionFlag
-> UserTypeCtxt
-> TcType
-> TcType
-> ([(Name, TcInvisTVBinder)] -> TcType -> TcM result)
-> TcM (HsWrapper, result)
forall result.
DeepSubsumptionFlag
-> UserTypeCtxt
-> TcType
-> TcType
-> ([(Name, TcInvisTVBinder)] -> TcType -> TcM result)
-> TcM (HsWrapper, result)
tcSkolemiseGeneral DeepSubsumptionFlag
Shallow UserTypeCtxt
GenSigCtxt TcType
exp_ty TcType
exp_ty (([(Name, TcInvisTVBinder)] -> TcType -> TcM result)
 -> TcM (HsWrapper, result))
-> ([(Name, TcInvisTVBinder)] -> TcType -> TcM result)
-> TcM (HsWrapper, result)
forall a b. (a -> b) -> a -> b
$ \[(Name, TcInvisTVBinder)]
tv_prs TcType
rho_ty ->
    [ExpPatType] -> TcType -> TcM result
thing_inside (((Name, TcInvisTVBinder) -> ExpPatType)
-> [(Name, TcInvisTVBinder)] -> [ExpPatType]
forall a b. (a -> b) -> [a] -> [b]
map (TcInvisTVBinder -> ExpPatType
mkInvisExpPatType (TcInvisTVBinder -> ExpPatType)
-> ((Name, TcInvisTVBinder) -> TcInvisTVBinder)
-> (Name, TcInvisTVBinder)
-> ExpPatType
forall b c a. (b -> c) -> (a -> b) -> a -> c
. (Name, TcInvisTVBinder) -> TcInvisTVBinder
forall a b. (a, b) -> b
snd) [(Name, TcInvisTVBinder)]
tv_prs) TcType
rho_ty

tcSkolemise :: DeepSubsumptionFlag -> UserTypeCtxt -> TcSigmaType
            -> (TcRhoType -> TcM result)
            -> TcM (HsWrapper, result)
tcSkolemise :: forall result.
DeepSubsumptionFlag
-> UserTypeCtxt
-> TcType
-> (TcType -> TcM result)
-> TcM (HsWrapper, result)
tcSkolemise DeepSubsumptionFlag
ds_flag UserTypeCtxt
ctxt TcType
expected_ty TcType -> TcM result
thing_inside
  = DeepSubsumptionFlag
-> UserTypeCtxt
-> TcType
-> TcType
-> ([(Name, TcInvisTVBinder)] -> TcType -> TcM result)
-> TcM (HsWrapper, result)
forall result.
DeepSubsumptionFlag
-> UserTypeCtxt
-> TcType
-> TcType
-> ([(Name, TcInvisTVBinder)] -> TcType -> TcM result)
-> TcM (HsWrapper, result)
tcSkolemiseGeneral DeepSubsumptionFlag
ds_flag UserTypeCtxt
ctxt TcType
expected_ty TcType
expected_ty (([(Name, TcInvisTVBinder)] -> TcType -> TcM result)
 -> TcM (HsWrapper, result))
-> ([(Name, TcInvisTVBinder)] -> TcType -> TcM result)
-> TcM (HsWrapper, result)
forall a b. (a -> b) -> a -> b
$ \[(Name, TcInvisTVBinder)]
_ TcType
rho_ty ->
    TcType -> TcM result
thing_inside TcType
rho_ty

checkConstraints :: SkolemInfoAnon
                 -> [TcTyVar]           -- Skolems
                 -> [EvVar]             -- Given
                 -> TcM result
                 -> TcM (TcEvBinds, result)
-- checkConstraints is careful to build an implication even if
-- `skol_tvs` and `given` are both empty, under certain circumstances
-- See Note [When to build an implication]
checkConstraints :: forall result.
SkolemInfoAnon
-> [TcTyVar] -> [TcTyVar] -> TcM result -> TcM (TcEvBinds, result)
checkConstraints SkolemInfoAnon
skol_info [TcTyVar]
skol_tvs [TcTyVar]
given TcM result
thing_inside
  = do { implication_needed <- SkolemInfoAnon -> [TcTyVar] -> [TcTyVar] -> TcM Bool
implicationNeeded SkolemInfoAnon
skol_info [TcTyVar]
skol_tvs [TcTyVar]
given

       ; if implication_needed
         then do { (tclvl, wanted, result) <- pushLevelAndCaptureConstraints thing_inside
                 ; (implics, ev_binds) <- buildImplicationFor tclvl skol_info skol_tvs given wanted
                 ; traceTc "checkConstraints" (ppr tclvl $$ ppr skol_tvs)
                 ; emitImplications implics
                 ; return (ev_binds, result) }

         else -- Fast path.  We check every function argument with tcCheckPolyExpr,
              -- which uses tcTopSkolemise and hence checkConstraints.
              -- So this fast path is well-exercised
              do { res <- thing_inside
                 ; return (emptyTcEvBinds, res) } }

checkTvConstraints :: SkolemInfo
                   -> [TcTyVar]          -- Skolem tyvars
                   -> TcM result
                   -> TcM result

checkTvConstraints :: forall result. SkolemInfo -> [TcTyVar] -> TcM result -> TcM result
checkTvConstraints SkolemInfo
skol_info [TcTyVar]
skol_tvs TcM result
thing_inside
  = do { (tclvl, wanted, result) <- TcM result -> TcM (TcLevel, WantedConstraints, result)
forall a. TcM a -> TcM (TcLevel, WantedConstraints, a)
pushLevelAndCaptureConstraints TcM result
thing_inside
       ; emitResidualTvConstraint skol_info skol_tvs tclvl wanted
       ; return result }

emitResidualTvConstraint :: SkolemInfo -> [TcTyVar]
                         -> TcLevel -> WantedConstraints -> TcM ()
emitResidualTvConstraint :: SkolemInfo -> [TcTyVar] -> TcLevel -> WantedConstraints -> TcM ()
emitResidualTvConstraint SkolemInfo
skol_info [TcTyVar]
skol_tvs TcLevel
tclvl WantedConstraints
wanted
  | Bool -> Bool
not (WantedConstraints -> Bool
isEmptyWC WantedConstraints
wanted) Bool -> Bool -> Bool
||
    SkolemInfoAnon -> Bool
checkTelescopeSkol SkolemInfoAnon
skol_info_anon
  = -- checkTelescopeSkol: in this case, /always/ emit this implication
    -- even if 'wanted' is empty. We need the implication so that we check
    -- for a bad telescope. See Note [Skolem escape and forall-types] in
    -- GHC.Tc.Gen.HsType
    do { implic <- SkolemInfoAnon
-> [TcTyVar] -> TcLevel -> WantedConstraints -> TcM Implication
buildTvImplication SkolemInfoAnon
skol_info_anon [TcTyVar]
skol_tvs TcLevel
tclvl WantedConstraints
wanted
       ; emitImplication implic }

  | Bool
otherwise  -- Empty 'wanted', emit nothing
  = () -> TcM ()
forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
forall (m :: * -> *) a. Monad m => a -> m a
return ()
  where
     skol_info_anon :: SkolemInfoAnon
skol_info_anon = SkolemInfo -> SkolemInfoAnon
getSkolemInfo SkolemInfo
skol_info

buildTvImplication :: SkolemInfoAnon -> [TcTyVar]
                   -> TcLevel -> WantedConstraints -> TcM Implication
buildTvImplication :: SkolemInfoAnon
-> [TcTyVar] -> TcLevel -> WantedConstraints -> TcM Implication
buildTvImplication SkolemInfoAnon
skol_info [TcTyVar]
skol_tvs TcLevel
tclvl WantedConstraints
wanted
  = Bool -> SDoc -> TcM Implication -> TcM Implication
forall a. HasCallStack => Bool -> SDoc -> a -> a
assertPpr ((TcTyVar -> Bool) -> [TcTyVar] -> Bool
forall (t :: * -> *) a. Foldable t => (a -> Bool) -> t a -> Bool
all (TcTyVar -> Bool
isSkolemTyVar (TcTyVar -> Bool) -> (TcTyVar -> Bool) -> TcTyVar -> Bool
forall (f :: * -> *). Applicative f => f Bool -> f Bool -> f Bool
<||> TcTyVar -> Bool
isTyVarTyVar) [TcTyVar]
skol_tvs) ([TcTyVar] -> SDoc
forall a. Outputable a => a -> SDoc
ppr [TcTyVar]
skol_tvs) (TcM Implication -> TcM Implication)
-> TcM Implication -> TcM Implication
forall a b. (a -> b) -> a -> b
$
    do { ev_binds <- TcM EvBindsVar
newNoTcEvBinds  -- Used for equalities only, so all the constraints
                                     -- are solved by filling in coercion holes, not
                                     -- by creating a value-level evidence binding
       ; implic   <- newImplication

       ; let implic' = Implication
implic { ic_tclvl     = tclvl
                              , ic_skols     = skol_tvs
                              , ic_given_eqs = NoGivenEqs
                              , ic_wanted    = wanted
                              , ic_binds     = ev_binds
                              , ic_info      = skol_info }

       ; checkImplicationInvariants implic'
       ; return implic' }

implicationNeeded :: SkolemInfoAnon -> [TcTyVar] -> [EvVar] -> TcM Bool
-- See Note [When to build an implication]
implicationNeeded :: SkolemInfoAnon -> [TcTyVar] -> [TcTyVar] -> TcM Bool
implicationNeeded SkolemInfoAnon
skol_info [TcTyVar]
skol_tvs [TcTyVar]
given
  | [TcTyVar] -> Bool
forall a. [a] -> Bool
forall (t :: * -> *) a. Foldable t => t a -> Bool
null [TcTyVar]
skol_tvs
  , [TcTyVar] -> Bool
forall a. [a] -> Bool
forall (t :: * -> *) a. Foldable t => t a -> Bool
null [TcTyVar]
given
  , Bool -> Bool
not (SkolemInfoAnon -> Bool
alwaysBuildImplication SkolemInfoAnon
skol_info)
  = -- Empty skolems and givens
    do { tc_lvl <- TcM TcLevel
getTcLevel
       ; if not (isTopTcLevel tc_lvl)  -- No implication needed if we are
         then return False             -- already inside an implication
         else
    do { dflags <- getDynFlags       -- If any deferral can happen,
                                     -- we must build an implication
       ; return (gopt Opt_DeferTypeErrors dflags ||
                 gopt Opt_DeferTypedHoles dflags ||
                 gopt Opt_DeferOutOfScopeVariables dflags) } }

  | Bool
otherwise     -- Non-empty skolems or givens
  = Bool -> TcM Bool
forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
forall (m :: * -> *) a. Monad m => a -> m a
return Bool
True   -- Definitely need an implication

alwaysBuildImplication :: SkolemInfoAnon -> Bool
-- See Note [When to build an implication]
alwaysBuildImplication :: SkolemInfoAnon -> Bool
alwaysBuildImplication SkolemInfoAnon
_ = Bool
False

{-  Commmented out for now while I figure out about error messages.
    See #14185

alwaysBuildImplication (SigSkol ctxt _ _)
  = case ctxt of
      FunSigCtxt {} -> True  -- RHS of a binding with a signature
      _             -> False
alwaysBuildImplication (RuleSkol {})      = True
alwaysBuildImplication (InstSkol {})      = True
alwaysBuildImplication (FamInstSkol {})   = True
alwaysBuildImplication _                  = False
-}

buildImplicationFor :: TcLevel -> SkolemInfoAnon -> [TcTyVar]
                   -> [EvVar] -> WantedConstraints
                   -> TcM (Bag Implication, TcEvBinds)
buildImplicationFor :: TcLevel
-> SkolemInfoAnon
-> [TcTyVar]
-> [TcTyVar]
-> WantedConstraints
-> TcM (Bag Implication, TcEvBinds)
buildImplicationFor TcLevel
tclvl SkolemInfoAnon
skol_info [TcTyVar]
skol_tvs [TcTyVar]
given WantedConstraints
wanted
  | WantedConstraints -> Bool
isEmptyWC WantedConstraints
wanted Bool -> Bool -> Bool
&& [TcTyVar] -> Bool
forall a. [a] -> Bool
forall (t :: * -> *) a. Foldable t => t a -> Bool
null [TcTyVar]
given
             -- Optimisation : if there are no wanteds, and no givens
             -- don't generate an implication at all.
             -- Reason for the (null given): we don't want to lose
             -- the "inaccessible alternative" error check
  = (Bag Implication, TcEvBinds) -> TcM (Bag Implication, TcEvBinds)
forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
forall (m :: * -> *) a. Monad m => a -> m a
return (Bag Implication
forall a. Bag a
emptyBag, TcEvBinds
emptyTcEvBinds)

  | Bool
otherwise
  = Bool
-> SDoc
-> TcM (Bag Implication, TcEvBinds)
-> TcM (Bag Implication, TcEvBinds)
forall a. HasCallStack => Bool -> SDoc -> a -> a
assertPpr ((TcTyVar -> Bool) -> [TcTyVar] -> Bool
forall (t :: * -> *) a. Foldable t => (a -> Bool) -> t a -> Bool
all (TcTyVar -> Bool
isSkolemTyVar (TcTyVar -> Bool) -> (TcTyVar -> Bool) -> TcTyVar -> Bool
forall (f :: * -> *). Applicative f => f Bool -> f Bool -> f Bool
<||> TcTyVar -> Bool
isTyVarTyVar) [TcTyVar]
skol_tvs) ([TcTyVar] -> SDoc
forall a. Outputable a => a -> SDoc
ppr [TcTyVar]
skol_tvs) (TcM (Bag Implication, TcEvBinds)
 -> TcM (Bag Implication, TcEvBinds))
-> TcM (Bag Implication, TcEvBinds)
-> TcM (Bag Implication, TcEvBinds)
forall a b. (a -> b) -> a -> b
$
      -- Why allow TyVarTvs? Because implicitly declared kind variables in
      -- non-CUSK type declarations are TyVarTvs, and we need to bring them
      -- into scope as a skolem in an implication. This is OK, though,
      -- because TyVarTvs will always remain tyvars, even after unification.
    do { ev_binds_var <- TcM EvBindsVar
newTcEvBinds
       ; implic <- newImplication
       ; let implic' = Implication
implic { ic_tclvl  = tclvl
                              , ic_skols  = skol_tvs
                              , ic_given  = given
                              , ic_wanted = wanted
                              , ic_binds  = ev_binds_var
                              , ic_info   = skol_info }
       ; checkImplicationInvariants implic'

       ; return (unitBag implic', TcEvBinds ev_binds_var) }

{- Note [When to build an implication]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Suppose we have some 'skolems' and some 'givens', and we are
considering whether to wrap the constraints in their scope into an
implication.  We must /always/ do so if either 'skolems' or 'givens' are
non-empty.  But what if both are empty?  You might think we could
always drop the implication.  Other things being equal, the fewer
implications the better.  Less clutter and overhead.  But we must
take care:

* If we have an unsolved [W] g :: a ~# b, and -fdefer-type-errors,
  we'll make a /term-level/ evidence binding for 'g = error "blah"'.
  We must have an EvBindsVar those bindings!, otherwise they end up as
  top-level unlifted bindings, which are verboten. This only matters
  at top level, so we check for that
  See also Note [Deferred errors for coercion holes] in GHC.Tc.Errors.
  cf #14149 for an example of what goes wrong.

* This is /necessary/ for top level but may be /desirable/ even for
  nested bindings, because if the deferred coercion is bound too far
  out it will be reported even if that thunk (say) is not evaluated.

* If you have
     f :: Int;  f = f_blah
     g :: Bool; g = g_blah
  If we don't build an implication for f or g (no tyvars, no givens),
  the constraints for f_blah and g_blah are solved together.  And that
  can yield /very/ confusing error messages, because we can get
      [W] C Int b1    -- from f_blah
      [W] C Int b2    -- from g_blan
  and fundeps can yield [W] b1 ~ b2, even though the two functions have
  literally nothing to do with each other.  #14185 is an example.
  Building an implication keeps them separate.

Note [Herald for matchExpectedFunTys]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The 'herald' always looks like:
   "The equation(s) for 'f' have"
   "The abstraction (\x.e) takes"
   "The section (+ x) expects"
   "The function 'f' is applied to"

This is used to construct a message of form

   The abstraction `\Just 1 -> ...' takes two arguments
   but its type `Maybe a -> a' has only one

   The equation(s) for `f' have two arguments
   but its type `Maybe a -> a' has only one

   The section `(f 3)' requires 'f' to take two arguments
   but its type `Int -> Int' has only one

   The function 'f' is applied to two arguments
   but its type `Int -> Int' has only one

When visible type applications (e.g., `f @Int 1 2`, as in #13902) enter the
picture, we have a choice in deciding whether to count the type applications as
proper arguments:

   The function 'f' is applied to one visible type argument
     and two value arguments
   but its type `forall a. a -> a` has only one visible type argument
     and one value argument

Or whether to include the type applications as part of the herald itself:

   The expression 'f @Int' is applied to two arguments
   but its type `Int -> Int` has only one

The latter is easier to implement and is arguably easier to understand, so we
choose to implement that option.

Note [matchExpectedFunTys]
~~~~~~~~~~~~~~~~~~~~~~~~~~
matchExpectedFunTys checks that a sigma has the form
of an n-ary function.  It passes the decomposed type to the
thing_inside, and returns a wrapper to coerce between the two types

It's used wherever a language construct must have a functional type,
namely:
        A lambda expression
        A function definition
     An operator section

This function must be written CPS'd because it needs to fill in the
ExpTypes produced for arguments before it can fill in the ExpType
passed in.

Note [Return arguments with a fixed RuntimeRep]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The functions

  - matchExpectedFunTys,
  - matchActualFunTy,
  - matchActualFunTys,

peel off argument types, as explained in Note [matchExpectedFunTys].
It's important that these functions return argument types that have
a fixed runtime representation, otherwise we would be in violation
of the representation-polymorphism invariants of
Note [Representation polymorphism invariants] in GHC.Core.

This is why all these functions have an additional invariant,
that the argument types they return all have a syntactically fixed RuntimeRep,
in the sense of Note [Fixed RuntimeRep] in GHC.Tc.Utils.Concrete.

Example:

  Suppose we have

    type F :: Type -> RuntimeRep
    type family F a where { F Int = LiftedRep }

    type Dual :: Type -> Type
    type family Dual a where
      Dual a = a -> ()

    f :: forall (a :: TYPE (F Int)). Dual a
    f = \ x -> ()

  The body of `f` is a lambda abstraction, so we must be able to split off
  one argument type from its type. This is handled by `matchExpectedFunTys`
  (see 'GHC.Tc.Gen.Match.tcLambdaMatches'). We end up with desugared Core that
  looks like this:

    f :: forall (a :: TYPE (F Int)). Dual (a |> (TYPE F[0]))
    f = \ @(a :: TYPE (F Int)) ->
          (\ (x :: (a |> (TYPE F[0]))) -> ())
          `cast`
          (Sub (Sym (Dual[0] <(a |> (TYPE F[0]))>)))

  Two important transformations took place:

    1. We inserted casts around the argument type to ensure that it has
       a fixed runtime representation, as required by invariant (I1) from
       Note [Representation polymorphism invariants] in GHC.Core.
    2. We inserted a cast around the whole lambda to make everything line up
       with the type signature.
-}

-- | Use this function to split off arguments types when you have an
-- \"expected\" type.
--
-- This function skolemises at each polytype.
--
-- Invariant: this function only applies the provided function
-- to a list of argument types which all have a syntactically fixed RuntimeRep
-- in the sense of Note [Fixed RuntimeRep] in GHC.Tc.Utils.Concrete.
-- See Note [Return arguments with a fixed RuntimeRep].
matchExpectedFunTys :: forall a.
                       ExpectedFunTyOrigin  -- See Note [Herald for matchExpectedFunTys]
                    -> UserTypeCtxt
                    -> VisArity
                    -> ExpSigmaType
                    -> ([ExpPatType] -> ExpRhoType -> TcM a)
                    -> TcM (HsWrapper, a)
-- If    matchExpectedFunTys n ty = (wrap, _)
-- then  wrap : (t1 -> ... -> tn -> ty_r) ~> ty,
--   where [t1, ..., tn], ty_r are passed to the thing_inside
--
-- Unconditionally concludes by skolemising any trailing invisible
-- binders and, if DeepSubsumption is on, it does so deeply.
--
-- Postcondition:
--   If exp_ty is Check {}, then [ExpPatType] and ExpRhoType results are all Check{}
--   If exp_ty is Infer {}, then [ExpPatType] and ExpRhoType results are all Infer{}
matchExpectedFunTys :: forall a.
ExpectedFunTyOrigin
-> UserTypeCtxt
-> Int
-> ExpRhoType
-> ([ExpPatType] -> ExpRhoType -> TcM a)
-> TcM (HsWrapper, a)
matchExpectedFunTys ExpectedFunTyOrigin
herald UserTypeCtxt
_ Int
arity (Infer InferResult
inf_res) [ExpPatType] -> ExpRhoType -> TcM a
thing_inside
  = do { arg_tys <- (Int -> IOEnv (Env TcGblEnv TcLclEnv) (Scaled ExpRhoType))
-> [Int] -> IOEnv (Env TcGblEnv TcLclEnv) [Scaled ExpRhoType]
forall (t :: * -> *) (m :: * -> *) a b.
(Traversable t, Monad m) =>
(a -> m b) -> t a -> m (t b)
forall (m :: * -> *) a b. Monad m => (a -> m b) -> [a] -> m [b]
mapM (ExpectedFunTyOrigin
-> Int -> IOEnv (Env TcGblEnv TcLclEnv) (Scaled ExpRhoType)
new_infer_arg_ty ExpectedFunTyOrigin
herald) [Int
1 .. Int
arity]
       ; res_ty  <- newInferExpType
       ; result  <- thing_inside (map ExpFunPatTy arg_tys) res_ty
       ; arg_tys <- mapM (\(Scaled TcType
m ExpRhoType
t) -> TcType -> TcType -> Scaled TcType
forall a. TcType -> a -> Scaled a
Scaled TcType
m (TcType -> Scaled TcType) -> TcM TcType -> TcM (Scaled TcType)
forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
<$> ExpRhoType -> TcM TcType
forall (m :: * -> *). MonadIO m => ExpRhoType -> m TcType
readExpType ExpRhoType
t) arg_tys
       ; res_ty  <- readExpType res_ty
       ; co <- fillInferResult (mkScaledFunTys arg_tys res_ty) inf_res
       ; return (mkWpCastN co, result) }

matchExpectedFunTys ExpectedFunTyOrigin
herald UserTypeCtxt
ctx Int
arity (Check TcType
top_ty) [ExpPatType] -> ExpRhoType -> TcM a
thing_inside
  = Int
-> [ExpPatType]
-> TcType
-> IOEnv (Env TcGblEnv TcLclEnv) (HsWrapper, a)
check Int
arity [] TcType
top_ty
  where
    check :: VisArity -> [ExpPatType] -> TcSigmaType -> TcM (HsWrapper, a)
    -- `check` is called only in the Check{} case
    -- It collects rev_pat_tys in reversed order
    -- n_req is the number of /visible/ arguments still needed

    ----------------------------
    -- Skolemise quantifiers, both visible (up to n_req) and invisible
    -- See Note [Visible type application and abstraction] in GHC.Tc.Gen.App
    check :: Int
-> [ExpPatType]
-> TcType
-> IOEnv (Env TcGblEnv TcLclEnv) (HsWrapper, a)
check Int
n_req [ExpPatType]
rev_pat_tys TcType
ty
      | TcType -> Bool
isSigmaTy TcType
ty                     -- An invisible quantifier at the top
        Bool -> Bool -> Bool
|| (Int
n_req Int -> Int -> Bool
forall a. Ord a => a -> a -> Bool
> Int
0 Bool -> Bool -> Bool
&& TcType -> Bool
isForAllTy TcType
ty)  -- A visible quantifier at top, and we need it
      = do { rec { (n_req', wrap_gen, tv_nms, bndrs, given, inner_ty) <- skolemiseRequired skol_info n_req ty
                 ; let sig_skol = UserTypeCtxt -> TcType -> [(Name, TcTyVar)] -> SkolemInfoAnon
SigSkol UserTypeCtxt
ctx TcType
top_ty ([Name]
tv_nms [Name] -> [TcTyVar] -> [(Name, TcTyVar)]
forall a b. [a] -> [b] -> [(a, b)]
`zip` [TcTyVar]
skol_tvs)
                       skol_tvs = [ForAllTyBinder] -> [TcTyVar]
forall tv argf. [VarBndr tv argf] -> [tv]
binderVars [ForAllTyBinder]
bndrs
                 ; skol_info <- mkSkolemInfo sig_skol }
             -- rec {..}: see Note [Keeping SkolemInfo inside a SkolemTv]
             --           in GHC.Tc.Utils.TcType
           ; (ev_binds, (wrap_res, result))
                  <- checkConstraints (getSkolemInfo skol_info) skol_tvs given $
                     check n_req'
                           (reverse (map ExpForAllPatTy bndrs) ++ rev_pat_tys)
                           inner_ty
           ; assertPpr (not (null bndrs && null given)) (ppr ty) $
                       -- The guard ensures that we made some progress
             return (wrap_gen <.> mkWpLet ev_binds <.> wrap_res, result) }

    ----------------------------
    -- Base case: (n_req == 0): no more args
    --    The earlier skolemisation ensurs that rho_ty has no top-level invisible quantifiers
    --    If there is deep subsumption, do deep skolemisation now
    check Int
n_req [ExpPatType]
rev_pat_tys TcType
rho_ty
      | Int
n_req Int -> Int -> Bool
forall a. Eq a => a -> a -> Bool
== Int
0
      = do { let pat_tys :: [ExpPatType]
pat_tys = [ExpPatType] -> [ExpPatType]
forall a. [a] -> [a]
reverse [ExpPatType]
rev_pat_tys
           ; ds_flag <- TcM DeepSubsumptionFlag
getDeepSubsumptionFlag
           ; case ds_flag of
               DeepSubsumptionFlag
Shallow -> do { res <- [ExpPatType] -> ExpRhoType -> TcM a
thing_inside [ExpPatType]
pat_tys (TcType -> ExpRhoType
mkCheckExpType TcType
rho_ty)
                             ; return (idHsWrapper, res) }
               DeepSubsumptionFlag
Deep    -> DeepSubsumptionFlag
-> UserTypeCtxt
-> TcType
-> TcType
-> ([(Name, TcInvisTVBinder)] -> TcType -> TcM a)
-> IOEnv (Env TcGblEnv TcLclEnv) (HsWrapper, a)
forall result.
DeepSubsumptionFlag
-> UserTypeCtxt
-> TcType
-> TcType
-> ([(Name, TcInvisTVBinder)] -> TcType -> TcM result)
-> TcM (HsWrapper, result)
tcSkolemiseGeneral DeepSubsumptionFlag
Deep UserTypeCtxt
ctx TcType
top_ty TcType
rho_ty (([(Name, TcInvisTVBinder)] -> TcType -> TcM a)
 -> IOEnv (Env TcGblEnv TcLclEnv) (HsWrapper, a))
-> ([(Name, TcInvisTVBinder)] -> TcType -> TcM a)
-> IOEnv (Env TcGblEnv TcLclEnv) (HsWrapper, a)
forall a b. (a -> b) -> a -> b
$ \[(Name, TcInvisTVBinder)]
_ TcType
rho_ty ->
                          -- "_" drop the /deeply/-skolemise binders
                          -- They do not line up with binders in the Match
                          [ExpPatType] -> ExpRhoType -> TcM a
thing_inside [ExpPatType]
pat_tys (TcType -> ExpRhoType
mkCheckExpType TcType
rho_ty) }

    ----------------------------
    -- Function types
    check Int
n_req [ExpPatType]
rev_pat_tys (FunTy { ft_af :: TcType -> FunTyFlag
ft_af = FunTyFlag
af, ft_mult :: TcType -> TcType
ft_mult = TcType
mult
                                   , ft_arg :: TcType -> TcType
ft_arg = TcType
arg_ty, ft_res :: TcType -> TcType
ft_res = TcType
res_ty })
      = Bool
-> IOEnv (Env TcGblEnv TcLclEnv) (HsWrapper, a)
-> IOEnv (Env TcGblEnv TcLclEnv) (HsWrapper, a)
forall a. HasCallStack => Bool -> a -> a
assert (FunTyFlag -> Bool
isVisibleFunArg FunTyFlag
af) (IOEnv (Env TcGblEnv TcLclEnv) (HsWrapper, a)
 -> IOEnv (Env TcGblEnv TcLclEnv) (HsWrapper, a))
-> IOEnv (Env TcGblEnv TcLclEnv) (HsWrapper, a)
-> IOEnv (Env TcGblEnv TcLclEnv) (HsWrapper, a)
forall a b. (a -> b) -> a -> b
$
        do { let arg_pos :: Int
arg_pos = Int
arity Int -> Int -> Int
forall a. Num a => a -> a -> a
- Int
n_req Int -> Int -> Int
forall a. Num a => a -> a -> a
+ Int
1   -- 1 for the first argument etc
           ; (arg_co, arg_ty) <- HasDebugCallStack =>
FixedRuntimeRepContext -> TcType -> TcM (Coercion, TcType)
FixedRuntimeRepContext -> TcType -> TcM (Coercion, TcType)
hasFixedRuntimeRep (ExpectedFunTyOrigin -> Int -> FixedRuntimeRepContext
FRRExpectedFunTy ExpectedFunTyOrigin
herald Int
arg_pos) TcType
arg_ty
           ; (wrap_res, result) <- check (n_req - 1)
                                         (mkCheckExpFunPatTy (Scaled mult arg_ty) : rev_pat_tys)
                                         res_ty
           ; let wrap_arg = Coercion -> HsWrapper
mkWpCastN Coercion
arg_co
                 fun_wrap = HsWrapper -> HsWrapper -> Scaled TcType -> TcType -> HsWrapper
mkWpFun HsWrapper
wrap_arg HsWrapper
wrap_res (TcType -> TcType -> Scaled TcType
forall a. TcType -> a -> Scaled a
Scaled TcType
mult TcType
arg_ty) TcType
res_ty
           ; return (fun_wrap, result) }

    ----------------------------
    -- Type variables
    check Int
n_req [ExpPatType]
rev_pat_tys ty :: TcType
ty@(TyVarTy TcTyVar
tv)
      | TcTyVar -> Bool
isMetaTyVar TcTyVar
tv
      = do { cts <- TcTyVar -> IOEnv (Env TcGblEnv TcLclEnv) MetaDetails
forall (m :: * -> *). MonadIO m => TcTyVar -> m MetaDetails
readMetaTyVar TcTyVar
tv
           ; case cts of
               Indirect TcType
ty' -> Int
-> [ExpPatType]
-> TcType
-> IOEnv (Env TcGblEnv TcLclEnv) (HsWrapper, a)
check Int
n_req [ExpPatType]
rev_pat_tys TcType
ty'
               MetaDetails
Flexi        -> Int
-> [ExpPatType]
-> TcType
-> IOEnv (Env TcGblEnv TcLclEnv) (HsWrapper, a)
defer Int
n_req [ExpPatType]
rev_pat_tys TcType
ty }

    ----------------------------
    -- NOW do coreView.  We didn't do it before, so that we do not unnecessarily
    -- unwrap a synonym in the returned rho_ty
    check Int
n_req [ExpPatType]
rev_pat_tys TcType
ty
      | Just TcType
ty' <- TcType -> Maybe TcType
coreView TcType
ty = Int
-> [ExpPatType]
-> TcType
-> IOEnv (Env TcGblEnv TcLclEnv) (HsWrapper, a)
check Int
n_req [ExpPatType]
rev_pat_tys TcType
ty'

       -- In all other cases we bale out into ordinary unification
       -- However unlike the meta-tyvar case, we are sure that the
       -- number of arguments doesn't match arity of the original
       -- type, so we can add a bit more context to the error message
       -- (cf #7869).
       --
       -- It is not always an error, because specialized type may have
       -- different arity, for example:
       --
       -- > f1 = f2 'a'
       -- > f2 :: Monad m => m Bool
       -- > f2 = undefined
       --
       -- But in that case we add specialized type into error context
       -- anyway, because it may be useful. See also #9605.
    check Int
n_req [ExpPatType]
rev_pat_tys TcType
res_ty
      = (TidyEnv -> ZonkM (TidyEnv, ErrCtxtMsg))
-> IOEnv (Env TcGblEnv TcLclEnv) (HsWrapper, a)
-> IOEnv (Env TcGblEnv TcLclEnv) (HsWrapper, a)
forall a.
(TidyEnv -> ZonkM (TidyEnv, ErrCtxtMsg)) -> TcM a -> TcM a
addErrCtxtM (ExpectedFunTyOrigin
-> (Int, TcType) -> TidyEnv -> ZonkM (TidyEnv, ErrCtxtMsg)
mkFunTysMsg ExpectedFunTyOrigin
herald (Int
arity, TcType
top_ty))  (IOEnv (Env TcGblEnv TcLclEnv) (HsWrapper, a)
 -> IOEnv (Env TcGblEnv TcLclEnv) (HsWrapper, a))
-> IOEnv (Env TcGblEnv TcLclEnv) (HsWrapper, a)
-> IOEnv (Env TcGblEnv TcLclEnv) (HsWrapper, a)
forall a b. (a -> b) -> a -> b
$
        Int
-> [ExpPatType]
-> TcType
-> IOEnv (Env TcGblEnv TcLclEnv) (HsWrapper, a)
defer Int
n_req [ExpPatType]
rev_pat_tys TcType
res_ty

    ------------
    defer :: VisArity -> [ExpPatType] -> TcRhoType -> TcM (HsWrapper, a)
    defer :: Int
-> [ExpPatType]
-> TcType
-> IOEnv (Env TcGblEnv TcLclEnv) (HsWrapper, a)
defer Int
n_req [ExpPatType]
rev_pat_tys TcType
fun_ty
      = do { more_arg_tys <- (Int -> TcM (Scaled TcType))
-> [Int] -> IOEnv (Env TcGblEnv TcLclEnv) [Scaled TcType]
forall (t :: * -> *) (m :: * -> *) a b.
(Traversable t, Monad m) =>
(a -> m b) -> t a -> m (t b)
forall (m :: * -> *) a b. Monad m => (a -> m b) -> [a] -> m [b]
mapM (ExpectedFunTyOrigin -> Int -> TcM (Scaled TcType)
new_check_arg_ty ExpectedFunTyOrigin
herald) [Int
arity Int -> Int -> Int
forall a. Num a => a -> a -> a
- Int
n_req Int -> Int -> Int
forall a. Num a => a -> a -> a
+ Int
1 .. Int
arity]
           ; let all_pats = [ExpPatType] -> [ExpPatType]
forall a. [a] -> [a]
reverse [ExpPatType]
rev_pat_tys [ExpPatType] -> [ExpPatType] -> [ExpPatType]
forall a. [a] -> [a] -> [a]
++ (Scaled TcType -> ExpPatType) -> [Scaled TcType] -> [ExpPatType]
forall a b. (a -> b) -> [a] -> [b]
map Scaled TcType -> ExpPatType
mkCheckExpFunPatTy [Scaled TcType]
more_arg_tys
           ; res_ty <- newOpenFlexiTyVarTy
           ; result <- thing_inside all_pats (mkCheckExpType res_ty)

           ; co <- unifyType Nothing (mkScaledFunTys more_arg_tys res_ty) fun_ty
           ; return (mkWpCastN co, result) }

new_infer_arg_ty :: ExpectedFunTyOrigin -> Int -> TcM (Scaled ExpSigmaTypeFRR)
new_infer_arg_ty :: ExpectedFunTyOrigin
-> Int -> IOEnv (Env TcGblEnv TcLclEnv) (Scaled ExpRhoType)
new_infer_arg_ty ExpectedFunTyOrigin
herald Int
arg_pos -- position for error messages only
  = do { mult     <- TcType -> TcM TcType
newFlexiTyVarTy TcType
multiplicityTy
       ; inf_hole <- newInferExpTypeFRR (FRRExpectedFunTy herald arg_pos)
       ; return (mkScaled mult inf_hole) }

new_check_arg_ty :: ExpectedFunTyOrigin -> Int -> TcM (Scaled TcType)
new_check_arg_ty :: ExpectedFunTyOrigin -> Int -> TcM (Scaled TcType)
new_check_arg_ty ExpectedFunTyOrigin
herald Int
arg_pos -- Position for error messages only, 1 for first arg
  = do { mult   <- TcType -> TcM TcType
newFlexiTyVarTy TcType
multiplicityTy
       ; arg_ty <- newOpenFlexiFRRTyVarTy (FRRExpectedFunTy herald arg_pos)
       ; return (mkScaled mult arg_ty) }

mkFunTysMsg :: ExpectedFunTyOrigin
            -> (VisArity, TcType)
            -> TidyEnv -> ZonkM (TidyEnv, ErrCtxtMsg)
-- See Note [Reporting application arity errors]
mkFunTysMsg :: ExpectedFunTyOrigin
-> (Int, TcType) -> TidyEnv -> ZonkM (TidyEnv, ErrCtxtMsg)
mkFunTysMsg ExpectedFunTyOrigin
herald (Int
n_vis_args_in_call, TcType
fun_ty) TidyEnv
env
  = do { (env', fun_ty) <- TidyEnv -> TcType -> ZonkM (TidyEnv, TcType)
zonkTidyTcType TidyEnv
env TcType
fun_ty

       ; let (pi_ty_bndrs, _) = splitPiTys fun_ty
             n_fun_args = (PiTyBinder -> Bool) -> [PiTyBinder] -> Int
forall a. (a -> Bool) -> [a] -> Int
count PiTyBinder -> Bool
isVisiblePiTyBinder [PiTyBinder]
pi_ty_bndrs

       ; return (env', FunTysCtxt herald fun_ty n_vis_args_in_call n_fun_args) }


{- Note [Reporting application arity errors]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider      f :: Int -> Int -> Int
and the call  foo = f 3 4 5
We'd like to get an error like:

    • Couldn't match expected type ‘t0 -> t’ with actual type ‘Int’
    • The function ‘f’ is applied to three visible arguments,           -- What are "visible" arguments?
        but its type ‘Int -> Int -> Int’ has only two                   -- See Note [Visibility and arity] in GHC.Types.Basic

That is what `mkFunTysMsg` tries to do.  But what is the "type of the function".
Most obviously, we can report its full, polymorphic type; that is simple and
explicable.  But sometimes a bit odd.  Consider
    f :: Bool -> t Int Int
    foo = f True 5 10
We get this error:
    • Couldn't match type ‘Int’ with ‘t0 -> t’
      Expected: Int -> t0 -> t
        Actual: Int -> Int
    • The function ‘f’ is applied to three visible arguments,
        but its type ‘Bool -> t Int Int’ has only one

That's not /quite/ right beause we can instantiate `t` to an arrow and get
two arrows (but not three!).  With that in mind, one could consider reporting
the /instantiated/ type, and GHC used to do so.  But it's more work, and in
some ways more confusing, especially when nested quantifiers are concerned, e.g.
    f :: Bool -> forall t. t Int Int

So we just keep it simple and report the original function type.


************************************************************************
*                                                                      *
                    Other matchExpected functions
*                                                                      *
********************************************************************* -}

matchExpectedListTy :: TcRhoType -> TcM (TcCoercionN, TcRhoType)
-- Special case for lists
matchExpectedListTy :: TcType -> TcM (Coercion, TcType)
matchExpectedListTy TcType
exp_ty
 = do { (co, [elt_ty]) <- TyCon -> TcType -> TcM (Coercion, [TcType])
matchExpectedTyConApp TyCon
listTyCon TcType
exp_ty
      ; return (co, elt_ty) }

---------------------
matchExpectedTyConApp :: TyCon                -- T :: forall kv1 ... kvm. k1 -> ... -> kn -> *
                      -> TcRhoType            -- orig_ty
                      -> TcM (TcCoercionN,    -- T k1 k2 k3 a b c ~N orig_ty
                              [TcSigmaType])  -- Element types, k1 k2 k3 a b c

-- It's used for wired-in tycons, so we call checkWiredInTyCon
-- Precondition: never called with FunTyCon
-- Precondition: input type :: *
-- Postcondition: (T k1 k2 k3 a b c) is well-kinded

matchExpectedTyConApp :: TyCon -> TcType -> TcM (Coercion, [TcType])
matchExpectedTyConApp TyCon
tc TcType
orig_ty
  = Bool
-> SDoc -> TcM (Coercion, [TcType]) -> TcM (Coercion, [TcType])
forall a. HasCallStack => Bool -> SDoc -> a -> a
assertPpr (TyCon -> Bool
isAlgTyCon TyCon
tc) (TyCon -> SDoc
forall a. Outputable a => a -> SDoc
ppr TyCon
tc) (TcM (Coercion, [TcType]) -> TcM (Coercion, [TcType]))
-> TcM (Coercion, [TcType]) -> TcM (Coercion, [TcType])
forall a b. (a -> b) -> a -> b
$
    TcType -> TcM (Coercion, [TcType])
go TcType
orig_ty
  where
    go :: TcType -> TcM (Coercion, [TcType])
go TcType
ty
       | Just TcType
ty' <- TcType -> Maybe TcType
coreView TcType
ty
       = TcType -> TcM (Coercion, [TcType])
go TcType
ty'

    go ty :: TcType
ty@(TyConApp TyCon
tycon [TcType]
args)
       | TyCon
tc TyCon -> TyCon -> Bool
forall a. Eq a => a -> a -> Bool
== TyCon
tycon  -- Common case
       = (Coercion, [TcType]) -> TcM (Coercion, [TcType])
forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
forall (m :: * -> *) a. Monad m => a -> m a
return (TcType -> Coercion
mkNomReflCo TcType
ty, [TcType]
args)

    go (TyVarTy TcTyVar
tv)
       | TcTyVar -> Bool
isMetaTyVar TcTyVar
tv
       = do { cts <- TcTyVar -> IOEnv (Env TcGblEnv TcLclEnv) MetaDetails
forall (m :: * -> *). MonadIO m => TcTyVar -> m MetaDetails
readMetaTyVar TcTyVar
tv
            ; case cts of
                Indirect TcType
ty -> TcType -> TcM (Coercion, [TcType])
go TcType
ty
                MetaDetails
Flexi       -> TcM (Coercion, [TcType])
defer }

    go TcType
_ = TcM (Coercion, [TcType])
defer

    -- If the common case does not occur, instantiate a template
    -- T k1 .. kn t1 .. tm, and unify with the original type
    -- Doing it this way ensures that the types we return are
    -- kind-compatible with T.  For example, suppose we have
    --       matchExpectedTyConApp T (f Maybe)
    -- where data T a = MkT a
    -- Then we don't want to instantiate T's data constructors with
    --    (a::*) ~ Maybe
    -- because that'll make types that are utterly ill-kinded.
    -- This happened in #7368
    defer :: TcM (Coercion, [TcType])
defer
      = do { (_, arg_tvs) <- [TcTyVar] -> TcM (Subst, [TcTyVar])
newMetaTyVars (TyCon -> [TcTyVar]
tyConTyVars TyCon
tc)
           ; traceTc "matchExpectedTyConApp" (ppr tc $$ ppr (tyConTyVars tc) $$ ppr arg_tvs)
           ; let args = [TcTyVar] -> [TcType]
mkTyVarTys [TcTyVar]
arg_tvs
                 tc_template = TyCon -> [TcType] -> TcType
mkTyConApp TyCon
tc [TcType]
args
           ; co <- unifyType Nothing tc_template orig_ty
           ; return (co, args) }

----------------------
matchExpectedAppTy :: TcRhoType                         -- orig_ty
                   -> TcM (TcCoercion,                   -- m a ~N orig_ty
                           (TcSigmaType, TcSigmaType))  -- Returns m, a
-- If the incoming type is a mutable type variable of kind k, then
-- matchExpectedAppTy returns a new type variable (m: * -> k); note the *.

matchExpectedAppTy :: TcType -> TcM (Coercion, (TcType, TcType))
matchExpectedAppTy TcType
orig_ty
  = TcType -> TcM (Coercion, (TcType, TcType))
go TcType
orig_ty
  where
    go :: TcType -> TcM (Coercion, (TcType, TcType))
go TcType
ty
      | Just TcType
ty' <- TcType -> Maybe TcType
coreView TcType
ty = TcType -> TcM (Coercion, (TcType, TcType))
go TcType
ty'

      | Just (TcType
fun_ty, TcType
arg_ty) <- TcType -> Maybe (TcType, TcType)
tcSplitAppTy_maybe TcType
ty
      = (Coercion, (TcType, TcType)) -> TcM (Coercion, (TcType, TcType))
forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
forall (m :: * -> *) a. Monad m => a -> m a
return (TcType -> Coercion
mkNomReflCo TcType
orig_ty, (TcType
fun_ty, TcType
arg_ty))

    go (TyVarTy TcTyVar
tv)
      | TcTyVar -> Bool
isMetaTyVar TcTyVar
tv
      = do { cts <- TcTyVar -> IOEnv (Env TcGblEnv TcLclEnv) MetaDetails
forall (m :: * -> *). MonadIO m => TcTyVar -> m MetaDetails
readMetaTyVar TcTyVar
tv
           ; case cts of
               Indirect TcType
ty -> TcType -> TcM (Coercion, (TcType, TcType))
go TcType
ty
               MetaDetails
Flexi       -> TcM (Coercion, (TcType, TcType))
defer }

    go TcType
_ = TcM (Coercion, (TcType, TcType))
defer

    -- Defer splitting by generating an equality constraint
    defer :: TcM (Coercion, (TcType, TcType))
defer
      = do { ty1 <- TcType -> TcM TcType
newFlexiTyVarTy TcType
kind1
           ; ty2 <- newFlexiTyVarTy kind2
           ; co <- unifyType Nothing (mkAppTy ty1 ty2) orig_ty
           ; return (co, (ty1, ty2)) }

    orig_kind :: TcType
orig_kind = HasDebugCallStack => TcType -> TcType
TcType -> TcType
typeKind TcType
orig_ty
    kind1 :: TcType
kind1 = HasDebugCallStack => TcType -> TcType -> TcType
TcType -> TcType -> TcType
mkVisFunTyMany TcType
liftedTypeKind TcType
orig_kind
    kind2 :: TcType
kind2 = TcType
liftedTypeKind    -- m :: * -> k
                              -- arg type :: *

{- **********************************************************************
*
                      fillInferResult
*
********************************************************************** -}

{- Note [inferResultToType]
~~~~~~~~~~~~~~~~~~~~~~~~~~~
expTypeToType and inferResultType convert an InferResult to a monotype.
It must be a monotype because if the InferResult isn't already filled in,
we fill it in with a unification variable (hence monotype).  So to preserve
order-independence we check for mono-type-ness even if it *is* filled in
already.

See also Note [TcLevel of ExpType] in GHC.Tc.Utils.TcType, and
Note [fillInferResult].
-}

-- | Fill an 'InferResult' with the given type.
--
-- If @co = fillInferResult t1 infer_res@, then @co :: t1 ~# t2@,
-- where @t2@ is the type stored in the 'ir_ref' field of @infer_res@.
--
-- This function enforces the following invariants:
--
--  - Level invariant.
--    The stored type @t2@ is at the same level as given by the
--    'ir_lvl' field.
--  - FRR invariant.
--    Whenever the 'ir_frr' field is not @Nothing@, @t2@ is guaranteed
--    to have a syntactically fixed RuntimeRep, in the sense of
--    Note [Fixed RuntimeRep] in GHC.Tc.Utils.Concrete.
fillInferResult :: TcType -> InferResult -> TcM TcCoercionN
fillInferResult :: TcType -> InferResult -> TcM Coercion
fillInferResult TcType
act_res_ty (IR { ir_uniq :: InferResult -> Unique
ir_uniq = Unique
u
                               , ir_lvl :: InferResult -> TcLevel
ir_lvl  = TcLevel
res_lvl
                               , ir_frr :: InferResult -> Maybe FixedRuntimeRepContext
ir_frr  = Maybe FixedRuntimeRepContext
mb_frr
                               , ir_ref :: InferResult -> IORef (Maybe TcType)
ir_ref  = IORef (Maybe TcType)
ref })
  = do { mb_exp_res_ty <- IORef (Maybe TcType)
-> IOEnv (Env TcGblEnv TcLclEnv) (Maybe TcType)
forall (m :: * -> *) a. MonadIO m => TcRef a -> m a
readTcRef IORef (Maybe TcType)
ref
       ; case mb_exp_res_ty of
            Just TcType
exp_res_ty
               -- We progressively refine the type stored in 'ref',
               -- for example when inferring types across multiple equations.
               --
               -- Example:
               --
               --  \ x -> case y of { True -> x ; False -> 3 :: Int }
               --
               -- When inferring the return type of this function, we will create
               -- an 'Infer' 'ExpType', which will first be filled by the type of 'x'
               -- after typechecking the first equation, and then filled again with
               -- the type 'Int', at which point we want to ensure that we unify
               -- the type of 'x' with 'Int'. This is what is happening below when
               -- we are "joining" several inferred 'ExpType's.
               -> do { String -> SDoc -> TcM ()
traceTc String
"Joining inferred ExpType" (SDoc -> TcM ()) -> SDoc -> TcM ()
forall a b. (a -> b) -> a -> b
$
                       Unique -> SDoc
forall a. Outputable a => a -> SDoc
ppr Unique
u SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<> SDoc
forall doc. IsLine doc => doc
colon SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> TcType -> SDoc
forall a. Outputable a => a -> SDoc
ppr TcType
act_res_ty SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> Char -> SDoc
forall doc. IsLine doc => Char -> doc
char Char
'~' SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> TcType -> SDoc
forall a. Outputable a => a -> SDoc
ppr TcType
exp_res_ty
                     ; cur_lvl <- TcM TcLevel
getTcLevel
                     ; unless (cur_lvl `sameDepthAs` res_lvl) $
                       ensureMonoType act_res_ty
                     ; unifyType Nothing act_res_ty exp_res_ty }
            Maybe TcType
Nothing
               -> do { String -> SDoc -> TcM ()
traceTc String
"Filling inferred ExpType" (SDoc -> TcM ()) -> SDoc -> TcM ()
forall a b. (a -> b) -> a -> b
$
                       Unique -> SDoc
forall a. Outputable a => a -> SDoc
ppr Unique
u SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> String -> SDoc
forall doc. IsLine doc => String -> doc
text String
":=" SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> TcType -> SDoc
forall a. Outputable a => a -> SDoc
ppr TcType
act_res_ty

                     -- Enforce the level invariant: ensure the TcLevel of
                     -- the type we are writing to 'ref' matches 'ir_lvl'.
                     ; (prom_co, act_res_ty) <- TcLevel -> TcType -> TcM (Coercion, TcType)
promoteTcType TcLevel
res_lvl TcType
act_res_ty

                     -- Enforce the FRR invariant: ensure the type has a syntactically
                     -- fixed RuntimeRep (if necessary, i.e. 'mb_frr' is not 'Nothing').
                     ; (frr_co, act_res_ty) <-
                         case mb_frr of
                           Maybe FixedRuntimeRepContext
Nothing       -> (Coercion, TcType) -> TcM (Coercion, TcType)
forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
forall (m :: * -> *) a. Monad m => a -> m a
return (TcType -> Coercion
mkNomReflCo TcType
act_res_ty, TcType
act_res_ty)
                           Just FixedRuntimeRepContext
frr_orig -> HasDebugCallStack =>
FixedRuntimeRepContext -> TcType -> TcM (Coercion, TcType)
FixedRuntimeRepContext -> TcType -> TcM (Coercion, TcType)
hasFixedRuntimeRep FixedRuntimeRepContext
frr_orig TcType
act_res_ty

                     -- Compose the two coercions.
                     ; let final_co = Coercion
prom_co HasDebugCallStack => Coercion -> Coercion -> Coercion
Coercion -> Coercion -> Coercion
`mkTransCo` Coercion
frr_co

                     ; writeTcRef ref (Just act_res_ty)

                     ; return final_co }
     }

{- Note [fillInferResult]
~~~~~~~~~~~~~~~~~~~~~~~~~
When inferring, we use fillInferResult to "fill in" the hole in InferResult
   data InferResult = IR { ir_uniq :: Unique
                         , ir_lvl  :: TcLevel
                         , ir_ref  :: IORef (Maybe TcType) }

There are two things to worry about:

1. What if it is under a GADT or existential pattern match?
   - GADTs: a unification variable (and Infer's hole is similar) is untouchable
   - Existentials: be careful about skolem-escape

2. What if it is filled in more than once?  E.g. multiple branches of a case
     case e of
        T1 -> e1
        T2 -> e2

Our typing rules are:

* The RHS of a existential or GADT alternative must always be a
  monotype, regardless of the number of alternatives.

* Multiple non-existential/GADT branches can have (the same)
  higher rank type (#18412).  E.g. this is OK:
      case e of
        True  -> hr
        False -> hr
  where hr:: (forall a. a->a) -> Int
  c.f. Section 7.1 of "Practical type inference for arbitrary-rank types"
       We use choice (2) in that Section.
       (GHC 8.10 and earlier used choice (1).)

  But note that
      case e of
        True  -> hr
        False -> \x -> hr x
  will fail, because we still /infer/ both branches, so the \x will get
  a (monotype) unification variable, which will fail to unify with
  (forall a. a->a)

For (1) we can detect the GADT/existential situation by seeing that
the current TcLevel is greater than that stored in ir_lvl of the Infer
ExpType.  We bump the level whenever we go past a GADT/existential match.

Then, before filling the hole use promoteTcType to promote the type
to the outer ir_lvl.  promoteTcType does this
  - create a fresh unification variable alpha at level ir_lvl
  - emits an equality alpha[ir_lvl] ~ ty
  - fills the hole with alpha
That forces the type to be a monotype (since unification variables can
only unify with monotypes); and catches skolem-escapes because the
alpha is untouchable until the equality floats out.

For (2), we simply look to see if the hole is filled already.
  - if not, we promote (as above) and fill the hole
  - if it is filled, we simply unify with the type that is
    already there

There is one wrinkle.  Suppose we have
   case e of
      T1 -> e1 :: (forall a. a->a) -> Int
      G2 -> e2
where T1 is not GADT or existential, but G2 is a GADT.  Then suppose the
T1 alternative fills the hole with (forall a. a->a) -> Int, which is fine.
But now the G2 alternative must not *just* unify with that else we'd risk
allowing through (e2 :: (forall a. a->a) -> Int).  If we'd checked G2 first
we'd have filled the hole with a unification variable, which enforces a
monotype.

So if we check G2 second, we still want to emit a constraint that restricts
the RHS to be a monotype. This is done by ensureMonoType, and it works
by simply generating a constraint (alpha ~ ty), where alpha is a fresh
unification variable.  We discard the evidence.

-}



{-
************************************************************************
*                                                                      *
                Subsumption checking
*                                                                      *
************************************************************************

Note [Subsumption checking: tcSubType]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
All the tcSubType calls have the form
                tcSubType actual_ty expected_ty
which checks
                actual_ty <= expected_ty

That is, that a value of type actual_ty is acceptable in
a place expecting a value of type expected_ty.  I.e. that

    actual ty   is more polymorphic than   expected_ty

It returns a wrapper function
        co_fn :: actual_ty ~ expected_ty
which takes an HsExpr of type actual_ty into one of type
expected_ty.

Note [Ambiguity check and deep subsumption]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider
   f :: (forall b. Eq b => a -> a) -> Int

Does `f` have an ambiguous type?   The ambiguity check usually checks
that this definition of f' would typecheck, where f' has the exact same
type as f:
   f' :: (forall b. Eq b => a -> a) -> Intp
   f' = f

This will be /rejected/ with DeepSubsumption but /accepted/ with
ShallowSubsumption.  On the other hand, this eta-expanded version f''
would be rejected both ways:
   f'' :: (forall b. Eq b => a -> a) -> Intp
   f'' x = f x

This is squishy in the same way as other examples in GHC.Tc.Validity
Note [The squishiness of the ambiguity check]

The situation in June 2022.  Since we have SimpleSubsumption at the moment,
we don't want introduce new breakage if you add -XDeepSubsumption, by
rejecting types as ambiguous that weren't ambiguous before.  So, as a
holding decision, we /always/ use SimpleSubsumption for the ambiguity check
(erring on the side accepting more programs). Hence tcSubTypeAmbiguity.
-}



-----------------
-- tcWrapResult needs both un-type-checked (for origins and error messages)
-- and type-checked (for wrapping) expressions
tcWrapResult :: HsExpr GhcRn -> HsExpr GhcTc -> TcSigmaType -> ExpRhoType
             -> TcM (HsExpr GhcTc)
tcWrapResult :: HsExpr GhcRn
-> HsExpr GhcTc -> TcType -> ExpRhoType -> TcM (HsExpr GhcTc)
tcWrapResult HsExpr GhcRn
rn_expr = CtOrigin
-> HsExpr GhcRn
-> HsExpr GhcTc
-> TcType
-> ExpRhoType
-> TcM (HsExpr GhcTc)
tcWrapResultO (HsExpr GhcRn -> CtOrigin
exprCtOrigin HsExpr GhcRn
rn_expr) HsExpr GhcRn
rn_expr

tcWrapResultO :: CtOrigin -> HsExpr GhcRn -> HsExpr GhcTc -> TcSigmaType -> ExpRhoType
               -> TcM (HsExpr GhcTc)
tcWrapResultO :: CtOrigin
-> HsExpr GhcRn
-> HsExpr GhcTc
-> TcType
-> ExpRhoType
-> TcM (HsExpr GhcTc)
tcWrapResultO CtOrigin
orig HsExpr GhcRn
rn_expr HsExpr GhcTc
expr TcType
actual_ty ExpRhoType
res_ty
  = do { String -> SDoc -> TcM ()
traceTc String
"tcWrapResult" ([SDoc] -> SDoc
forall doc. IsDoc doc => [doc] -> doc
vcat [ String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"Actual:  " SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> TcType -> SDoc
forall a. Outputable a => a -> SDoc
ppr TcType
actual_ty
                                      , String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"Expected:" SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> ExpRhoType -> SDoc
forall a. Outputable a => a -> SDoc
ppr ExpRhoType
res_ty ])
       ; wrap <- CtOrigin
-> UserTypeCtxt
-> Maybe TypedThing
-> TcType
-> ExpRhoType
-> TcM HsWrapper
tcSubTypeNC CtOrigin
orig UserTypeCtxt
GenSigCtxt (TypedThing -> Maybe TypedThing
forall a. a -> Maybe a
Just (TypedThing -> Maybe TypedThing) -> TypedThing -> Maybe TypedThing
forall a b. (a -> b) -> a -> b
$ HsExpr GhcRn -> TypedThing
HsExprRnThing HsExpr GhcRn
rn_expr) TcType
actual_ty ExpRhoType
res_ty
       ; return (mkHsWrap wrap expr) }

tcWrapResultMono :: HsExpr GhcRn -> HsExpr GhcTc
                 -> TcRhoType   -- Actual -- a rho-type not a sigma-type
                 -> ExpRhoType  -- Expected
                 -> TcM (HsExpr GhcTc)
-- A version of tcWrapResult to use when the actual type is a
-- rho-type, so nothing to instantiate; just go straight to unify.
-- It means we don't need to pass in a CtOrigin
tcWrapResultMono :: HsExpr GhcRn
-> HsExpr GhcTc -> TcType -> ExpRhoType -> TcM (HsExpr GhcTc)
tcWrapResultMono HsExpr GhcRn
rn_expr HsExpr GhcTc
expr TcType
act_ty ExpRhoType
res_ty
  = Bool -> SDoc -> TcM (HsExpr GhcTc) -> TcM (HsExpr GhcTc)
forall a. HasCallStack => Bool -> SDoc -> a -> a
assertPpr (TcType -> Bool
isRhoTy TcType
act_ty) (TcType -> SDoc
forall a. Outputable a => a -> SDoc
ppr TcType
act_ty SDoc -> SDoc -> SDoc
forall doc. IsDoc doc => doc -> doc -> doc
$$ HsExpr GhcRn -> SDoc
forall a. Outputable a => a -> SDoc
ppr HsExpr GhcRn
rn_expr) (TcM (HsExpr GhcTc) -> TcM (HsExpr GhcTc))
-> TcM (HsExpr GhcTc) -> TcM (HsExpr GhcTc)
forall a b. (a -> b) -> a -> b
$
    do { co <- HsExpr GhcRn -> TcType -> ExpRhoType -> TcM Coercion
unifyExpectedType HsExpr GhcRn
rn_expr TcType
act_ty ExpRhoType
res_ty
       ; return (mkHsWrapCo co expr) }

unifyExpectedType :: HsExpr GhcRn
                  -> TcRhoType   -- Actual -- a rho-type not a sigma-type
                  -> ExpRhoType  -- Expected
                  -> TcM TcCoercionN
unifyExpectedType :: HsExpr GhcRn -> TcType -> ExpRhoType -> TcM Coercion
unifyExpectedType HsExpr GhcRn
rn_expr TcType
act_ty ExpRhoType
exp_ty
  = case ExpRhoType
exp_ty of
      Infer InferResult
inf_res -> TcType -> InferResult -> TcM Coercion
fillInferResult TcType
act_ty InferResult
inf_res
      Check TcType
exp_ty  -> Maybe TypedThing -> TcType -> TcType -> TcM Coercion
unifyType (TypedThing -> Maybe TypedThing
forall a. a -> Maybe a
Just (TypedThing -> Maybe TypedThing) -> TypedThing -> Maybe TypedThing
forall a b. (a -> b) -> a -> b
$ HsExpr GhcRn -> TypedThing
HsExprRnThing HsExpr GhcRn
rn_expr) TcType
act_ty TcType
exp_ty

------------------------
tcSubTypePat :: CtOrigin -> UserTypeCtxt
            -> ExpSigmaType -> TcSigmaType -> TcM HsWrapper
-- Used in patterns; polarity is backwards compared
--   to tcSubType
-- If wrap = tc_sub_type_et t1 t2
--    => wrap :: t1 ~> t2
tcSubTypePat :: CtOrigin -> UserTypeCtxt -> ExpRhoType -> TcType -> TcM HsWrapper
tcSubTypePat CtOrigin
inst_orig UserTypeCtxt
ctxt (Check TcType
ty_actual) TcType
ty_expected
  = (TcType -> TcType -> TcM Coercion)
-> CtOrigin -> UserTypeCtxt -> TcType -> TcType -> TcM HsWrapper
tc_sub_type TcType -> TcType -> TcM Coercion
unifyTypeET CtOrigin
inst_orig UserTypeCtxt
ctxt TcType
ty_actual TcType
ty_expected

tcSubTypePat CtOrigin
_ UserTypeCtxt
_ (Infer InferResult
inf_res) TcType
ty_expected
  = do { co <- TcType -> InferResult -> TcM Coercion
fillInferResult TcType
ty_expected InferResult
inf_res
               -- In patterns we do not instantatiate

       ; return (mkWpCastN (mkSymCo co)) }

---------------
tcSubType :: CtOrigin -> UserTypeCtxt
          -> TcSigmaType  -- ^ Actual
          -> ExpRhoType   -- ^ Expected
          -> TcM HsWrapper
-- Checks that 'actual' is more polymorphic than 'expected'
tcSubType :: CtOrigin -> UserTypeCtxt -> TcType -> ExpRhoType -> TcM HsWrapper
tcSubType CtOrigin
orig UserTypeCtxt
ctxt TcType
ty_actual ExpRhoType
ty_expected
  = TcType -> ExpRhoType -> TcM HsWrapper -> TcM HsWrapper
forall a. TcType -> ExpRhoType -> TcM a -> TcM a
addSubTypeCtxt TcType
ty_actual ExpRhoType
ty_expected (TcM HsWrapper -> TcM HsWrapper) -> TcM HsWrapper -> TcM HsWrapper
forall a b. (a -> b) -> a -> b
$
    do { String -> SDoc -> TcM ()
traceTc String
"tcSubType" ([SDoc] -> SDoc
forall doc. IsDoc doc => [doc] -> doc
vcat [UserTypeCtxt -> SDoc
pprUserTypeCtxt UserTypeCtxt
ctxt, TcType -> SDoc
forall a. Outputable a => a -> SDoc
ppr TcType
ty_actual, ExpRhoType -> SDoc
forall a. Outputable a => a -> SDoc
ppr ExpRhoType
ty_expected])
       ; CtOrigin
-> UserTypeCtxt
-> Maybe TypedThing
-> TcType
-> ExpRhoType
-> TcM HsWrapper
tcSubTypeNC CtOrigin
orig UserTypeCtxt
ctxt Maybe TypedThing
forall a. Maybe a
Nothing TcType
ty_actual ExpRhoType
ty_expected }

---------------
tcSubTypeDS :: HsExpr GhcRn
            -> TcRhoType   -- Actual type -- a rho-type not a sigma-type
            -> TcRhoType   -- Expected type
                           -- DeepSubsumption <=> when checking, this type
                           --                     is deeply skolemised
            -> TcM HsWrapper
-- Similar signature to unifyExpectedType; does deep subsumption
-- Only one call site, in GHC.Tc.Gen.App.tcApp
tcSubTypeDS :: HsExpr GhcRn -> TcType -> TcType -> TcM HsWrapper
tcSubTypeDS HsExpr GhcRn
rn_expr TcType
act_rho TcType
exp_rho
  = HasDebugCallStack =>
(TcType -> TcType -> TcM Coercion)
-> CtOrigin -> UserTypeCtxt -> TcType -> TcType -> TcM HsWrapper
(TcType -> TcType -> TcM Coercion)
-> CtOrigin -> UserTypeCtxt -> TcType -> TcType -> TcM HsWrapper
tc_sub_type_deep (HsExpr GhcRn -> TcType -> TcType -> TcM Coercion
unifyExprType HsExpr GhcRn
rn_expr) CtOrigin
orig UserTypeCtxt
GenSigCtxt TcType
act_rho TcType
exp_rho
  where
    orig :: CtOrigin
orig = HsExpr GhcRn -> CtOrigin
exprCtOrigin HsExpr GhcRn
rn_expr

---------------
tcSubTypeNC :: CtOrigin          -- ^ Used when instantiating
            -> UserTypeCtxt      -- ^ Used when skolemising
            -> Maybe TypedThing -- ^ The expression that has type 'actual' (if known)
            -> TcSigmaType       -- ^ Actual type
            -> ExpRhoType        -- ^ Expected type
            -> TcM HsWrapper
tcSubTypeNC :: CtOrigin
-> UserTypeCtxt
-> Maybe TypedThing
-> TcType
-> ExpRhoType
-> TcM HsWrapper
tcSubTypeNC CtOrigin
inst_orig UserTypeCtxt
ctxt Maybe TypedThing
m_thing TcType
ty_actual ExpRhoType
res_ty
  = case ExpRhoType
res_ty of
      Check TcType
ty_expected -> (TcType -> TcType -> TcM Coercion)
-> CtOrigin -> UserTypeCtxt -> TcType -> TcType -> TcM HsWrapper
tc_sub_type (Maybe TypedThing -> TcType -> TcType -> TcM Coercion
unifyType Maybe TypedThing
m_thing) CtOrigin
inst_orig UserTypeCtxt
ctxt
                                       TcType
ty_actual TcType
ty_expected

      Infer InferResult
inf_res -> do { (wrap, rho) <- CtOrigin -> TcType -> TcM (HsWrapper, TcType)
topInstantiate CtOrigin
inst_orig TcType
ty_actual
                                   -- See Note [Instantiation of InferResult]
                          ; co <- fillInferResult rho inf_res
                          ; return (mkWpCastN co <.> wrap) }

---------------
tcSubTypeSigma :: CtOrigin       -- where did the actual type arise / why are we
                                 -- doing this subtype check?
               -> UserTypeCtxt   -- where did the expected type arise?
               -> TcSigmaType -> TcSigmaType -> TcM HsWrapper
-- External entry point, but no ExpTypes on either side
-- Checks that actual <= expected
-- Returns HsWrapper :: actual ~ expected
tcSubTypeSigma :: CtOrigin -> UserTypeCtxt -> TcType -> TcType -> TcM HsWrapper
tcSubTypeSigma CtOrigin
orig UserTypeCtxt
ctxt TcType
ty_actual TcType
ty_expected
  = (TcType -> TcType -> TcM Coercion)
-> CtOrigin -> UserTypeCtxt -> TcType -> TcType -> TcM HsWrapper
tc_sub_type (Maybe TypedThing -> TcType -> TcType -> TcM Coercion
unifyType Maybe TypedThing
forall a. Maybe a
Nothing) CtOrigin
orig UserTypeCtxt
ctxt TcType
ty_actual TcType
ty_expected

---------------
tcSubTypeAmbiguity :: UserTypeCtxt   -- Where did this type arise
                   -> TcSigmaType -> TcSigmaType -> TcM HsWrapper
-- See Note [Ambiguity check and deep subsumption]
tcSubTypeAmbiguity :: UserTypeCtxt -> TcType -> TcType -> TcM HsWrapper
tcSubTypeAmbiguity UserTypeCtxt
ctxt TcType
ty_actual TcType
ty_expected
  = DeepSubsumptionFlag
-> (TcType -> TcType -> TcM Coercion)
-> CtOrigin
-> UserTypeCtxt
-> TcType
-> TcType
-> TcM HsWrapper
tc_sub_type_ds DeepSubsumptionFlag
Shallow (Maybe TypedThing -> TcType -> TcType -> TcM Coercion
unifyType Maybe TypedThing
forall a. Maybe a
Nothing)
                           (UserTypeCtxt -> CtOrigin
AmbiguityCheckOrigin UserTypeCtxt
ctxt)
                           UserTypeCtxt
ctxt TcType
ty_actual TcType
ty_expected

---------------
addSubTypeCtxt :: TcType -> ExpType -> TcM a -> TcM a
addSubTypeCtxt :: forall a. TcType -> ExpRhoType -> TcM a -> TcM a
addSubTypeCtxt TcType
ty_actual ExpRhoType
ty_expected TcM a
thing_inside
 | TcType -> Bool
isRhoTy TcType
ty_actual        -- If there is no polymorphism involved, the
 , ExpRhoType -> Bool
isRhoExpTy ExpRhoType
ty_expected   -- TypeEqOrigin stuff (added by the _NC functions)
 = TcM a
thing_inside             -- gives enough context by itself
 | Bool
otherwise
 = (TidyEnv -> ZonkM (TidyEnv, ErrCtxtMsg)) -> TcM a -> TcM a
forall a.
(TidyEnv -> ZonkM (TidyEnv, ErrCtxtMsg)) -> TcM a -> TcM a
addErrCtxtM TidyEnv -> ZonkM (TidyEnv, ErrCtxtMsg)
mk_msg TcM a
thing_inside
  where
    mk_msg :: TidyEnv -> ZonkM (TidyEnv, ErrCtxtMsg)
mk_msg TidyEnv
tidy_env
      = do { (tidy_env, ty_actual)   <- TidyEnv -> TcType -> ZonkM (TidyEnv, TcType)
zonkTidyTcType TidyEnv
tidy_env TcType
ty_actual
           ; ty_expected             <- readExpType ty_expected
                   -- A worry: might not be filled if we're debugging. Ugh.
           ; (tidy_env, ty_expected) <- zonkTidyTcType tidy_env ty_expected
           ; return (tidy_env, SubTypeCtxt ty_expected ty_actual) }


{- Note [Instantiation of InferResult]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
We now always instantiate before filling in InferResult, so that
the result is a TcRhoType: see #17173 for discussion.

For example:

1. Consider
    f x = (*)
   We want to instantiate the type of (*) before returning, else we
   will infer the type
     f :: forall {a}. a -> forall b. Num b => b -> b -> b
   This is surely confusing for users.

   And worse, the monomorphism restriction won't work properly. The MR is
   dealt with in simplifyInfer, and simplifyInfer has no way of
   instantiating. This could perhaps be worked around, but it may be
   hard to know even when instantiation should happen.

2. Another reason.  Consider
       f :: (?x :: Int) => a -> a
       g y = let ?x = 3::Int in f
   Here want to instantiate f's type so that the ?x::Int constraint
  gets discharged by the enclosing implicit-parameter binding.

3. Suppose one defines plus = (+). If we instantiate lazily, we will
   infer plus :: forall a. Num a => a -> a -> a. However, the monomorphism
   restriction compels us to infer
      plus :: Integer -> Integer -> Integer
   (or similar monotype). Indeed, the only way to know whether to apply
   the monomorphism restriction at all is to instantiate

There is one place where we don't want to instantiate eagerly,
namely in GHC.Tc.Module.tcRnExpr, which implements GHCi's :type
command. See Note [Implementing :type] in GHC.Tc.Module.
-}

---------------
tc_sub_type :: (TcType -> TcType -> TcM TcCoercionN)  -- How to unify
            -> CtOrigin       -- Used when instantiating
            -> UserTypeCtxt   -- Used when skolemising
            -> TcSigmaType    -- Actual; a sigma-type
            -> TcSigmaType    -- Expected; also a sigma-type
            -> TcM HsWrapper
-- Checks that actual_ty is more polymorphic than expected_ty
-- If wrap = tc_sub_type t1 t2
--    => wrap :: t1 ~> t2
--
-- The "how to unify argument" is always a call to `uType TypeLevel orig`,
-- but with different ways of constructing the CtOrigin `orig` from
-- the argument types and context.

----------------------
tc_sub_type :: (TcType -> TcType -> TcM Coercion)
-> CtOrigin -> UserTypeCtxt -> TcType -> TcType -> TcM HsWrapper
tc_sub_type TcType -> TcType -> TcM Coercion
unify CtOrigin
inst_orig UserTypeCtxt
ctxt TcType
ty_actual TcType
ty_expected
  = do { ds_flag <- TcM DeepSubsumptionFlag
getDeepSubsumptionFlag
       ; tc_sub_type_ds ds_flag unify inst_orig ctxt ty_actual ty_expected }

----------------------
tc_sub_type_ds :: DeepSubsumptionFlag
               -> (TcType -> TcType -> TcM TcCoercionN)
               -> CtOrigin -> UserTypeCtxt -> TcSigmaType
               -> TcSigmaType -> TcM HsWrapper
-- tc_sub_type_ds is the main subsumption worker function
-- It takes an explicit DeepSubsumptionFlag
tc_sub_type_ds :: DeepSubsumptionFlag
-> (TcType -> TcType -> TcM Coercion)
-> CtOrigin
-> UserTypeCtxt
-> TcType
-> TcType
-> TcM HsWrapper
tc_sub_type_ds DeepSubsumptionFlag
ds_flag TcType -> TcType -> TcM Coercion
unify CtOrigin
inst_orig UserTypeCtxt
ctxt TcType
ty_actual TcType
ty_expected
  | TcType -> Bool
definitely_poly TcType
ty_expected   -- See Note [Don't skolemise unnecessarily]
  , DeepSubsumptionFlag -> TcType -> Bool
isRhoTyDS DeepSubsumptionFlag
ds_flag TcType
ty_actual
  = do { String -> SDoc -> TcM ()
traceTc String
"tc_sub_type (drop to equality)" (SDoc -> TcM ()) -> SDoc -> TcM ()
forall a b. (a -> b) -> a -> b
$
         [SDoc] -> SDoc
forall doc. IsDoc doc => [doc] -> doc
vcat [ String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"ty_actual   =" SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> TcType -> SDoc
forall a. Outputable a => a -> SDoc
ppr TcType
ty_actual
              , String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"ty_expected =" SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> TcType -> SDoc
forall a. Outputable a => a -> SDoc
ppr TcType
ty_expected ]
       ; Coercion -> HsWrapper
mkWpCastN (Coercion -> HsWrapper) -> TcM Coercion -> TcM HsWrapper
forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
<$>
         TcType -> TcType -> TcM Coercion
unify TcType
ty_actual TcType
ty_expected }

  | Bool
otherwise   -- This is the general case
  = do { String -> SDoc -> TcM ()
traceTc String
"tc_sub_type (general case)" (SDoc -> TcM ()) -> SDoc -> TcM ()
forall a b. (a -> b) -> a -> b
$
         [SDoc] -> SDoc
forall doc. IsDoc doc => [doc] -> doc
vcat [ String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"ty_actual   =" SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> TcType -> SDoc
forall a. Outputable a => a -> SDoc
ppr TcType
ty_actual
              , String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"ty_expected =" SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> TcType -> SDoc
forall a. Outputable a => a -> SDoc
ppr TcType
ty_expected ]

       ; (sk_wrap, inner_wrap)
            <- DeepSubsumptionFlag
-> UserTypeCtxt
-> TcType
-> (TcType -> TcM HsWrapper)
-> TcM (HsWrapper, HsWrapper)
forall result.
DeepSubsumptionFlag
-> UserTypeCtxt
-> TcType
-> (TcType -> TcM result)
-> TcM (HsWrapper, result)
tcSkolemise DeepSubsumptionFlag
ds_flag UserTypeCtxt
ctxt TcType
ty_expected ((TcType -> TcM HsWrapper) -> TcM (HsWrapper, HsWrapper))
-> (TcType -> TcM HsWrapper) -> TcM (HsWrapper, HsWrapper)
forall a b. (a -> b) -> a -> b
$ \TcType
sk_rho ->
               case DeepSubsumptionFlag
ds_flag of
                 DeepSubsumptionFlag
Deep    -> HasDebugCallStack =>
(TcType -> TcType -> TcM Coercion)
-> CtOrigin -> UserTypeCtxt -> TcType -> TcType -> TcM HsWrapper
(TcType -> TcType -> TcM Coercion)
-> CtOrigin -> UserTypeCtxt -> TcType -> TcType -> TcM HsWrapper
tc_sub_type_deep TcType -> TcType -> TcM Coercion
unify CtOrigin
inst_orig UserTypeCtxt
ctxt TcType
ty_actual TcType
sk_rho
                 DeepSubsumptionFlag
Shallow -> (TcType -> TcType -> TcM Coercion)
-> CtOrigin -> TcType -> TcType -> TcM HsWrapper
tc_sub_type_shallow TcType -> TcType -> TcM Coercion
unify CtOrigin
inst_orig TcType
ty_actual TcType
sk_rho

       ; return (sk_wrap <.> inner_wrap) }

----------------------
tc_sub_type_shallow :: (TcType -> TcType -> TcM TcCoercionN)
                    -> CtOrigin
                    -> TcSigmaType
                    -> TcRhoType   -- Skolemised (shallow-ly)
                    -> TcM HsWrapper
tc_sub_type_shallow :: (TcType -> TcType -> TcM Coercion)
-> CtOrigin -> TcType -> TcType -> TcM HsWrapper
tc_sub_type_shallow TcType -> TcType -> TcM Coercion
unify CtOrigin
inst_orig TcType
ty_actual TcType
sk_rho
  = do { (wrap, rho_a) <- CtOrigin -> TcType -> TcM (HsWrapper, TcType)
topInstantiate CtOrigin
inst_orig TcType
ty_actual
       ; cow           <- unify rho_a sk_rho
       ; return (mkWpCastN cow <.> wrap) }

----------------------
definitely_poly :: TcType -> Bool
-- A very conservative test:
-- see Note [Don't skolemise unnecessarily]
definitely_poly :: TcType -> Bool
definitely_poly TcType
ty
  | ([TcTyVar]
tvs, [TcType]
theta, TcType
tau) <- TcType -> ([TcTyVar], [TcType], TcType)
tcSplitSigmaTy TcType
ty
  , (TcTyVar
tv:[TcTyVar]
_) <- [TcTyVar]
tvs   -- At least one tyvar
  , [TcType] -> Bool
forall a. [a] -> Bool
forall (t :: * -> *) a. Foldable t => t a -> Bool
null [TcType]
theta      -- No constraints; see (DP1)
  , TcTyVar
tv TcTyVar -> TcType -> Bool
`isInjectiveInType` TcType
tau
       -- The tyvar actually occurs (DP2),
       -- and occurs in an injective position (DP3).
  = Bool
True
  | Bool
otherwise
  = Bool
False

{- Note [Don't skolemise unnecessarily]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Suppose we are trying to solve
     ty_actual   <= ty_expected
    (Char->Char) <= (forall a. a->a)
We could skolemise the 'forall a', and then complain
that (Char ~ a) is insoluble; but that's a pretty obscure
error.  It's better to say that
    (Char->Char) ~ (forall a. a->a)
fails.

If we prematurely go to equality we'll reject a program we should
accept (e.g. #13752).  So the test (which is only to improve error
message) is very conservative:

 * ty_actual   is /definitely/ monomorphic: see `definitely_mono`
   This definitely_mono test comes in "shallow" and "deep" variants

 * ty_expected is /definitely/ polymorphic: see `definitely_poly`
   This definitely_poly test is more subtle than you might think.
   Here are three cases where expected_ty looks polymorphic, but
   isn't, and where it would be /wrong/ to switch to equality:

   (DP1)  (Char->Char) <= (forall a. (a~Char) => a -> a)

   (DP2)  (Char->Char) <= (forall a. Char -> Char)

   (DP3)  (Char->Char) <= (forall a. F [a] Char -> Char)
                          where type instance F [x] t = t


Note [Coercion errors in tcSubMult]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
At the moment, we insist that all sub-multiplicity tests turn out
(once the typechecker has finished its work) to be equalities,
i.e. implementable by ReflCo.  Why?  Because our type system has
no way to express non-Refl sub-multiplicities.

How can we check that every call to `tcSubMult` returns `Refl`?
It might not be `Refl` *yet*.

[TODO: add counterexample #25130]

So we take the following approach

* In `tcEqMult`:
  - Emit a perfectly ordinary Wanted equality constraint for the equality,
    returning a coercion.

  - Wrap that coercion with `DE_Multiplicity` to make a `DelayedError`, and put
    that delayed error into `wc_errors` of the current WantedConstraints.  This
    is done by `ensureReflMultiplicityCo`.

* When solving constraints, discard any `DE_Multiplicity` errors that wrap a
  reflective coercion, of kind `ty ~ ty`.  This is done in
  `GHC.Tc.Solver.simplifyDelayedErrors`

* After constraint solving is complete report an error if there are any
  remaining `DE_Multiplicity` errors.  See
  `GHC.Tc.Errors.reportMultiplicityCoercionErrs`

Wrinkles

(DME1) If the multiplicity constraint is /solved/, but with a non-reflective
   coercion, we'll have just a `DE_Multiplicity` error left over.

   But if the multiplicity constraint is /unsolved/ (e.g. ManyTy ~ OneTy), we
   will have /both/ an unsolved Wanted in `wc_simple`, /and/ a `DE_Multiplicity`
   in `wc_errors`.  We don't want to report both.  Solution: suppress all
   `DE_Multiplicity` constraints if there are any unsolved wanted.

   This way, the delayed error is indeed only reported when the constraint is
   solved with a non-reflexivity coercion.

An alternative would be to have a kind of constraint which can only produce
trivial evidence. This would allow such checks to happen in the constraint
solver (#18756).  This would be similar to the existing setup for Concrete, see
Note [The Concrete mechanism] in GHC.Tc.Utils.Concrete (PHASE 1 in particular).

-}

tcSubMult :: CtOrigin -> Mult -> Mult -> TcM ()
tcSubMult :: CtOrigin -> TcType -> TcType -> TcM ()
tcSubMult CtOrigin
origin TcType
w_actual TcType
w_expected
  | Just (TcType
w1, TcType
w2) <- TcType -> Maybe (TcType, TcType)
isMultMul TcType
w_actual =
  do { CtOrigin -> TcType -> TcType -> TcM ()
tcSubMult CtOrigin
origin TcType
w1 TcType
w_expected
     ; CtOrigin -> TcType -> TcType -> TcM ()
tcSubMult CtOrigin
origin TcType
w2 TcType
w_expected }
  -- Currently, we consider p*q and sup p q to be equal.  Therefore, p*q <= r is
  -- equivalent to p <= r and q <= r.  For other cases, we approximate p <= q by p
  -- ~ q.  This is not complete, but it's sound. See also Note [Overapproximating
  -- multiplicities] in Multiplicity.
tcSubMult CtOrigin
origin TcType
w_actual TcType
w_expected =
  case TcType -> TcType -> IsSubmult
submult TcType
w_actual TcType
w_expected of
    IsSubmult
Submult -> () -> TcM ()
forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
forall (m :: * -> *) a. Monad m => a -> m a
return ()
    IsSubmult
Unknown -> CtOrigin -> TcType -> TcType -> TcM ()
tcEqMult CtOrigin
origin TcType
w_actual TcType
w_expected

tcEqMult :: CtOrigin -> Mult -> Mult -> TcM ()
tcEqMult :: CtOrigin -> TcType -> TcType -> TcM ()
tcEqMult CtOrigin
origin TcType
w_actual TcType
w_expected = do
  {
  -- Note that here we do not call to `submult`, so we check
  -- for strict equality.
  ; coercion <- TypeOrKind -> CtOrigin -> TcType -> TcType -> TcM Coercion
unifyTypeAndEmit TypeOrKind
TypeLevel CtOrigin
origin TcType
w_actual TcType
w_expected
  -- See Note [Coercion errors in tcSubMult].
  ; ensureReflMultiplicityCo coercion origin }


{- *********************************************************************
*                                                                      *
                    Deep subsumption
*                                                                      *
********************************************************************* -}

{- Note [Deep subsumption]
~~~~~~~~~~~~~~~~~~~~~~~~~~
The DeepSubsumption extension, documented here

    https://github.com/ghc-proposals/ghc-proposals/pull/511.

makes a best-efforts attempt implement deep subsumption as it was
prior to the Simplify Subsumption proposal:

    https://github.com/ghc-proposals/ghc-proposals/pull/287

The effects are in these main places:

1. In the subsumption check, tcSubType, we must do deep skolemisation:
   see the call to tcSkolemise Deep in tc_sub_type_deep

2. In tcPolyExpr we must do deep skolemisation:
   see the call to tcSkolemise in tcSkolemiseExpType

3. for expression type signatures (e :: ty), and functions with type
   signatures (e.g. f :: ty; f = e), we must deeply skolemise the type;
   see the call to tcDeeplySkolemise in tcSkolemiseScoped.

4. In GHC.Tc.Gen.App.tcApp we call tcSubTypeDS to match the result
   type. Without deep subsumption, unifyExpectedType would be sufficent.

In all these cases note that the deep skolemisation must be done /first/.
Consider (1)
     (forall a. Int -> a -> a)  <=  Int -> (forall b. b -> b)
We must skolemise the `forall b` before instantiating the `forall a`.
See also Note [Deep skolemisation].

Wrinkles:

(DS1) Note that we /always/ use shallow subsumption in the ambiguity check.
      See Note [Ambiguity check and deep subsumption].

(DS2) Deep subsumption requires deep instantiation too.
      See Note [The need for deep instantiation]

(DS3) The interaction between deep subsumption and required foralls
      (forall a -> ty) is a bit subtle.  See #24696 and
      Note [Deep subsumption and required foralls]

Note [Deep skolemisation]
~~~~~~~~~~~~~~~~~~~~~~~~~
deeplySkolemise decomposes and skolemises a type, returning a type
with all its arrows visible (ie not buried under foralls)

Examples:

  deeplySkolemise (Int -> forall a. Ord a => blah)
    =  ( wp, [a], [d:Ord a], Int -> blah )
    where wp = \x:Int. /\a. \(d:Ord a). <hole> x

  deeplySkolemise  (forall a. Ord a => Maybe a -> forall b. Eq b => blah)
    =  ( wp, [a,b], [d1:Ord a,d2:Eq b], Maybe a -> blah )
    where wp = /\a.\(d1:Ord a).\(x:Maybe a)./\b.\(d2:Ord b). <hole> x

In general,
  if      deeplySkolemise ty = (wrap, tvs, evs, rho)
    and   e :: rho
  then    wrap e :: ty
    and   'wrap' binds tvs, evs

ToDo: this eta-abstraction plays fast and loose with termination,
      because it can introduce extra lambdas.  Maybe add a `seq` to
      fix this

Note [Setting the argument context]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider we are doing the ambiguity check for the (bogus)
  f :: (forall a b. C b => a -> a) -> Int

We'll call
   tcSubType ((forall a b. C b => a->a) -> Int )
             ((forall a b. C b => a->a) -> Int )

with a UserTypeCtxt of (FunSigCtxt "f").  Then we'll do the co/contra thing
on the argument type of the (->) -- and at that point we want to switch
to a UserTypeCtxt of GenSigCtxt.  Why?

* Error messages.  If we stick with FunSigCtxt we get errors like
     * Could not deduce: C b
       from the context: C b0
        bound by the type signature for:
            f :: forall a b. C b => a->a
  But of course f does not have that type signature!
  Example tests: T10508, T7220a, Simple14

* Implications. We may decide to build an implication for the whole
  ambiguity check, but we don't need one for each level within it,
  and TcUnify.alwaysBuildImplication checks the UserTypeCtxt.
  See Note [When to build an implication]

Note [Multiplicity in deep subsumption]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider
   t1 ->{mt} t2  <=   s1 ->{ms} s2

At the moment we /unify/ ms~mt, via tcEqMult.

Arguably we should use `tcSubMult`. But then if mt=m0 (a unification
variable) and ms=Many, `tcSubMult` is a no-op (since anything is a
sub-multiplicty of Many).  But then `m0` may never get unified with
anything.  It is then skolemised by the zonker; see GHC.HsToCore.Binds
Note [Free tyvars on rule LHS].  So we in RULE foldr/app in GHC.Base
we get this

 "foldr/app"     [1] forall ys m1 m2. foldr (\x{m1} \xs{m2}. (:) x xs) ys
                                       = \xs -> xs ++ ys

where we eta-expanded that (:).  But now foldr expects an argument
with ->{Many} and gets an argument with ->{m1} or ->{m2}, and Lint
complains.

The easiest solution was to use tcEqMult in tc_sub_type_deep, and
insist on equality. This is only in the DeepSubsumption code anyway.

Note [The need for deep instantiation]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider this, without Quick Look, but with Deep Subsumption:
   f :: ∀a b c. a b c -> Int
   g :: Bool -> ∀d. d -> d
Consider the application (f g).  We need to do the subsumption test

  (Bool -> ∀ d. d->d)   <=   (alpha beta gamma)

where alpha, beta, gamma are the unification variables that instantiate a,b,c,
respectively.  We must not drop down to unification, or we will reject the call.
Rather we must deeply instantiate the LHS to get

  (Bool -> delta -> delta)   <=   (alpha beta gamma)

and now we can unify to get

   alpha = (->)
   beta = Bool
   gamma = delta -> delta

Hence the call to `deeplyInstantiate` in `tc_sub_type_deep`.

See typecheck/should_compile/T11305 for an example of when this is important.

Note [Deep subsumption and required foralls]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
A required forall, (forall a -> ty) behaves like a "rho-type", one with no
top-level quantification.  In particular, it is neither implicitly instantiated nor
skolemised.  So

  rid1 :: forall a -> a -> a
  rid1 = id

  rid2 :: forall a -> a -> a
  rid2 a = id

Here `rid2` wll typecheck, but `rid1` will not, because we don't implicitly skolemise
the  type.

This "no implicit subsumption nor skolemisation" applies during subsumption.
For example
   (forall a. a->a)  <=  (forall a -> a -> a)  -- NOT!
does /not/ hold, because that would require implicitly skoleming the (forall a->).

Note also that, in Core, `eqType` distinguishes between
   (forall a. blah) and forall a -> blah)
See discussion on #22762 and these Notes in GHC.Core.TyCo.Compare
  * Note [ForAllTy and type equality]
  * Note [Comparing visibility]

So during deep subsumption we simply stop (and drop down to equality) when we encounter
a (forall a->).  This is a little odd:
* Deep subsumption looks inside invisible foralls (forall a. ty)
* Deep subsumption looks inside arrows (t1 -> t2)
* But it does not look inside required foralls (forall a -> ty)

There is discussion on #24696.  How is this implemented?

* In `tc_sub_type_deep`, the calls to `topInstantiate` and `deeplyInstantiate`
  instantiate only /invisible/ binders.
* In `tc_sub_type_ds`, the call to `tcSkolemise` skolemises only /invisible/
  binders.

Here is a slightly more powerful alternative
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 In the story above, if we have
    (forall a -> Eq a => a -> a)  <=  (forall a -> Ord a => a -> a)
we'll reject it, because both are rho-types but they aren't equal.  But in the
"drop to equality" stage we could instead see if both rho-types are headed with
(forall a ->) and if so strip that off and go back into deep subsumption.

This is a bit more powerful, but also a bit more complicated, so GHC
doesn't do it yet, awaiting credible user demand.  See #24696.
-}

data DeepSubsumptionFlag = Deep | Shallow

instance Outputable DeepSubsumptionFlag where
    ppr :: DeepSubsumptionFlag -> SDoc
ppr DeepSubsumptionFlag
Deep    = String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"Deep"
    ppr DeepSubsumptionFlag
Shallow = String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"Shallow"

getDeepSubsumptionFlag :: TcM DeepSubsumptionFlag
getDeepSubsumptionFlag :: TcM DeepSubsumptionFlag
getDeepSubsumptionFlag = do { ds <- Extension -> TcM Bool
forall gbl lcl. Extension -> TcRnIf gbl lcl Bool
xoptM Extension
LangExt.DeepSubsumption
                            ; if ds then return Deep else return Shallow }

tc_sub_type_deep :: HasDebugCallStack
                 => (TcType -> TcType -> TcM TcCoercionN)  -- How to unify
                 -> CtOrigin       -- Used when instantiating
                 -> UserTypeCtxt   -- Used when skolemising
                 -> TcSigmaType    -- Actual; a sigma-type
                 -> TcRhoType      -- Expected; deeply skolemised
                 -> TcM HsWrapper

-- If wrap = tc_sub_type_deep t1 t2
--    => wrap :: t1 ~> t2
-- Here is where the work actually happens!
-- Precondition: ty_expected is deeply skolemised

tc_sub_type_deep :: HasDebugCallStack =>
(TcType -> TcType -> TcM Coercion)
-> CtOrigin -> UserTypeCtxt -> TcType -> TcType -> TcM HsWrapper
tc_sub_type_deep TcType -> TcType -> TcM Coercion
unify CtOrigin
inst_orig UserTypeCtxt
ctxt TcType
ty_actual TcType
ty_expected
  = Bool -> SDoc -> TcM HsWrapper -> TcM HsWrapper
forall a. HasCallStack => Bool -> SDoc -> a -> a
assertPpr (TcType -> Bool
isDeepRhoTy TcType
ty_expected) (TcType -> SDoc
forall a. Outputable a => a -> SDoc
ppr TcType
ty_expected) (TcM HsWrapper -> TcM HsWrapper) -> TcM HsWrapper -> TcM HsWrapper
forall a b. (a -> b) -> a -> b
$
    do { String -> SDoc -> TcM ()
traceTc String
"tc_sub_type_deep" (SDoc -> TcM ()) -> SDoc -> TcM ()
forall a b. (a -> b) -> a -> b
$
         [SDoc] -> SDoc
forall doc. IsDoc doc => [doc] -> doc
vcat [ String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"ty_actual   =" SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> TcType -> SDoc
forall a. Outputable a => a -> SDoc
ppr TcType
ty_actual
              , String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"ty_expected =" SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> TcType -> SDoc
forall a. Outputable a => a -> SDoc
ppr TcType
ty_expected ]
       ; TcType -> TcType -> TcM HsWrapper
go TcType
ty_actual TcType
ty_expected }
  where
    -- NB: 'go' is not recursive, except for doing coreView
    go :: TcType -> TcType -> TcM HsWrapper
go TcType
ty_a TcType
ty_e | Just TcType
ty_a' <- TcType -> Maybe TcType
coreView TcType
ty_a = TcType -> TcType -> TcM HsWrapper
go TcType
ty_a' TcType
ty_e
                 | Just TcType
ty_e' <- TcType -> Maybe TcType
coreView TcType
ty_e = TcType -> TcType -> TcM HsWrapper
go TcType
ty_a  TcType
ty_e'

    go (TyVarTy TcTyVar
tv_a) TcType
ty_e
      = do { lookup_res <- TcTyVar -> IOEnv (Env TcGblEnv TcLclEnv) (Maybe TcType)
isFilledMetaTyVar_maybe TcTyVar
tv_a
           ; case lookup_res of
               Just TcType
ty_a' ->
                 do { String -> SDoc -> TcM ()
traceTc String
"tc_sub_type_deep following filled meta-tyvar:"
                        (TcTyVar -> SDoc
forall a. Outputable a => a -> SDoc
ppr TcTyVar
tv_a SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"-->" SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> TcType -> SDoc
forall a. Outputable a => a -> SDoc
ppr TcType
ty_a')
                    ; HasDebugCallStack =>
(TcType -> TcType -> TcM Coercion)
-> CtOrigin -> UserTypeCtxt -> TcType -> TcType -> TcM HsWrapper
(TcType -> TcType -> TcM Coercion)
-> CtOrigin -> UserTypeCtxt -> TcType -> TcType -> TcM HsWrapper
tc_sub_type_deep TcType -> TcType -> TcM Coercion
unify CtOrigin
inst_orig UserTypeCtxt
ctxt TcType
ty_a' TcType
ty_e }
               Maybe TcType
Nothing -> TcType -> TcType -> TcM HsWrapper
just_unify TcType
ty_actual TcType
ty_expected }

    go ty_a :: TcType
ty_a@(FunTy { ft_af :: TcType -> FunTyFlag
ft_af = FunTyFlag
af1, ft_mult :: TcType -> TcType
ft_mult = TcType
act_mult, ft_arg :: TcType -> TcType
ft_arg = TcType
act_arg, ft_res :: TcType -> TcType
ft_res = TcType
act_res })
       ty_e :: TcType
ty_e@(FunTy { ft_af :: TcType -> FunTyFlag
ft_af = FunTyFlag
af2, ft_mult :: TcType -> TcType
ft_mult = TcType
exp_mult, ft_arg :: TcType -> TcType
ft_arg = TcType
exp_arg, ft_res :: TcType -> TcType
ft_res = TcType
exp_res })
      | FunTyFlag -> Bool
isVisibleFunArg FunTyFlag
af1, FunTyFlag -> Bool
isVisibleFunArg FunTyFlag
af2
      = if (TcType -> Bool
isTauTy TcType
ty_a Bool -> Bool -> Bool
&& TcType -> Bool
isTauTy TcType
ty_e)       -- Short cut common case to avoid
        then TcType -> TcType -> TcM HsWrapper
just_unify TcType
ty_actual TcType
ty_expected   -- unnecessary eta expansion
        else
        -- This is where we do the co/contra thing, and generate a WpFun, which in turn
        -- causes eta-expansion, which we don't like; hence encouraging NoDeepSubsumption
        do { arg_wrap  <- DeepSubsumptionFlag
-> (TcType -> TcType -> TcM Coercion)
-> CtOrigin
-> UserTypeCtxt
-> TcType
-> TcType
-> TcM HsWrapper
tc_sub_type_ds DeepSubsumptionFlag
Deep TcType -> TcType -> TcM Coercion
unify CtOrigin
given_orig UserTypeCtxt
GenSigCtxt TcType
exp_arg TcType
act_arg
                          -- GenSigCtxt: See Note [Setting the argument context]
           ; res_wrap  <- tc_sub_type_deep unify inst_orig ctxt act_res exp_res
           ; tcEqMult inst_orig act_mult exp_mult
             -- See Note [Multiplicity in deep subsumption]
           ; return (mkWpFun arg_wrap res_wrap (Scaled exp_mult exp_arg) exp_res) }
                     -- arg_wrap :: exp_arg ~> act_arg
                     -- res_wrap :: act-res ~> exp_res
      where
        given_orig :: CtOrigin
given_orig = SkolemInfoAnon -> CtOrigin
GivenOrigin (UserTypeCtxt -> TcType -> [(Name, TcTyVar)] -> SkolemInfoAnon
SigSkol UserTypeCtxt
GenSigCtxt TcType
exp_arg [])

    go TcType
ty_a TcType
ty_e
      | let ([TcTyVar]
tvs, [TcType]
theta, TcType
_) = TcType -> ([TcTyVar], [TcType], TcType)
tcSplitSigmaTy TcType
ty_a
      , Bool -> Bool
not ([TcTyVar] -> Bool
forall a. [a] -> Bool
forall (t :: * -> *) a. Foldable t => t a -> Bool
null [TcTyVar]
tvs Bool -> Bool -> Bool
&& [TcType] -> Bool
forall a. [a] -> Bool
forall (t :: * -> *) a. Foldable t => t a -> Bool
null [TcType]
theta)
      = do { (in_wrap, in_rho) <- CtOrigin -> TcType -> TcM (HsWrapper, TcType)
topInstantiate CtOrigin
inst_orig TcType
ty_a
           ; body_wrap <- tc_sub_type_deep unify inst_orig ctxt in_rho ty_e
           ; return (body_wrap <.> in_wrap) }

      | Bool
otherwise   -- Revert to unification
      = do { -- It's still possible that ty_actual has nested foralls. Instantiate
             -- these, as there's no way unification will succeed with them in.
             -- See Note [The need for deep instantiation]
             (inst_wrap, rho_a) <- CtOrigin -> TcType -> TcM (HsWrapper, TcType)
deeplyInstantiate CtOrigin
inst_orig TcType
ty_actual
           ; unify_wrap         <- just_unify rho_a ty_expected
           ; return (unify_wrap <.> inst_wrap) }

    just_unify :: TcType -> TcType -> TcM HsWrapper
just_unify TcType
ty_a TcType
ty_e = do { cow <- TcType -> TcType -> TcM Coercion
unify TcType
ty_a TcType
ty_e
                              ; return (mkWpCastN cow) }

-----------------------
deeplySkolemise :: SkolemInfo -> TcSigmaType
                -> TcM ( HsWrapper
                       , [(Name,TcInvisTVBinder)]     -- All skolemised variables
                       , [EvVar]                      -- All "given"s
                       , TcRhoType )
-- See Note [Deep skolemisation]
deeplySkolemise :: SkolemInfo
-> TcType
-> IOEnv
     (Env TcGblEnv TcLclEnv)
     (HsWrapper, [(Name, TcInvisTVBinder)], [TcTyVar], TcType)
deeplySkolemise SkolemInfo
skol_info TcType
ty
  = Subst
-> TcType
-> IOEnv
     (Env TcGblEnv TcLclEnv)
     (HsWrapper, [(Name, TcInvisTVBinder)], [TcTyVar], TcType)
go Subst
init_subst TcType
ty
  where
    init_subst :: Subst
init_subst = InScopeSet -> Subst
mkEmptySubst (VarSet -> InScopeSet
mkInScopeSet (TcType -> VarSet
tyCoVarsOfType TcType
ty))

    go :: Subst
-> TcType
-> IOEnv
     (Env TcGblEnv TcLclEnv)
     (HsWrapper, [(Name, TcInvisTVBinder)], [TcTyVar], TcType)
go Subst
subst TcType
ty
      | Just ([Scaled TcType]
arg_tys, [TcInvisTVBinder]
bndrs, [TcType]
theta, TcType
ty') <- TcType
-> Maybe ([Scaled TcType], [TcInvisTVBinder], [TcType], TcType)
tcDeepSplitSigmaTy_maybe TcType
ty
      = do { let arg_tys' :: [Scaled TcType]
arg_tys' = HasDebugCallStack => Subst -> [Scaled TcType] -> [Scaled TcType]
Subst -> [Scaled TcType] -> [Scaled TcType]
substScaledTys Subst
subst [Scaled TcType]
arg_tys
           ; ids1             <- FastString -> [Scaled TcType] -> TcRnIf TcGblEnv TcLclEnv [TcTyVar]
forall (t :: * -> *) gbl lcl.
Traversable t =>
FastString -> t (Scaled TcType) -> TcRnIf gbl lcl (t TcTyVar)
newSysLocalIds (String -> FastString
fsLit String
"dk") [Scaled TcType]
arg_tys'
           ; (subst', bndrs1) <- tcInstSkolTyVarBndrsX skol_info subst bndrs
           ; ev_vars1         <- newEvVars (substTheta subst' theta)
           ; (wrap, tvs_prs2, ev_vars2, rho) <- go subst' ty'
           ; let tvs     = [TcInvisTVBinder] -> [TcTyVar]
forall tv argf. [VarBndr tv argf] -> [tv]
binderVars [TcInvisTVBinder]
bndrs
                 tvs1    = [TcInvisTVBinder] -> [TcTyVar]
forall tv argf. [VarBndr tv argf] -> [tv]
binderVars [TcInvisTVBinder]
bndrs1
                 tv_prs1 = (TcTyVar -> Name) -> [TcTyVar] -> [Name]
forall a b. (a -> b) -> [a] -> [b]
map TcTyVar -> Name
tyVarName [TcTyVar]
tvs [Name] -> [TcInvisTVBinder] -> [(Name, TcInvisTVBinder)]
forall a b. [a] -> [b] -> [(a, b)]
`zip` [TcInvisTVBinder]
bndrs1
           ; return ( mkWpEta ids1 (mkWpTyLams tvs1
                                    <.> mkWpEvLams ev_vars1
                                    <.> wrap)
                    , tv_prs1  ++ tvs_prs2
                    , ev_vars1 ++ ev_vars2
                    , mkScaledFunTys arg_tys' rho ) }

      | Bool
otherwise
      = (HsWrapper, [(Name, TcInvisTVBinder)], [TcTyVar], TcType)
-> IOEnv
     (Env TcGblEnv TcLclEnv)
     (HsWrapper, [(Name, TcInvisTVBinder)], [TcTyVar], TcType)
forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
forall (m :: * -> *) a. Monad m => a -> m a
return (HsWrapper
idHsWrapper, [], [], HasDebugCallStack => Subst -> TcType -> TcType
Subst -> TcType -> TcType
substTy Subst
subst TcType
ty)
        -- substTy is a quick no-op on an empty substitution

deeplyInstantiate :: CtOrigin -> TcType -> TcM (HsWrapper, Type)
deeplyInstantiate :: CtOrigin -> TcType -> TcM (HsWrapper, TcType)
deeplyInstantiate CtOrigin
orig TcType
ty
  = Subst -> TcType -> TcM (HsWrapper, TcType)
go Subst
init_subst TcType
ty
  where
    init_subst :: Subst
init_subst = InScopeSet -> Subst
mkEmptySubst (VarSet -> InScopeSet
mkInScopeSet (TcType -> VarSet
tyCoVarsOfType TcType
ty))

    go :: Subst -> TcType -> TcM (HsWrapper, TcType)
go Subst
subst TcType
ty
      | Just ([Scaled TcType]
arg_tys, [TcInvisTVBinder]
bndrs, [TcType]
theta, TcType
rho) <- TcType
-> Maybe ([Scaled TcType], [TcInvisTVBinder], [TcType], TcType)
tcDeepSplitSigmaTy_maybe TcType
ty
      = do { let tvs :: [TcTyVar]
tvs = [TcInvisTVBinder] -> [TcTyVar]
forall tv argf. [VarBndr tv argf] -> [tv]
binderVars [TcInvisTVBinder]
bndrs
           ; (subst', tvs') <- Subst -> [TcTyVar] -> TcM (Subst, [TcTyVar])
newMetaTyVarsX Subst
subst [TcTyVar]
tvs
           ; let arg_tys' = HasDebugCallStack => Subst -> [Scaled TcType] -> [Scaled TcType]
Subst -> [Scaled TcType] -> [Scaled TcType]
substScaledTys   Subst
subst' [Scaled TcType]
arg_tys
                 theta'   = HasDebugCallStack => Subst -> [TcType] -> [TcType]
Subst -> [TcType] -> [TcType]
substTheta Subst
subst' [TcType]
theta
           ; ids1  <- newSysLocalIds (fsLit "di") arg_tys'
           ; wrap1 <- instCall orig (mkTyVarTys tvs') theta'
           ; (wrap2, rho2) <- go subst' rho
           ; return (mkWpEta ids1 (wrap2 <.> wrap1),
                     mkScaledFunTys arg_tys' rho2) }

      | Bool
otherwise
      = do { let ty' :: TcType
ty' = HasDebugCallStack => Subst -> TcType -> TcType
Subst -> TcType -> TcType
substTy Subst
subst TcType
ty
           ; (HsWrapper, TcType) -> TcM (HsWrapper, TcType)
forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
forall (m :: * -> *) a. Monad m => a -> m a
return (HsWrapper
idHsWrapper, TcType
ty') }

tcDeepSplitSigmaTy_maybe
  :: TcSigmaType -> Maybe ([Scaled TcType], [TcInvisTVBinder], ThetaType, TcSigmaType)
-- Looks for a *non-trivial* quantified type, under zero or more function arrows
-- By "non-trivial" we mean either tyvars or constraints are non-empty
tcDeepSplitSigmaTy_maybe :: TcType
-> Maybe ([Scaled TcType], [TcInvisTVBinder], [TcType], TcType)
tcDeepSplitSigmaTy_maybe TcType
ty
  = TcType
-> Maybe ([Scaled TcType], [TcInvisTVBinder], [TcType], TcType)
go TcType
ty
  where
  go :: TcType
-> Maybe ([Scaled TcType], [TcInvisTVBinder], [TcType], TcType)
go TcType
ty | Just (Scaled TcType
arg_ty, TcType
res_ty)           <- TcType -> Maybe (Scaled TcType, TcType)
tcSplitFunTy_maybe TcType
ty
        , Just ([Scaled TcType]
arg_tys, [TcInvisTVBinder]
tvs, [TcType]
theta, TcType
rho) <- TcType
-> Maybe ([Scaled TcType], [TcInvisTVBinder], [TcType], TcType)
go TcType
res_ty
        = ([Scaled TcType], [TcInvisTVBinder], [TcType], TcType)
-> Maybe ([Scaled TcType], [TcInvisTVBinder], [TcType], TcType)
forall a. a -> Maybe a
Just (Scaled TcType
arg_tyScaled TcType -> [Scaled TcType] -> [Scaled TcType]
forall a. a -> [a] -> [a]
:[Scaled TcType]
arg_tys, [TcInvisTVBinder]
tvs, [TcType]
theta, TcType
rho)

        | ([TcInvisTVBinder]
tvs, [TcType]
theta, TcType
rho) <- TcType -> ([TcInvisTVBinder], [TcType], TcType)
tcSplitSigmaTyBndrs TcType
ty
        , Bool -> Bool
not ([TcInvisTVBinder] -> Bool
forall a. [a] -> Bool
forall (t :: * -> *) a. Foldable t => t a -> Bool
null [TcInvisTVBinder]
tvs Bool -> Bool -> Bool
&& [TcType] -> Bool
forall a. [a] -> Bool
forall (t :: * -> *) a. Foldable t => t a -> Bool
null [TcType]
theta)
        = ([Scaled TcType], [TcInvisTVBinder], [TcType], TcType)
-> Maybe ([Scaled TcType], [TcInvisTVBinder], [TcType], TcType)
forall a. a -> Maybe a
Just ([], [TcInvisTVBinder]
tvs, [TcType]
theta, TcType
rho)

        | Bool
otherwise = Maybe ([Scaled TcType], [TcInvisTVBinder], [TcType], TcType)
forall a. Maybe a
Nothing

isDeepRhoTy :: TcType -> Bool
-- True if there are no foralls or (=>) at the top, or nested under
-- arrows to the right.  e.g
--    forall a. a                  False
--    Int -> forall a. a           False
--    (forall a. a) -> Int         True
-- Returns True iff tcDeepSplitSigmaTy_maybe returns Nothing
isDeepRhoTy :: TcType -> Bool
isDeepRhoTy TcType
ty
  | Bool -> Bool
not (TcType -> Bool
isRhoTy TcType
ty)                       = Bool
False  -- Foralls or (=>) at top
  | Just (Scaled TcType
_, TcType
res) <- TcType -> Maybe (Scaled TcType, TcType)
tcSplitFunTy_maybe TcType
ty = TcType -> Bool
isDeepRhoTy TcType
res
  | Bool
otherwise                              = Bool
True   -- No forall, (=>), or (->) at top

isRhoTyDS :: DeepSubsumptionFlag -> TcType -> Bool
isRhoTyDS :: DeepSubsumptionFlag -> TcType -> Bool
isRhoTyDS DeepSubsumptionFlag
ds_flag TcType
ty
  = case DeepSubsumptionFlag
ds_flag of
      DeepSubsumptionFlag
Shallow -> TcType -> Bool
isRhoTy TcType
ty      -- isRhoTy: no top level forall or (=>)
      DeepSubsumptionFlag
Deep    -> TcType -> Bool
isDeepRhoTy TcType
ty  -- "deep" version: no nested forall or (=>)

{-
************************************************************************
*                                                                      *
                Boxy unification
*                                                                      *
************************************************************************

The exported functions are all defined as versions of some
non-exported generic functions.
-}

unifyExprType :: HsExpr GhcRn -> TcType -> TcType -> TcM TcCoercionN
unifyExprType :: HsExpr GhcRn -> TcType -> TcType -> TcM Coercion
unifyExprType HsExpr GhcRn
rn_expr TcType
ty1 TcType
ty2
  = Maybe TypedThing -> TcType -> TcType -> TcM Coercion
unifyType (TypedThing -> Maybe TypedThing
forall a. a -> Maybe a
Just (HsExpr GhcRn -> TypedThing
HsExprRnThing HsExpr GhcRn
rn_expr)) TcType
ty1 TcType
ty2

unifyType :: Maybe TypedThing  -- ^ If present, the thing that has type ty1
          -> TcTauType -> TcTauType    -- ty1 (actual), ty2 (expected)
          -> TcM TcCoercionN           -- :: ty1 ~# ty2
-- Actual and expected types
-- Returns a coercion : ty1 ~ ty2
unifyType :: Maybe TypedThing -> TcType -> TcType -> TcM Coercion
unifyType Maybe TypedThing
thing TcType
ty1 TcType
ty2
  = TypeOrKind -> CtOrigin -> TcType -> TcType -> TcM Coercion
unifyTypeAndEmit TypeOrKind
TypeLevel CtOrigin
origin TcType
ty1 TcType
ty2
  where
    origin :: CtOrigin
origin = TypeEqOrigin { uo_actual :: TcType
uo_actual   = TcType
ty1
                          , uo_expected :: TcType
uo_expected = TcType
ty2
                          , uo_thing :: Maybe TypedThing
uo_thing    = Maybe TypedThing
thing
                          , uo_visible :: Bool
uo_visible  = Bool
True }

unifyInvisibleType :: TcTauType -> TcTauType    -- ty1 (actual), ty2 (expected)
                   -> TcM TcCoercionN           -- :: ty1 ~# ty2
-- Actual and expected types
-- Returns a coercion : ty1 ~ ty2
unifyInvisibleType :: TcType -> TcType -> TcM Coercion
unifyInvisibleType TcType
ty1 TcType
ty2
  = TypeOrKind -> CtOrigin -> TcType -> TcType -> TcM Coercion
unifyTypeAndEmit TypeOrKind
TypeLevel CtOrigin
origin TcType
ty1 TcType
ty2
  where
    origin :: CtOrigin
origin = TypeEqOrigin { uo_actual :: TcType
uo_actual   = TcType
ty1
                          , uo_expected :: TcType
uo_expected = TcType
ty2
                          , uo_thing :: Maybe TypedThing
uo_thing    = Maybe TypedThing
forall a. Maybe a
Nothing
                          , uo_visible :: Bool
uo_visible  = Bool
False }  -- This is the "invisible" bit

unifyTypeET :: TcTauType -> TcTauType -> TcM CoercionN
-- Like unifyType, but swap expected and actual in error messages
-- This is used when typechecking patterns
unifyTypeET :: TcType -> TcType -> TcM Coercion
unifyTypeET TcType
ty1 TcType
ty2
  = TypeOrKind -> CtOrigin -> TcType -> TcType -> TcM Coercion
unifyTypeAndEmit TypeOrKind
TypeLevel CtOrigin
origin TcType
ty1 TcType
ty2
  where
    origin :: CtOrigin
origin = TypeEqOrigin { uo_actual :: TcType
uo_actual   = TcType
ty2   -- NB swapped
                          , uo_expected :: TcType
uo_expected = TcType
ty1   -- NB swapped
                          , uo_thing :: Maybe TypedThing
uo_thing    = Maybe TypedThing
forall a. Maybe a
Nothing
                          , uo_visible :: Bool
uo_visible  = Bool
True }


unifyKind :: Maybe TypedThing -> TcKind -> TcKind -> TcM CoercionN
unifyKind :: Maybe TypedThing -> TcType -> TcType -> TcM Coercion
unifyKind Maybe TypedThing
mb_thing TcType
ty1 TcType
ty2
  = TypeOrKind -> CtOrigin -> TcType -> TcType -> TcM Coercion
unifyTypeAndEmit TypeOrKind
KindLevel CtOrigin
origin TcType
ty1 TcType
ty2
  where
    origin :: CtOrigin
origin = TypeEqOrigin { uo_actual :: TcType
uo_actual   = TcType
ty1
                          , uo_expected :: TcType
uo_expected = TcType
ty2
                          , uo_thing :: Maybe TypedThing
uo_thing    = Maybe TypedThing
mb_thing
                          , uo_visible :: Bool
uo_visible  = Bool
True }

unifyTypeAndEmit :: TypeOrKind -> CtOrigin -> TcType -> TcType -> TcM CoercionN
-- Make a ref-cell, unify, emit the collected constraints
unifyTypeAndEmit :: TypeOrKind -> CtOrigin -> TcType -> TcType -> TcM Coercion
unifyTypeAndEmit TypeOrKind
t_or_k CtOrigin
orig TcType
ty1 TcType
ty2
  = do { ref <- Bag Ct -> IOEnv (Env TcGblEnv TcLclEnv) (TcRef (Bag Ct))
forall (m :: * -> *) a. MonadIO m => a -> m (TcRef a)
newTcRef Bag Ct
forall a. Bag a
emptyBag
       ; loc <- getCtLocM orig (Just t_or_k)
       ; let env = UE { u_loc :: CtLoc
u_loc = CtLoc
loc, u_role :: Role
u_role = Role
Nominal
                      , u_rewriters :: RewriterSet
u_rewriters = RewriterSet
emptyRewriterSet  -- ToDo: check this
                      , u_defer :: TcRef (Bag Ct)
u_defer = TcRef (Bag Ct)
ref, u_unified :: Maybe (TcRef [TcTyVar])
u_unified = Maybe (TcRef [TcTyVar])
forall a. Maybe a
Nothing }

       -- The hard work happens here
       ; co <- uType env ty1 ty2

       ; cts <- readTcRef ref
       ; unless (null cts) (emitSimples cts)
       ; return co }

{-
%************************************************************************
%*                                                                      *
                 uType and friends
%*                                                                      *
%************************************************************************

Note [The eager unifier]
~~~~~~~~~~~~~~~~~~~~~~~~
The eager unifier, `uType`, is called by

  * The constraint generator (e.g. in GHC.Tc.Gen.Expr),
    via the wrappers `unifyType`, `unifyKind` etc

  * The constraint solver (e.g. in GHC.Tc.Solver.Equality),
    via `GHC.Tc.Solver.Monad.wrapUnifierTcS`.

`uType` runs in the TcM monad, but it carries a UnifyEnv that tells it
what to do when unifying a variable or deferring a constraint. Specifically,
  * it collects deferred constraints in `u_defer`, and
  * it records which unification variables it has unified in `u_unified`
Then it is up to the wrappers (one for the constraint generator, one for
the constraint solver) to deal with these collected sets.

Although `uType` runs in the TcM monad for convenience, really it could
operate just with the ability to
  * write to the accumulators of deferred constraints
    and unification variables in UnifyEnv.
  * read and update existing unification variables
  * zonk types befire unifying (`zonkTcType` in `uUnfilledVar`, and
    `zonkTyCoVarKind` in `uUnfilledVar1`
  * create fresh coercion holes (`newCoercionHole`)
  * emit tracing info for debugging
  * look at the ambient TcLevel: `getTcLevel`
A job for the future.
-}

data UnifyEnv
  = UE { UnifyEnv -> Role
u_role      :: Role
       , UnifyEnv -> CtLoc
u_loc       :: CtLoc
       , UnifyEnv -> RewriterSet
u_rewriters :: RewriterSet

         -- Deferred constraints
       , UnifyEnv -> TcRef (Bag Ct)
u_defer     :: TcRef (Bag Ct)

         -- Which variables are unified;
         -- if Nothing, we don't care
       , UnifyEnv -> Maybe (TcRef [TcTyVar])
u_unified :: Maybe (TcRef [TcTyVar])
    }

setUEnvRole :: UnifyEnv -> Role -> UnifyEnv
setUEnvRole :: UnifyEnv -> Role -> UnifyEnv
setUEnvRole UnifyEnv
uenv Role
role = UnifyEnv
uenv { u_role = role }

updUEnvLoc :: UnifyEnv -> (CtLoc -> CtLoc) -> UnifyEnv
updUEnvLoc :: UnifyEnv -> (CtLoc -> CtLoc) -> UnifyEnv
updUEnvLoc uenv :: UnifyEnv
uenv@(UE { u_loc :: UnifyEnv -> CtLoc
u_loc = CtLoc
loc }) CtLoc -> CtLoc
upd = UnifyEnv
uenv { u_loc = upd loc }

mkKindEnv :: UnifyEnv -> TcType -> TcType -> UnifyEnv
-- Modify the UnifyEnv to be right for unifing
-- the kinds of these two types
mkKindEnv :: UnifyEnv -> TcType -> TcType -> UnifyEnv
mkKindEnv env :: UnifyEnv
env@(UE { u_loc :: UnifyEnv -> CtLoc
u_loc = CtLoc
ctloc }) TcType
ty1 TcType
ty2
  = UnifyEnv
env { u_role = Nominal, u_loc = mkKindEqLoc ty1 ty2 ctloc }

uType, uType_defer
  :: UnifyEnv
  -> TcType    -- ty1 is the *actual* type
  -> TcType    -- ty2 is the *expected* type
  -> TcM CoercionN

-- It is always safe to defer unification to the main constraint solver
-- See Note [Deferred unification]
uType_defer :: UnifyEnv -> TcType -> TcType -> TcM Coercion
uType_defer (UE { u_loc :: UnifyEnv -> CtLoc
u_loc = CtLoc
loc, u_defer :: UnifyEnv -> TcRef (Bag Ct)
u_defer = TcRef (Bag Ct)
ref
                , u_role :: UnifyEnv -> Role
u_role = Role
role, u_rewriters :: UnifyEnv -> RewriterSet
u_rewriters = RewriterSet
rewriters })
            TcType
ty1 TcType
ty2  -- ty1 is "actual", ty2 is "expected"
  = do { let pred_ty :: TcType
pred_ty = Role -> TcType -> TcType -> TcType
mkEqPredRole Role
role TcType
ty1 TcType
ty2
       ; hole <- CtLoc -> TcType -> TcM CoercionHole
newCoercionHole CtLoc
loc TcType
pred_ty
       ; let ct = CtEvidence -> Ct
mkNonCanonical (CtEvidence -> Ct) -> CtEvidence -> Ct
forall a b. (a -> b) -> a -> b
$
                  CtWanted { ctev_pred :: TcType
ctev_pred      = TcType
pred_ty
                           , ctev_dest :: TcEvDest
ctev_dest      = CoercionHole -> TcEvDest
HoleDest CoercionHole
hole
                           , ctev_loc :: CtLoc
ctev_loc       = CtLoc
loc
                           , ctev_rewriters :: RewriterSet
ctev_rewriters = RewriterSet
rewriters }
             co = CoercionHole -> Coercion
HoleCo CoercionHole
hole
       ; updTcRef ref (`snocBag` ct)
         -- snocBag: see Note [Work-list ordering] in GHC.Tc.Solver.Equality

       -- Error trace only
       -- NB. do *not* call mkErrCtxt unless tracing is on,
       --     because it is hugely expensive (#5631)
       ; whenDOptM Opt_D_dump_tc_trace $
         do { ctxt     <- getErrCtxt
            ; err_ctxt <- mkErrCtxt emptyTidyEnv ctxt
            ; traceTc "utype_defer" $
                vcat ( ppr role
                     : debugPprType ty1
                     : debugPprType ty2
                     : map pprErrCtxtMsg err_ctxt )
            ; traceTc "utype_defer2" (ppr co) }

       ; return co }


--------------
uType :: UnifyEnv -> TcType -> TcType -> TcM Coercion
uType env :: UnifyEnv
env@(UE { u_role :: UnifyEnv -> Role
u_role = Role
role }) TcType
orig_ty1 TcType
orig_ty2
  | Role
Phantom <- Role
role
  = do { kind_co <- UnifyEnv -> TcType -> TcType -> TcM Coercion
uType (UnifyEnv -> TcType -> TcType -> UnifyEnv
mkKindEnv UnifyEnv
env TcType
orig_ty1 TcType
orig_ty2)
                          (HasDebugCallStack => TcType -> TcType
TcType -> TcType
typeKind TcType
orig_ty1) (HasDebugCallStack => TcType -> TcType
TcType -> TcType
typeKind TcType
orig_ty2)
       ; return (mkPhantomCo kind_co orig_ty1 orig_ty2) }

  | Bool
otherwise
  = do { tclvl <- TcM TcLevel
getTcLevel
       ; traceTc "u_tys" $ vcat
              [ text "tclvl" <+> ppr tclvl
              , sep [ ppr orig_ty1, text "~" <> ppr role, ppr orig_ty2] ]
       ; co <- go orig_ty1 orig_ty2
       ; if isReflCo co
            then traceTc "u_tys yields no coercion" Outputable.empty
            else traceTc "u_tys yields coercion:" (ppr co)
       ; return co }
  where
    go :: TcType -> TcType -> TcM CoercionN
        -- The arguments to 'go' are always semantically identical
        -- to orig_ty{1,2} except for looking through type synonyms

     -- Unwrap casts before looking for variables. This way, we can easily
     -- recognize (t |> co) ~ (t |> co), which is nice. Previously, we
     -- didn't do it this way, and then the unification above was deferred.
    go :: TcType -> TcType -> TcM Coercion
go (CastTy TcType
t1 Coercion
co1) TcType
t2
      = do { co_tys <- UnifyEnv -> TcType -> TcType -> TcM Coercion
uType UnifyEnv
env TcType
t1 TcType
t2
           ; return (mkCoherenceLeftCo role t1 co1 co_tys) }

    go TcType
t1 (CastTy TcType
t2 Coercion
co2)
      = do { co_tys <- UnifyEnv -> TcType -> TcType -> TcM Coercion
uType UnifyEnv
env TcType
t1 TcType
t2
           ; return (mkCoherenceRightCo role t2 co2 co_tys) }

        -- Variables; go for uUnfilledVar
        -- Note that we pass in *original* (before synonym expansion),
        -- so that type variables tend to get filled in with
        -- the most informative version of the type
    go (TyVarTy TcTyVar
tv1) TcType
ty2
      = do { lookup_res <- TcTyVar -> IOEnv (Env TcGblEnv TcLclEnv) (Maybe TcType)
isFilledMetaTyVar_maybe TcTyVar
tv1
           ; case lookup_res of
               Just TcType
ty1 -> do { String -> SDoc -> TcM ()
traceTc String
"found filled tyvar" (TcTyVar -> SDoc
forall a. Outputable a => a -> SDoc
ppr TcTyVar
tv1 SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> String -> SDoc
forall doc. IsLine doc => String -> doc
text String
":->" SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> TcType -> SDoc
forall a. Outputable a => a -> SDoc
ppr TcType
ty1)
                              ; UnifyEnv -> TcType -> TcType -> TcM Coercion
uType UnifyEnv
env TcType
ty1 TcType
orig_ty2 }
               Maybe TcType
Nothing -> UnifyEnv -> SwapFlag -> TcTyVar -> TcType -> TcM Coercion
uUnfilledVar UnifyEnv
env SwapFlag
NotSwapped TcTyVar
tv1 TcType
ty2 }

    go TcType
ty1 (TyVarTy TcTyVar
tv2)
      = do { lookup_res <- TcTyVar -> IOEnv (Env TcGblEnv TcLclEnv) (Maybe TcType)
isFilledMetaTyVar_maybe TcTyVar
tv2
           ; case lookup_res of
               Just TcType
ty2 -> do { String -> SDoc -> TcM ()
traceTc String
"found filled tyvar" (TcTyVar -> SDoc
forall a. Outputable a => a -> SDoc
ppr TcTyVar
tv2 SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> String -> SDoc
forall doc. IsLine doc => String -> doc
text String
":->" SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> TcType -> SDoc
forall a. Outputable a => a -> SDoc
ppr TcType
ty2)
                              ; UnifyEnv -> TcType -> TcType -> TcM Coercion
uType UnifyEnv
env TcType
orig_ty1 TcType
ty2 }
               Maybe TcType
Nothing -> UnifyEnv -> SwapFlag -> TcTyVar -> TcType -> TcM Coercion
uUnfilledVar UnifyEnv
env SwapFlag
IsSwapped TcTyVar
tv2 TcType
ty1 }

      -- See Note [Unifying type synonyms] in GHC.Core.Unify
    go ty1 :: TcType
ty1@(TyConApp TyCon
tc1 []) (TyConApp TyCon
tc2 [])
      | TyCon
tc1 TyCon -> TyCon -> Bool
forall a. Eq a => a -> a -> Bool
== TyCon
tc2
      = Coercion -> TcM Coercion
forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
forall (m :: * -> *) a. Monad m => a -> m a
return (Coercion -> TcM Coercion) -> Coercion -> TcM Coercion
forall a b. (a -> b) -> a -> b
$ Role -> TcType -> Coercion
mkReflCo Role
role TcType
ty1

        -- Now expand synonyms
        -- See Note [Expanding synonyms during unification]
        --
        -- Also NB that we recurse to 'go' so that we don't push a
        -- new item on the origin stack. As a result if we have
        --   type Foo = Int
        -- and we try to unify  Foo ~ Bool
        -- we'll end up saying "can't match Foo with Bool"
        -- rather than "can't match "Int with Bool".  See #4535.
    go TcType
ty1 TcType
ty2
      | Just TcType
ty1' <- TcType -> Maybe TcType
coreView TcType
ty1 = TcType -> TcType -> TcM Coercion
go TcType
ty1' TcType
ty2
      | Just TcType
ty2' <- TcType -> Maybe TcType
coreView TcType
ty2 = TcType -> TcType -> TcM Coercion
go TcType
ty1  TcType
ty2'

    -- Functions (t1 -> t2) just check the two parts
    go (FunTy { ft_af :: TcType -> FunTyFlag
ft_af = FunTyFlag
af1, ft_mult :: TcType -> TcType
ft_mult = TcType
w1, ft_arg :: TcType -> TcType
ft_arg = TcType
arg1, ft_res :: TcType -> TcType
ft_res = TcType
res1 })
       (FunTy { ft_af :: TcType -> FunTyFlag
ft_af = FunTyFlag
af2, ft_mult :: TcType -> TcType
ft_mult = TcType
w2, ft_arg :: TcType -> TcType
ft_arg = TcType
arg2, ft_res :: TcType -> TcType
ft_res = TcType
res2 })
      | FunTyFlag -> Bool
isVisibleFunArg FunTyFlag
af1  -- Do not attempt (c => t); just defer
      , FunTyFlag
af1 FunTyFlag -> FunTyFlag -> Bool
forall a. Eq a => a -> a -> Bool
== FunTyFlag
af2           -- Important!  See #21530
      = do { co_w <- UnifyEnv -> TcType -> TcType -> TcM Coercion
uType (UnifyEnv
env { u_role = funRole role SelMult }) TcType
w1   TcType
w2
           ; co_l <- uType (env { u_role = funRole role SelArg })  arg1 arg2
           ; co_r <- uType (env { u_role = funRole role SelRes })  res1 res2
           ; return $ mkNakedFunCo role af1 co_w co_l co_r }

        -- Always defer if a type synonym family (type function)
        -- is involved.  (Data families behave rigidly.)
    go ty1 :: TcType
ty1@(TyConApp TyCon
tc1 [TcType]
_) TcType
ty2
      | TyCon -> Bool
isTypeFamilyTyCon TyCon
tc1 = TcType -> TcType -> TcM Coercion
defer TcType
ty1 TcType
ty2
    go TcType
ty1 ty2 :: TcType
ty2@(TyConApp TyCon
tc2 [TcType]
_)
      | TyCon -> Bool
isTypeFamilyTyCon TyCon
tc2 = TcType -> TcType -> TcM Coercion
defer TcType
ty1 TcType
ty2

    go (TyConApp TyCon
tc1 [TcType]
tys1) (TyConApp TyCon
tc2 [TcType]
tys2)
      -- See Note [Mismatched type lists and application decomposition]
      | TyCon
tc1 TyCon -> TyCon -> Bool
forall a. Eq a => a -> a -> Bool
== TyCon
tc2, [TcType] -> [TcType] -> Bool
forall a b. [a] -> [b] -> Bool
equalLength [TcType]
tys1 [TcType]
tys2
      , TyCon -> Role -> Bool
isInjectiveTyCon TyCon
tc1 Role
role -- don't look under newtypes at Rep equality
      = Bool -> SDoc -> TcM Coercion -> TcM Coercion
forall a. HasCallStack => Bool -> SDoc -> a -> a
assertPpr (TyCon -> Role -> Bool
isGenerativeTyCon TyCon
tc1 Role
role) (TyCon -> SDoc
forall a. Outputable a => a -> SDoc
ppr TyCon
tc1) (TcM Coercion -> TcM Coercion) -> TcM Coercion -> TcM Coercion
forall a b. (a -> b) -> a -> b
$
        do { String -> SDoc -> TcM ()
traceTc String
"go-tycon" (TyCon -> SDoc
forall a. Outputable a => a -> SDoc
ppr TyCon
tc1 SDoc -> SDoc -> SDoc
forall doc. IsDoc doc => doc -> doc -> doc
$$ [TcType] -> SDoc
forall a. Outputable a => a -> SDoc
ppr [TcType]
tys1 SDoc -> SDoc -> SDoc
forall doc. IsDoc doc => doc -> doc -> doc
$$ [TcType] -> SDoc
forall a. Outputable a => a -> SDoc
ppr [TcType]
tys2 SDoc -> SDoc -> SDoc
forall doc. IsDoc doc => doc -> doc -> doc
$$ [Role] -> SDoc
forall a. Outputable a => a -> SDoc
ppr (Int -> [Role] -> [Role]
forall a. Int -> [a] -> [a]
take Int
10 (Role -> TyCon -> [Role]
tyConRoleListX Role
role TyCon
tc1)))
           ; cos <- (Bool -> Role -> TcType -> TcType -> TcM Coercion)
-> [Bool]
-> [Role]
-> [TcType]
-> [TcType]
-> IOEnv (Env TcGblEnv TcLclEnv) [Coercion]
forall (m :: * -> *) a b c d e.
Monad m =>
(a -> b -> c -> d -> m e) -> [a] -> [b] -> [c] -> [d] -> m [e]
zipWith4M Bool -> Role -> TcType -> TcType -> TcM Coercion
u_tc_arg (TyCon -> [Bool]
tyConVisibilities TyCon
tc1)   -- Infinite
                                       (Role -> TyCon -> [Role]
tyConRoleListX Role
role TyCon
tc1) -- Infinite
                                       [TcType]
tys1 [TcType]
tys2
           ; return $ mkTyConAppCo role tc1 cos }

    go (LitTy TyLit
m) ty :: TcType
ty@(LitTy TyLit
n)
      | TyLit
m TyLit -> TyLit -> Bool
forall a. Eq a => a -> a -> Bool
== TyLit
n
      = Coercion -> TcM Coercion
forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
forall (m :: * -> *) a. Monad m => a -> m a
return (Coercion -> TcM Coercion) -> Coercion -> TcM Coercion
forall a b. (a -> b) -> a -> b
$ Role -> TcType -> Coercion
mkReflCo Role
role TcType
ty

        -- See Note [Care with type applications]
        -- Do not decompose FunTy against App;
        -- it's often a type error, so leave it for the constraint solver
    go ty1 :: TcType
ty1@(AppTy TcType
s1 TcType
t1) ty2 :: TcType
ty2@(AppTy TcType
s2 TcType
t2)
      = Bool
-> TcType
-> TcType
-> TcType
-> TcType
-> TcType
-> TcType
-> TcM Coercion
go_app (TcType -> Bool
isNextArgVisible TcType
s1) TcType
ty1 TcType
s1 TcType
t1 TcType
ty2 TcType
s2 TcType
t2

    go ty1 :: TcType
ty1@(AppTy TcType
s1 TcType
t1) ty2 :: TcType
ty2@(TyConApp TyCon
tc2 [TcType]
ts2)
      | Just ([TcType]
ts2', TcType
t2') <- [TcType] -> Maybe ([TcType], TcType)
forall a. [a] -> Maybe ([a], a)
snocView [TcType]
ts2
      = Bool -> TcM Coercion -> TcM Coercion
forall a. HasCallStack => Bool -> a -> a
assert (Bool -> Bool
not (TyCon -> Bool
tyConMustBeSaturated TyCon
tc2)) (TcM Coercion -> TcM Coercion) -> TcM Coercion -> TcM Coercion
forall a b. (a -> b) -> a -> b
$
        Bool
-> TcType
-> TcType
-> TcType
-> TcType
-> TcType
-> TcType
-> TcM Coercion
go_app (TyCon -> [TcType] -> Bool
isNextTyConArgVisible TyCon
tc2 [TcType]
ts2')
               TcType
ty1 TcType
s1 TcType
t1 TcType
ty2 (TyCon -> [TcType] -> TcType
TyConApp TyCon
tc2 [TcType]
ts2') TcType
t2'

    go ty1 :: TcType
ty1@(TyConApp TyCon
tc1 [TcType]
ts1) ty2 :: TcType
ty2@(AppTy TcType
s2 TcType
t2)
      | Just ([TcType]
ts1', TcType
t1') <- [TcType] -> Maybe ([TcType], TcType)
forall a. [a] -> Maybe ([a], a)
snocView [TcType]
ts1
      = Bool -> TcM Coercion -> TcM Coercion
forall a. HasCallStack => Bool -> a -> a
assert (Bool -> Bool
not (TyCon -> Bool
tyConMustBeSaturated TyCon
tc1)) (TcM Coercion -> TcM Coercion) -> TcM Coercion -> TcM Coercion
forall a b. (a -> b) -> a -> b
$
        Bool
-> TcType
-> TcType
-> TcType
-> TcType
-> TcType
-> TcType
-> TcM Coercion
go_app (TyCon -> [TcType] -> Bool
isNextTyConArgVisible TyCon
tc1 [TcType]
ts1')
               TcType
ty1 (TyCon -> [TcType] -> TcType
TyConApp TyCon
tc1 [TcType]
ts1') TcType
t1' TcType
ty2 TcType
s2 TcType
t2

    go ty1 :: TcType
ty1@(CoercionTy Coercion
co1) ty2 :: TcType
ty2@(CoercionTy Coercion
co2)
      = do { kco <- UnifyEnv -> TcType -> TcType -> TcM Coercion
uType (UnifyEnv -> TcType -> TcType -> UnifyEnv
mkKindEnv UnifyEnv
env TcType
ty1 TcType
ty2)
                          (Coercion -> TcType
coercionType Coercion
co1) (Coercion -> TcType
coercionType Coercion
co2)
           ; return $ mkProofIrrelCo role kco co1 co2 }

        -- Anything else fails
        -- E.g. unifying for-all types, which is relative unusual
    go TcType
ty1 TcType
ty2 = TcType -> TcType -> TcM Coercion
defer TcType
ty1 TcType
ty2

    ------------------
    defer :: TcType -> TcType -> TcM Coercion
defer TcType
ty1 TcType
ty2   -- See Note [Check for equality before deferring]
      | TcType
ty1 HasDebugCallStack => TcType -> TcType -> Bool
TcType -> TcType -> Bool
`tcEqType` TcType
ty2 = Coercion -> TcM Coercion
forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
forall (m :: * -> *) a. Monad m => a -> m a
return (Role -> TcType -> Coercion
mkReflCo Role
role TcType
ty1)
      | Bool
otherwise          = UnifyEnv -> TcType -> TcType -> TcM Coercion
uType_defer UnifyEnv
env TcType
orig_ty1 TcType
orig_ty2


    ------------------
    u_tc_arg :: Bool -> Role -> TcType -> TcType -> TcM Coercion
u_tc_arg Bool
is_vis Role
role TcType
ty1 TcType
ty2
      = do { String -> SDoc -> TcM ()
traceTc String
"u_tc_arg" (Role -> SDoc
forall a. Outputable a => a -> SDoc
ppr Role
role SDoc -> SDoc -> SDoc
forall doc. IsDoc doc => doc -> doc -> doc
$$ TcType -> SDoc
forall a. Outputable a => a -> SDoc
ppr TcType
ty1 SDoc -> SDoc -> SDoc
forall doc. IsDoc doc => doc -> doc -> doc
$$ TcType -> SDoc
forall a. Outputable a => a -> SDoc
ppr TcType
ty2)
           ; UnifyEnv -> TcType -> TcType -> TcM Coercion
uType UnifyEnv
env_arg TcType
ty1 TcType
ty2 }
      where
        env_arg :: UnifyEnv
env_arg = UnifyEnv
env { u_loc = adjustCtLoc is_vis False (u_loc env)
                      , u_role = role }

    ------------------
    -- For AppTy, decompose only nominal equalities
    -- See Note [Decomposing AppTy equalities] in GHC.Tc.Solver.Equality
    go_app :: Bool
-> TcType
-> TcType
-> TcType
-> TcType
-> TcType
-> TcType
-> TcM Coercion
go_app Bool
vis TcType
ty1 TcType
s1 TcType
t1 TcType
ty2 TcType
s2 TcType
t2
      | Role
Nominal <- Role
role
      = -- Unify arguments t1/t2 before function s1/s2, because
        -- the former have smaller kinds, and hence simpler error messages
        -- c.f. GHC.Tc.Solver.Equality.can_eq_app
        -- Example: test T8603
        do { let env_arg :: UnifyEnv
env_arg = UnifyEnv
env { u_loc = adjustCtLoc vis False (u_loc env) }
           ; co_t <- UnifyEnv -> TcType -> TcType -> TcM Coercion
uType UnifyEnv
env_arg TcType
t1 TcType
t2
           ; co_s <- uType env s1 s2
           ; return $ mkAppCo co_s co_t }
      | Bool
otherwise
      = TcType -> TcType -> TcM Coercion
defer TcType
ty1 TcType
ty2

{- Note [Check for equality before deferring]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Particularly in ambiguity checks we can get equalities like (ty ~ ty).
If ty involves a type function we may defer, which isn't very sensible.
An egregious example of this was in test T9872a, which has a type signature
       Proxy :: Proxy (Solutions Cubes)
Doing the ambiguity check on this signature generates the equality
   Solutions Cubes ~ Solutions Cubes
and currently the constraint solver normalises both sides at vast cost.
This little short-cut in 'defer' helps quite a bit.

Note [Care with type applications]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Note: type applications need a bit of care!
They can match FunTy and TyConApp, so use splitAppTy_maybe
NB: we've already dealt with type variables and Notes,
so if one type is an App the other one jolly well better be too

Note [Mismatched type lists and application decomposition]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
When we find two TyConApps, you might think that the argument lists
are guaranteed equal length.  But they aren't. Consider matching
        w (T x) ~ Foo (T x y)
We do match (w ~ Foo) first, but in some circumstances we simply create
a deferred constraint; and then go ahead and match (T x ~ T x y).
This came up in #3950.

So either
   (a) either we must check for identical argument kinds
       when decomposing applications,

   (b) or we must be prepared for ill-kinded unification sub-problems

Currently we adopt (b) since it seems more robust -- no need to maintain
a global invariant.

Note [Expanding synonyms during unification]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
We expand synonyms during unification, but:
 * We expand *after* the variable case so that we tend to unify
   variables with un-expanded type synonym. This just makes it
   more likely that the inferred types will mention type synonyms
   understandable to the user

 * Similarly, we expand *after* the CastTy case, just in case the
   CastTy wraps a variable.

 * We expand *before* the TyConApp case.  For example, if we have
      type Phantom a = Int
   and are unifying
      Phantom Int ~ Phantom Char
   it is *wrong* to unify Int and Char.

 * The problem case immediately above can happen only with arguments
   to the tycon. So we check for nullary tycons *before* expanding.
   This is particularly helpful when checking (* ~ *), because * is
   now a type synonym.  See Note [Unifying type synonyms] in GHC.Core.Unify.

Note [Deferred unification]
~~~~~~~~~~~~~~~~~~~~~~~~~~~
We may encounter a unification ty1 ~ ty2 that cannot be performed syntactically,
and yet its consistency is undetermined. Previously, there was no way to still
make it consistent. So a mismatch error was issued.

Now these unifications are deferred until constraint simplification, where type
family instances and given equations may (or may not) establish the consistency.
Deferred unifications are of the form
                F ... ~ ...
or              x ~ ...
where F is a type function and x is a type variable.
E.g.
        id :: x ~ y => x -> y
        id e = e

involves the unification x = y. It is deferred until we bring into account the
context x ~ y to establish that it holds.

If available, we defer original types (rather than those where closed type
synonyms have already been expanded via tcCoreView).  This is, as usual, to
improve error messages.

************************************************************************
*                                                                      *
                 uUnfilledVar and friends
*                                                                      *
************************************************************************

@uunfilledVar@ is called when at least one of the types being unified is a
variable.  It does {\em not} assume that the variable is a fixed point
of the substitution; rather, notice that @uVar@ (defined below) nips
back into @uTys@ if it turns out that the variable is already bound.
-}

----------
uUnfilledVar, uUnfilledVar1
    :: UnifyEnv
    -> SwapFlag
    -> TcTyVar        -- Tyvar 1: not necessarily a meta-tyvar
                      --    definitely not a /filled/ meta-tyvar
    -> TcTauType      -- Type 2
    -> TcM CoercionN
-- "Unfilled" means that the variable is definitely not a filled-in meta tyvar
--            It might be a skolem, or untouchable, or meta
uUnfilledVar :: UnifyEnv -> SwapFlag -> TcTyVar -> TcType -> TcM Coercion
uUnfilledVar UnifyEnv
env SwapFlag
swapped TcTyVar
tv1 TcType
ty2
  | Role
Nominal <- UnifyEnv -> Role
u_role UnifyEnv
env
  = do { ty2 <- ZonkM TcType -> TcM TcType
forall a. ZonkM a -> TcM a
liftZonkM (ZonkM TcType -> TcM TcType) -> ZonkM TcType -> TcM TcType
forall a b. (a -> b) -> a -> b
$ TcType -> ZonkM TcType
zonkTcType TcType
ty2
                  -- Zonk to expose things to the occurs check, and so
                  -- that if ty2 looks like a type variable then it
                  -- /is/ a type variable
       ; uUnfilledVar1 env swapped tv1 ty2 }

  | Bool
otherwise  -- See Note [Do not unify representational equalities]
               -- in GHC.Tc.Solver.Equality
  = SwapFlag
-> (TcType -> TcType -> TcM Coercion)
-> TcType
-> TcType
-> TcM Coercion
forall a b. SwapFlag -> (a -> a -> b) -> a -> a -> b
unSwap SwapFlag
swapped (UnifyEnv -> TcType -> TcType -> TcM Coercion
uType_defer UnifyEnv
env) (TcTyVar -> TcType
mkTyVarTy TcTyVar
tv1) TcType
ty2

uUnfilledVar1 :: UnifyEnv -> SwapFlag -> TcTyVar -> TcType -> TcM Coercion
uUnfilledVar1 UnifyEnv
env       -- Precondition: u_role==Nominal
              SwapFlag
swapped
              TcTyVar
tv1
              TcType
ty2       -- ty2 is zonked
  | Just TcTyVar
tv2 <- TcType -> Maybe TcTyVar
getTyVar_maybe TcType
ty2
  = TcTyVar -> TcM Coercion
go TcTyVar
tv2

  | Bool
otherwise
  = UnifyEnv -> SwapFlag -> TcTyVar -> TcType -> TcM Coercion
uUnfilledVar2 UnifyEnv
env SwapFlag
swapped TcTyVar
tv1 TcType
ty2

  where
    -- 'go' handles the case where both are
    -- tyvars so we might want to swap
    -- E.g. maybe tv2 is a meta-tyvar and tv1 is not
    go :: TcTyVar -> TcM Coercion
go TcTyVar
tv2 | TcTyVar
tv1 TcTyVar -> TcTyVar -> Bool
forall a. Eq a => a -> a -> Bool
== TcTyVar
tv2  -- Same type variable => no-op
           = Coercion -> TcM Coercion
forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
forall (m :: * -> *) a. Monad m => a -> m a
return (TcType -> Coercion
mkNomReflCo (TcTyVar -> TcType
mkTyVarTy TcTyVar
tv1))

           | Bool -> TcTyVar -> TcTyVar -> Bool
swapOverTyVars Bool
False TcTyVar
tv1 TcTyVar
tv2   -- Distinct type variables
               -- Swap meta tyvar to the left if poss
           = do { tv1 <- ZonkM TcTyVar -> TcM TcTyVar
forall a. ZonkM a -> TcM a
liftZonkM (ZonkM TcTyVar -> TcM TcTyVar) -> ZonkM TcTyVar -> TcM TcTyVar
forall a b. (a -> b) -> a -> b
$ TcTyVar -> ZonkM TcTyVar
zonkTyCoVarKind TcTyVar
tv1
                     -- We must zonk tv1's kind because that might
                     -- not have happened yet, and it's an invariant of
                     -- uUnfilledTyVar2 that ty2 is fully zonked
                     -- Omitting this caused #16902
                ; uUnfilledVar2 env (flipSwap swapped) tv2 (mkTyVarTy tv1) }

           | Bool
otherwise
           = UnifyEnv -> SwapFlag -> TcTyVar -> TcType -> TcM Coercion
uUnfilledVar2 UnifyEnv
env SwapFlag
swapped TcTyVar
tv1 TcType
ty2

----------
uUnfilledVar2 :: UnifyEnv       -- Precondition: u_role==Nominal
              -> SwapFlag
              -> TcTyVar        -- Tyvar 1: not necessarily a meta-tyvar
                                --    definitely not a /filled/ meta-tyvar
              -> TcTauType      -- Type 2, zonked
              -> TcM CoercionN
uUnfilledVar2 :: UnifyEnv -> SwapFlag -> TcTyVar -> TcType -> TcM Coercion
uUnfilledVar2 env :: UnifyEnv
env@(UE { u_defer :: UnifyEnv -> TcRef (Bag Ct)
u_defer = TcRef (Bag Ct)
def_eq_ref }) SwapFlag
swapped TcTyVar
tv1 TcType
ty2
  = do { cur_lvl <- TcM TcLevel
getTcLevel
           -- See Note [Unification preconditions], (UNTOUCHABLE) wrinkles
           -- Here we don't know about given equalities here; so we treat
           -- /any/ level outside this one as untouchable.  Hence cur_lvl.
       ; if not (touchabilityAndShapeTest cur_lvl tv1 ty2
                 && simpleUnifyCheck UC_OnTheFly tv1 ty2)
         then not_ok_so_defer cur_lvl
         else
    do { def_eqs <- readTcRef def_eq_ref  -- Capture current state of def_eqs

       -- Attempt to unify kinds
       -- When doing so, be careful to preserve orientation;
       --    see Note [Kind Equality Orientation] in GHC.Tc.Solver.Equality
       --    and wrinkle (W2) in Note [Fundeps with instances, and equality orientation]
       --        in GHC.Tc.Solver.Dict
       -- Failing to preserve orientation led to #25597.
       ; let kind_env = SwapFlag
-> (TcType -> TcType -> UnifyEnv) -> TcType -> TcType -> UnifyEnv
forall a b. SwapFlag -> (a -> a -> b) -> a -> a -> b
unSwap SwapFlag
swapped (UnifyEnv -> TcType -> TcType -> UnifyEnv
mkKindEnv UnifyEnv
env) TcType
ty1 TcType
ty2
       ; co_k <- unSwap swapped (uType kind_env) (tyVarKind tv1) (typeKind ty2)

       ; traceTc "uUnfilledVar2 ok" $
         vcat [ ppr tv1 <+> dcolon <+> ppr (tyVarKind tv1)
              , ppr ty2 <+> dcolon <+> ppr (typeKind  ty2)
              , ppr (isReflCo co_k), ppr co_k ]

       ; if isReflCo co_k
           -- Only proceed if the kinds match
           -- NB: tv1 should still be unfilled, despite the kind unification
           --     because tv1 is not free in ty2' (or, hence, in its kind)
         then do { liftZonkM $ writeMetaTyVar tv1 ty2
                 ; case u_unified env of
                     Maybe (TcRef [TcTyVar])
Nothing -> () -> TcM ()
forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
forall (m :: * -> *) a. Monad m => a -> m a
return ()
                     Just TcRef [TcTyVar]
uref -> TcRef [TcTyVar] -> ([TcTyVar] -> [TcTyVar]) -> TcM ()
forall (m :: * -> *) a. MonadIO m => TcRef a -> (a -> a) -> m ()
updTcRef TcRef [TcTyVar]
uref (TcTyVar
tv1 TcTyVar -> [TcTyVar] -> [TcTyVar]
forall a. a -> [a] -> [a]
:)
                 ; return (mkNomReflCo ty2) }  -- Unification is always Nominal

         else -- The kinds don't match yet, so defer instead.
              do { writeTcRef def_eq_ref def_eqs
                     -- Since we are discarding co_k, also discard any constraints
                     -- emitted by kind unification; they are just useless clutter.
                     -- Do this dicarding by simply restoring the previous state
                     -- of def_eqs; a bit imperative/yukky but works fine.
                 ; defer }
         }}
  where
    ty1 :: TcType
ty1 = TcTyVar -> TcType
mkTyVarTy TcTyVar
tv1
    defer :: TcM Coercion
defer = SwapFlag
-> (TcType -> TcType -> TcM Coercion)
-> TcType
-> TcType
-> TcM Coercion
forall a b. SwapFlag -> (a -> a -> b) -> a -> a -> b
unSwap SwapFlag
swapped (UnifyEnv -> TcType -> TcType -> TcM Coercion
uType_defer UnifyEnv
env) TcType
ty1 TcType
ty2

    not_ok_so_defer :: TcLevel -> TcM Coercion
not_ok_so_defer TcLevel
cur_lvl =
      do { String -> SDoc -> TcM ()
traceTc String
"uUnfilledVar2 not ok" (SDoc -> TcM ()) -> SDoc -> TcM ()
forall a b. (a -> b) -> a -> b
$
             [SDoc] -> SDoc
forall doc. IsDoc doc => [doc] -> doc
vcat [ String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"tv1:" SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> TcTyVar -> SDoc
forall a. Outputable a => a -> SDoc
ppr TcTyVar
tv1
                  , String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"ty2:" SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> TcType -> SDoc
forall a. Outputable a => a -> SDoc
ppr TcType
ty2
                  , String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"simple-unify-chk:" SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> Bool -> SDoc
forall a. Outputable a => a -> SDoc
ppr (UnifyCheckCaller -> TcTyVar -> TcType -> Bool
simpleUnifyCheck UnifyCheckCaller
UC_OnTheFly TcTyVar
tv1 TcType
ty2)
                  , String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"touchability:" SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> Bool -> SDoc
forall a. Outputable a => a -> SDoc
ppr (TcLevel -> TcTyVar -> TcType -> Bool
touchabilityAndShapeTest TcLevel
cur_lvl TcTyVar
tv1 TcType
ty2)]
               -- Occurs check or an untouchable: just defer
               -- NB: occurs check isn't necessarily fatal:
               --     eg tv1 occurred in type family parameter
          ; TcM Coercion
defer }

swapOverTyVars :: Bool -> TcTyVar -> TcTyVar -> Bool
swapOverTyVars :: Bool -> TcTyVar -> TcTyVar -> Bool
swapOverTyVars Bool
is_given TcTyVar
tv1 TcTyVar
tv2
  -- See Note [Unification variables on the left]
  | Bool -> Bool
not Bool
is_given, Int
pri1 Int -> Int -> Bool
forall a. Eq a => a -> a -> Bool
== Int
0, Int
pri2 Int -> Int -> Bool
forall a. Ord a => a -> a -> Bool
> Int
0 = Bool
True
  | Bool -> Bool
not Bool
is_given, Int
pri2 Int -> Int -> Bool
forall a. Eq a => a -> a -> Bool
== Int
0, Int
pri1 Int -> Int -> Bool
forall a. Ord a => a -> a -> Bool
> Int
0 = Bool
False

  -- Level comparison: see Note [TyVar/TyVar orientation]
  | TcLevel
lvl1 TcLevel -> TcLevel -> Bool
`strictlyDeeperThan` TcLevel
lvl2 = Bool
False
  | TcLevel
lvl2 TcLevel -> TcLevel -> Bool
`strictlyDeeperThan` TcLevel
lvl1 = Bool
True

  -- Priority: see Note [TyVar/TyVar orientation]
  | Int
pri1 Int -> Int -> Bool
forall a. Ord a => a -> a -> Bool
> Int
pri2 = Bool
False
  | Int
pri2 Int -> Int -> Bool
forall a. Ord a => a -> a -> Bool
> Int
pri1 = Bool
True

  -- Names: see Note [TyVar/TyVar orientation]
  | Name -> Bool
isSystemName Name
tv2_name, Bool -> Bool
not (Name -> Bool
isSystemName Name
tv1_name) = Bool
True

  | Bool
otherwise = Bool
False

  where
    lvl1 :: TcLevel
lvl1 = TcTyVar -> TcLevel
tcTyVarLevel TcTyVar
tv1
    lvl2 :: TcLevel
lvl2 = TcTyVar -> TcLevel
tcTyVarLevel TcTyVar
tv2
    pri1 :: Int
pri1 = TcTyVar -> Int
lhsPriority TcTyVar
tv1
    pri2 :: Int
pri2 = TcTyVar -> Int
lhsPriority TcTyVar
tv2
    tv1_name :: Name
tv1_name = TcTyVar -> Name
Var.varName TcTyVar
tv1
    tv2_name :: Name
tv2_name = TcTyVar -> Name
Var.varName TcTyVar
tv2


lhsPriority :: TcTyVar -> Int
-- Higher => more important to be on the LHS
--        => more likely to be eliminated
-- Only used when the levels are identical
-- See Note [TyVar/TyVar orientation]
lhsPriority :: TcTyVar -> Int
lhsPriority TcTyVar
tv
  = Bool -> SDoc -> Int -> Int
forall a. HasCallStack => Bool -> SDoc -> a -> a
assertPpr (TcTyVar -> Bool
isTyVar TcTyVar
tv) (TcTyVar -> SDoc
forall a. Outputable a => a -> SDoc
ppr TcTyVar
tv) (Int -> Int) -> Int -> Int
forall a b. (a -> b) -> a -> b
$
    case TcTyVar -> TcTyVarDetails
tcTyVarDetails TcTyVar
tv of
      TcTyVarDetails
RuntimeUnk  -> Int
0
      SkolemTv {} -> Int
0
      MetaTv { mtv_info :: TcTyVarDetails -> MetaInfo
mtv_info = MetaInfo
info, mtv_tclvl :: TcTyVarDetails -> TcLevel
mtv_tclvl = TcLevel
lvl }
        | TcLevel
QLInstVar <- TcLevel
lvl
        -> Int
5  -- Eliminate instantiation variables first
        | Bool
otherwise
        -> case MetaInfo
info of
             MetaInfo
CycleBreakerTv -> Int
0
             MetaInfo
TyVarTv        -> Int
1
             ConcreteTv {}  -> Int
2
             MetaInfo
TauTv          -> Int
3
             MetaInfo
RuntimeUnkTv   -> Int
4

{- Note [Unification preconditions]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Question: given a homogeneous equality (alpha ~# ty), when is it OK to
unify alpha := ty?

This note only applied to /homogeneous/ equalities, in which both
sides have the same kind.

There are five reasons not to unify:

1. (SKOL-ESC) Skolem-escape
   Consider the constraint
        forall[2] a[2]. alpha[1] ~ Maybe a[2]
   If we unify alpha := Maybe a, the skolem 'a' may escape its scope.
   The level alpha[1] says that alpha may be used outside this constraint,
   where 'a' is not in scope at all.  So we must not unify.

   Bottom line: when looking at a constraint alpha[n] := ty, do not unify
   if any free variable of 'ty' has level deeper (greater) than n

2. (UNTOUCHABLE) Untouchable unification variables
   Consider the constraint
        forall[2] a[2]. b[1] ~ Int => alpha[1] ~ Int
   There is no (SKOL-ESC) problem with unifying alpha := Int, but it might
   not be the principal solution. Perhaps the "right" solution is alpha := b.
   We simply can't tell.  See "OutsideIn(X): modular type inference with local
   assumptions", section 2.2.  We say that alpha[1] is "untouchable" inside
   this implication.

   Bottom line: at ambient level 'l', when looking at a constraint
   alpha[n] ~ ty, do not unify alpha := ty if there are any given equalities
   between levels 'n' and 'l'.

   Exactly what is a "given equality" for the purpose of (UNTOUCHABLE)?
   Answer: see Note [Tracking Given equalities] in GHC.Tc.Solver.InertSet

3. (TYVAR-TV) Unifying TyVarTvs and CycleBreakerTvs
   This precondition looks at the MetaInfo of the unification variable:

   * TyVarTv: When considering alpha{tyv} ~ ty, if alpha{tyv} is a
     TyVarTv it can only unify with a type variable, not with a
     structured type.  So if 'ty' is a structured type, such as (Maybe x),
     don't unify.

   * CycleBreakerTv: never unified, except by restoreTyVarCycles.

4. (CONCRETE) A ConcreteTv can only unify with a concrete type,
    by definition.

    That is, if we have `rr[conc] ~ F Int`, we can't unify
    `rr` with `F Int`, so we hold off on unifying.
    Note however that the equality might get rewritten; for instance
    if we can rewrite `F Int` to a concrete type, say `FloatRep`,
    then we will have `rr[conc] ~ FloatRep` and we can unify `rr ~ FloatRep`.

    Note that we can still make progress on unification even if
    we can't fully solve an equality, e.g.

      alpha[conc] ~# TupleRep '[ beta[tau], F gamma[tau] ]

    we can fill beta[tau] := beta[conc]. This is why we call
    'makeTypeConcrete' in startSolvingByUnification.

5. (COERCION-HOLE) Confusing coercion holes
   Suppose our equality is
     (alpha :: k) ~ (Int |> {co})
   where co :: Type ~ k is an unsolved wanted. Note that this equality
   is homogeneous; both sides have kind k. We refrain from unifying here, because
   of the coercion hole in the RHS -- see Wrinkle (EIK2) in
   Note [Equalities with incompatible kinds] in GHC.Solver.Equality.

Needless to say, all there are wrinkles:

* (SKOL-ESC) Promotion.  Given alpha[n] ~ ty, what if beta[k] is free
  in 'ty', where beta is a unification variable, and k>n?  'beta'
  stands for a monotype, and since it is part of a level-n type
  (equal to alpha[n]), we must /promote/ beta to level n.  Just make
  up a fresh gamma[n], and unify beta[k] := gamma[n].

* (TYVAR-TV) Unification variables.  Suppose alpha[tyv,n] is a level-n
  TyVarTv (see Note [TyVarTv] in GHC.Tc.Types.TcMType)? Now
  consider alpha[tyv,n] ~ Bool.  We don't want to unify because that
  would break the TyVarTv invariant.

  What about alpha[tyv,n] ~ beta[tau,n], where beta is an ordinary
  TauTv?  Again, don't unify, because beta might later be unified
  with, say Bool.  (If levels permit, we reverse the orientation here;
  see Note [TyVar/TyVar orientation].)

* (UNTOUCHABLE) Untouchability.  When considering (alpha[n] ~ ty), how
  do we know whether there are any given equalities between level n
  and the ambient level?  We answer in two ways:

  * In the eager unifier, we only unify if l=n.  If not, alpha may be
    untouchable, and defer to the constraint solver.  This check is
    made in GHC.Tc.Utils.uUnifilledVar2, in the guard
    isTouchableMetaTyVar.

  * In the constraint solver, we track where Given equalities occur
    and use that to guard unification in
    GHC.Tc.Utils.Unify.touchabilityAndShapeTest. More details in
    Note [Tracking Given equalities] in GHC.Tc.Solver.InertSet

    Historical note: in the olden days (pre 2021) the constraint solver
    also used to unify only if l=n.  Equalities were "floated" out of the
    implication in a separate step, so that they would become touchable.
    But the float/don't-float question turned out to be very delicate,
    as you can see if you look at the long series of Notes associated with
    GHC.Tc.Solver.floatEqualities, around Nov 2020.  It's much easier
    to unify in-place, with no floating.

Note [TyVar/TyVar orientation]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
See also Note [Fundeps with instances, and equality orientation]
where the kind equality orientation is important

Given (a ~ b), should we orient the equality as (a~b) or (b~a)?
This is a surprisingly tricky question!

The question is answered by swapOverTyVars, which is used
  - in the eager unifier, in GHC.Tc.Utils.Unify.uUnfilledVar1
  - in the constraint solver, in GHC.Tc.Solver.Equality.canEqCanLHS2

First note: only swap if you have to!
   See Note [Avoid unnecessary swaps]

So we look for a positive reason to swap, using a three-step test:

* Level comparison. If 'a' has deeper level than 'b',
  put 'a' on the left.  See Note [Deeper level on the left]

* Priority.  If the levels are the same, look at what kind of
  type variable it is, using 'lhsPriority'.

  Generally speaking we always try to put a MetaTv on the left in
  preference to SkolemTv or RuntimeUnkTv, because the MetaTv may be
  touchable and can be unified.

  Tie-breaking rules for MetaTvs:
  - CycleBreakerTv: This is essentially a stand-in for another type;
       it's untouchable and should have the same priority as a skolem: 0.

  - TyVarTv: These can unify only with another tyvar, but we can't unify
       a TyVarTv with a TauTv, because then the TyVarTv could (transitively)
       get a non-tyvar type. So give these a low priority: 1.

  - ConcreteTv: These are like TauTv, except they can only unify with
    a concrete type. So we want to be able to write to them, but not quite
    as much as TauTvs: 2.

  - TauTv: This is the common case; we want these on the left so that they
       can be written to: 3.

  - RuntimeUnkTv: These aren't really meta-variables used in type inference,
       but just a convenience in the implementation of the GHCi debugger.
       Eagerly write to these: 4. See Note [RuntimeUnkTv] in
       GHC.Runtime.Heap.Inspect.

* Names. If the level and priority comparisons are all
  equal, try to eliminate a TyVar with a System Name in
  favour of ones with a Name derived from a user type signature

* Age.  At one point in the past we tried to break any remaining
  ties by eliminating the younger type variable, based on their
  Uniques.  See Note [Eliminate younger unification variables]
  (which also explains why we don't do this any more)

Note [Unification variables on the left]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
For wanteds, but not givens, swap (skolem ~ meta-tv) regardless of
level, so that the unification variable is on the left.

* We /don't/ want this for Givens because if we ave
    [G] a[2] ~ alpha[1]
    [W] Bool ~ a[2]
  we want to rewrite the wanted to Bool ~ alpha[1],
  so we can float the constraint and solve it.

* But for Wanteds putting the unification variable on
  the left means an easier job when floating, and when
  reporting errors -- just fewer cases to consider.

  In particular, we get better skolem-escape messages:
  see #18114

Note [Deeper level on the left]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The most important thing is that we want to put tyvars with
the deepest level on the left.  The reason to do so differs for
Wanteds and Givens, but either way, deepest wins!  Simple.

* Wanteds.  Putting the deepest variable on the left maximise the
  chances that it's a touchable meta-tyvar which can be solved.

* Givens. Suppose we have something like
     forall a[2]. b[1] ~ a[2] => beta[1] ~ a[2]

  If we orient the Given a[2] on the left, we'll rewrite the Wanted to
  (beta[1] ~ b[1]), and that can float out of the implication.
  Otherwise it can't.  By putting the deepest variable on the left
  we maximise our changes of eliminating skolem capture.

  See also GHC.Tc.Solver.InertSet Note [Let-bound skolems] for another reason
  to orient with the deepest skolem on the left.

  IMPORTANT NOTE: this test does a level-number comparison on
  skolems, so it's important that skolems have (accurate) level
  numbers.

See #15009 for an further analysis of why "deepest on the left"
is a good plan.

Note [Avoid unnecessary swaps]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
If we swap without actually improving matters, we can get an infinite loop.
Consider
    work item:  a ~ b
   inert item:  b ~ c
We canonicalise the work-item to (a ~ c).  If we then swap it before
adding to the inert set, we'll add (c ~ a), and therefore kick out the
inert guy, so we get
   new work item:  b ~ c
   inert item:     c ~ a
And now the cycle just repeats

Historical Note [Eliminate younger unification variables]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Given a choice of unifying
     alpha := beta   or   beta := alpha
we try, if possible, to eliminate the "younger" one, as determined
by `ltUnique`.  Reason: the younger one is less likely to appear free in
an existing inert constraint, and hence we are less likely to be forced
into kicking out and rewriting inert constraints.

This is a performance optimisation only.  It turns out to fix
#14723 all by itself, but clearly not reliably so!

It's simple to implement (see nicer_to_update_tv2 in swapOverTyVars).
But, to my surprise, it didn't seem to make any significant difference
to the compiler's performance, so I didn't take it any further.  Still
it seemed too nice to discard altogether, so I'm leaving these
notes.  SLPJ Jan 18.

Note [Prevent unification with type families]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
We prevent unification with type families because of an uneasy compromise.
It's perfectly sound to unify with type families, and it even improves the
error messages in the testsuite. It also modestly improves performance, at
least in some cases. But it's disastrous for test case perf/compiler/T3064.
Here is the problem: Suppose we have (F ty) where we also have [G] F ty ~ a.
What do we do? Do we reduce F? Or do we use the given? Hard to know what's
best. GHC reduces. This is a disaster for T3064, where the type's size
spirals out of control during reduction. If we prevent
unification with type families, then the solver happens to use the equality
before expanding the type family.

It would be lovely in the future to revisit this problem and remove this
extra, unnecessary check. But we retain it for now as it seems to work
better in practice.

Revisited in Nov '20, along with removing flattening variables. Problem
is still present, and the solution is still the same.

Note [Non-TcTyVars in GHC.Tc.Utils.Unify]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Because the same code is now shared between unifying types and unifying
kinds, we sometimes will see proper TyVars floating around the unifier.
Example (from test case polykinds/PolyKinds12):

    type family Apply (f :: k1 -> k2) (x :: k1) :: k2
    type instance Apply g y = g y

When checking the instance declaration, we first *kind-check* the LHS
and RHS, discovering that the instance really should be

    type instance Apply k3 k4 (g :: k3 -> k4) (y :: k3) = g y

During this kind-checking, all the tyvars will be TcTyVars. Then, however,
as a second pass, we desugar the RHS (which is done in functions prefixed
with "tc" in GHC.Tc.TyCl"). By this time, all the kind-vars are proper
TyVars, not TcTyVars, get some kind unification must happen.

Thus, we always check if a TyVar is a TcTyVar before asking if it's a
meta-tyvar.

This used to not be necessary for type-checking (that is, before * :: *)
because expressions get desugared via an algorithm separate from
type-checking (with wrappers, etc.). Types get desugared very differently,
causing this wibble in behavior seen here.
-}

-- | Breaks apart a function kind into its pieces.
matchExpectedFunKind
  :: TypedThing     -- ^ type, only for errors
  -> Arity           -- ^ n: number of desired arrows
  -> TcKind          -- ^ fun_kind
  -> TcM Coercion    -- ^ co :: fun_kind ~ (arg1 -> ... -> argn -> res)

matchExpectedFunKind :: TypedThing -> Int -> TcType -> TcM Coercion
matchExpectedFunKind TypedThing
hs_ty Int
n TcType
k = Int -> TcType -> TcM Coercion
go Int
n TcType
k
  where
    go :: Int -> TcType -> TcM Coercion
go Int
0 TcType
k = Coercion -> TcM Coercion
forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
forall (m :: * -> *) a. Monad m => a -> m a
return (TcType -> Coercion
mkNomReflCo TcType
k)

    go Int
n TcType
k | Just TcType
k' <- TcType -> Maybe TcType
coreView TcType
k = Int -> TcType -> TcM Coercion
go Int
n TcType
k'

    go Int
n k :: TcType
k@(TyVarTy TcTyVar
kvar)
      | TcTyVar -> Bool
isMetaTyVar TcTyVar
kvar
      = do { maybe_kind <- TcTyVar -> IOEnv (Env TcGblEnv TcLclEnv) MetaDetails
forall (m :: * -> *). MonadIO m => TcTyVar -> m MetaDetails
readMetaTyVar TcTyVar
kvar
           ; case maybe_kind of
                Indirect TcType
fun_kind -> Int -> TcType -> TcM Coercion
go Int
n TcType
fun_kind
                MetaDetails
Flexi ->             Int -> TcType -> TcM Coercion
defer Int
n TcType
k }

    go Int
n (FunTy { ft_af :: TcType -> FunTyFlag
ft_af = FunTyFlag
af, ft_mult :: TcType -> TcType
ft_mult = TcType
w, ft_arg :: TcType -> TcType
ft_arg = TcType
arg, ft_res :: TcType -> TcType
ft_res = TcType
res })
      | FunTyFlag -> Bool
isVisibleFunArg FunTyFlag
af
      = do { co <- Int -> TcType -> TcM Coercion
go (Int
nInt -> Int -> Int
forall a. Num a => a -> a -> a
-Int
1) TcType
res
           ; return (mkNakedFunCo Nominal af (mkNomReflCo w) (mkNomReflCo arg) co) }

    go Int
n TcType
other
     = Int -> TcType -> TcM Coercion
defer Int
n TcType
other

    defer :: Int -> TcType -> TcM Coercion
defer Int
n TcType
k
      = do { arg_kinds <- Int -> TcM [TcType]
newMetaKindVars Int
n
           ; res_kind  <- newMetaKindVar
           ; let new_fun = [TcType] -> TcType -> TcType
mkVisFunTysMany [TcType]
arg_kinds TcType
res_kind
                 origin  = TypeEqOrigin { uo_actual :: TcType
uo_actual   = TcType
k
                                        , uo_expected :: TcType
uo_expected = TcType
new_fun
                                        , uo_thing :: Maybe TypedThing
uo_thing    = TypedThing -> Maybe TypedThing
forall a. a -> Maybe a
Just TypedThing
hs_ty
                                        , uo_visible :: Bool
uo_visible  = Bool
True
                                        }
           ; unifyTypeAndEmit KindLevel origin k new_fun }

{- *********************************************************************
*                                                                      *
                 Checking alpha ~ ty
              for the on-the-fly unifier
*                                                                      *
********************************************************************* -}

data UnifyCheckCaller
  = UC_OnTheFly   -- Called from the on-the-fly unifier
  | UC_QuickLook  -- Called from Quick Look
  | UC_Solver     -- Called from constraint solver
  | UC_Defaulting -- Called when doing top-level defaulting

simpleUnifyCheck :: UnifyCheckCaller -> TcTyVar -> TcType -> Bool
-- simpleUnifyCheck does a fast check: True <=> unification is OK
-- If it says 'False' then unification might still be OK, but
-- it'll take more work to do -- use the full checkTypeEq
--
-- * Rejects if lhs_tv occurs in rhs_ty (occurs check)
-- * Rejects foralls unless
--      lhs_tv is RuntimeUnk (used by GHCi debugger)
--          or is a QL instantiation variable
-- * Rejects a non-concrete type if lhs_tv is concrete
-- * Rejects type families unless fam_ok=True
-- * Does a level-check for type variables, to avoid skolem escape
--
-- This function is pretty heavily used, so it's optimised not to allocate
simpleUnifyCheck :: UnifyCheckCaller -> TcTyVar -> TcType -> Bool
simpleUnifyCheck UnifyCheckCaller
caller TcTyVar
lhs_tv TcType
rhs
  = TcType -> Bool
go TcType
rhs
  where

    !(TcType -> Bool
occ_in_ty, Coercion -> Bool
occ_in_co) = Name -> (TcType -> Bool, Coercion -> Bool)
mkOccFolders (TcTyVar -> Name
tyVarName TcTyVar
lhs_tv)

    lhs_tv_lvl :: TcLevel
lhs_tv_lvl         = TcTyVar -> TcLevel
tcTyVarLevel TcTyVar
lhs_tv
    lhs_tv_is_concrete :: Bool
lhs_tv_is_concrete = TcTyVar -> Bool
isConcreteTyVar TcTyVar
lhs_tv

    forall_ok :: Bool
forall_ok = case UnifyCheckCaller
caller of
                   UnifyCheckCaller
UC_QuickLook -> TcTyVar -> Bool
isQLInstTyVar TcTyVar
lhs_tv
                   UnifyCheckCaller
_            -> TcTyVar -> Bool
isRuntimeUnkTyVar TcTyVar
lhs_tv

    -- This fam_ok thing relates to a very specific perf problem
    -- See Note [Prevent unification with type families]
    -- A couple of QuickLook regression tests rely on unifying with type
    --   families, so we let it through there (not very principled, but let's
    --   see if it bites us)
    fam_ok :: Bool
fam_ok = case UnifyCheckCaller
caller of
               UnifyCheckCaller
UC_Solver     -> Bool
True
               UnifyCheckCaller
UC_QuickLook  -> Bool
True
               UnifyCheckCaller
UC_OnTheFly   -> Bool
False
               UnifyCheckCaller
UC_Defaulting -> Bool
True

    go :: TcType -> Bool
go (TyVarTy TcTyVar
tv)
      | TcTyVar
lhs_tv TcTyVar -> TcTyVar -> Bool
forall a. Eq a => a -> a -> Bool
== TcTyVar
tv                                    = Bool
False
      | TcTyVar -> TcLevel
tcTyVarLevel TcTyVar
tv TcLevel -> TcLevel -> Bool
`strictlyDeeperThan` TcLevel
lhs_tv_lvl = Bool
False
      | Bool
lhs_tv_is_concrete, Bool -> Bool
not (TcTyVar -> Bool
isConcreteTyVar TcTyVar
tv)    = Bool
False
      | TcType -> Bool
occ_in_ty (TcType -> Bool) -> TcType -> Bool
forall a b. (a -> b) -> a -> b
$! (TcTyVar -> TcType
tyVarKind TcTyVar
tv)                     = Bool
False
      | Bool
otherwise                                       = Bool
True

    go (FunTy {ft_af :: TcType -> FunTyFlag
ft_af = FunTyFlag
af, ft_mult :: TcType -> TcType
ft_mult = TcType
w, ft_arg :: TcType -> TcType
ft_arg = TcType
a, ft_res :: TcType -> TcType
ft_res = TcType
r})
      | Bool -> Bool
not Bool
forall_ok, FunTyFlag -> Bool
isInvisibleFunArg FunTyFlag
af = Bool
False
      | Bool
otherwise                           = TcType -> Bool
go TcType
w Bool -> Bool -> Bool
&& TcType -> Bool
go TcType
a Bool -> Bool -> Bool
&& TcType -> Bool
go TcType
r

    go (TyConApp TyCon
tc [TcType]
tys)
      | Bool
lhs_tv_is_concrete, Bool -> Bool
not (TyCon -> Bool
isConcreteTyCon TyCon
tc) = Bool
False
      | Bool -> Bool
not Bool
forall_ok, Bool -> Bool
not (TyCon -> Bool
isTauTyCon TyCon
tc)           = Bool
False
      | Bool -> Bool
not Bool
fam_ok,    Bool -> Bool
not (TyCon -> Bool
isFamFreeTyCon TyCon
tc)       = Bool
False
      | Bool
otherwise                                    = (TcType -> Bool) -> [TcType] -> Bool
forall (t :: * -> *) a. Foldable t => (a -> Bool) -> t a -> Bool
all TcType -> Bool
go [TcType]
tys

    go (ForAllTy (Bndr TcTyVar
tv ForAllTyFlag
_) TcType
ty)
      | Bool
forall_ok = TcType -> Bool
go (TcTyVar -> TcType
tyVarKind TcTyVar
tv) Bool -> Bool -> Bool
&& (TcTyVar
tv TcTyVar -> TcTyVar -> Bool
forall a. Eq a => a -> a -> Bool
== TcTyVar
lhs_tv Bool -> Bool -> Bool
|| TcType -> Bool
go TcType
ty)
      | Bool
otherwise = Bool
False

    go (AppTy TcType
t1 TcType
t2)    = TcType -> Bool
go TcType
t1 Bool -> Bool -> Bool
&& TcType -> Bool
go TcType
t2
    go (CastTy TcType
ty Coercion
co)   = Bool -> Bool
not (Coercion -> Bool
occ_in_co Coercion
co) Bool -> Bool -> Bool
&& TcType -> Bool
go TcType
ty
    go (CoercionTy Coercion
co)  = Bool -> Bool
not (Coercion -> Bool
occ_in_co Coercion
co)
    go (LitTy {})       = Bool
True


mkOccFolders :: Name -> (TcType -> Bool, TcCoercion -> Bool)
-- These functions return True
--   * if lhs_tv occurs (incl deeply, in the kind of variable)
--   * if there is a coercion hole
-- No expansion of type synonyms
mkOccFolders :: Name -> (TcType -> Bool, Coercion -> Bool)
mkOccFolders Name
lhs_tv = (Any -> Bool
getAny (Any -> Bool) -> (TcType -> Any) -> TcType -> Bool
forall b c a. (b -> c) -> (a -> b) -> a -> c
. TcType -> Any
check_ty, Any -> Bool
getAny (Any -> Bool) -> (Coercion -> Any) -> Coercion -> Bool
forall b c a. (b -> c) -> (a -> b) -> a -> c
. Coercion -> Any
check_co)
  where
    !(TcType -> Any
check_ty, [TcType] -> Any
_, Coercion -> Any
check_co, [Coercion] -> Any
_) = TyCoFolder VarSet Any
-> VarSet
-> (TcType -> Any, [TcType] -> Any, Coercion -> Any,
    [Coercion] -> Any)
forall a env.
Monoid a =>
TyCoFolder env a
-> env
-> (TcType -> a, [TcType] -> a, Coercion -> a, [Coercion] -> a)
foldTyCo TyCoFolder VarSet Any
occ_folder VarSet
emptyVarSet
    occ_folder :: TyCoFolder VarSet Any
occ_folder = TyCoFolder { tcf_view :: TcType -> Maybe TcType
tcf_view  = TcType -> Maybe TcType
noView  -- Don't expand synonyms
                            , tcf_tyvar :: VarSet -> TcTyVar -> Any
tcf_tyvar = VarSet -> TcTyVar -> Any
do_tcv, tcf_covar :: VarSet -> TcTyVar -> Any
tcf_covar = VarSet -> TcTyVar -> Any
do_tcv
                            , tcf_hole :: VarSet -> CoercionHole -> Any
tcf_hole  = VarSet -> CoercionHole -> Any
forall {p} {p}. p -> p -> Any
do_hole
                            , tcf_tycobinder :: VarSet -> TcTyVar -> ForAllTyFlag -> VarSet
tcf_tycobinder = VarSet -> TcTyVar -> ForAllTyFlag -> VarSet
forall {p}. VarSet -> TcTyVar -> p -> VarSet
do_bndr }

    do_tcv :: VarSet -> TcTyVar -> Any
do_tcv VarSet
is TcTyVar
v = Bool -> Any
Any (Bool -> Bool
not (TcTyVar
v TcTyVar -> VarSet -> Bool
`elemVarSet` VarSet
is) Bool -> Bool -> Bool
&& TcTyVar -> Name
tyVarName TcTyVar
v Name -> Name -> Bool
forall a. Eq a => a -> a -> Bool
== Name
lhs_tv)
                  Any -> Any -> Any
forall a. Monoid a => a -> a -> a
`mappend` TcType -> Any
check_ty (TcTyVar -> TcType
varType TcTyVar
v)

    do_bndr :: VarSet -> TcTyVar -> p -> VarSet
do_bndr VarSet
is TcTyVar
tcv p
_faf = VarSet -> TcTyVar -> VarSet
extendVarSet VarSet
is TcTyVar
tcv
    do_hole :: p -> p -> Any
do_hole p
_is p
_hole = Bool -> Any
DM.Any Bool
True  -- Reject coercion holes

{- *********************************************************************
*                                                                      *
                 Equality invariant checking
*                                                                      *
********************************************************************* -}


{-  Note [Checking for foralls]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
We never want to unify
    alpha ~ (forall a. a->a) -> Int
So we look for foralls hidden inside the type, and it's convenient
to do that at the same time as the occurs check (which looks for
occurrences of alpha).

However, it's not just a question of looking for foralls /anywhere/!
Consider
   (alpha :: forall k. k->*)  ~  (beta :: forall k. k->*)
This is legal; e.g. dependent/should_compile/T11635.

We don't want to reject it because of the forall in beta's kind, but
(see Note [Occurrence checking: look inside kinds] in GHC.Core.Type)
we do need to look in beta's kind.  So we carry a flag saying if a
'forall' is OK, and switch the flag on when stepping inside a kind.

Why is it OK?  Why does it not count as impredicative polymorphism?
The reason foralls are bad is because we reply on "seeing" foralls
when doing implicit instantiation.  But the forall inside the kind is
fine.  We'll generate a kind equality constraint
  (forall k. k->*) ~ (forall k. k->*)
to check that the kinds of lhs and rhs are compatible.  If alpha's
kind had instead been
  (alpha :: kappa)
then this kind equality would rightly complain about unifying kappa
with (forall k. k->*)

Note [Forgetful synonyms in checkTyConApp]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider
   type S a b = b   -- Forgets 'a'

   [W] alpha[2] ~ Maybe (S beta[4] gamma[2])

We don't want to promote beta to level 2; rather, we should
expand the synonym. (Currently, in checkTypeEqRhs promotion
is irrevocable, by side effect.)

To avoid this risk we eagerly expand forgetful synonyms.
This also means we won't get an occurs check in
   a ~ Maybe (S a b)

The annoyance is that we might expand the synonym unnecessarily,
something we generally try to avoid.  But for now, this seems
simple.

In a forgetful case like a ~ Maybe (S a b), `checkTyEqRhs` returns
a Reduction that looks
    Reduction { reductionCoercion    = Refl
              , reductionReducedType = Maybe b }
We must jolly well use that reductionReduced type, even though the
reductionCoercion is Refl.  See `canEqCanLHSFinish_no_unification`.
-}

data PuResult a b = PuFail CheckTyEqResult
                  | PuOK (Bag a) b
  deriving stock ((forall a b. (a -> b) -> PuResult a a -> PuResult a b)
-> (forall a b. a -> PuResult a b -> PuResult a a)
-> Functor (PuResult a)
forall a b. a -> PuResult a b -> PuResult a a
forall a b. (a -> b) -> PuResult a a -> PuResult a b
forall a a b. a -> PuResult a b -> PuResult a a
forall a a b. (a -> b) -> PuResult a a -> PuResult a b
forall (f :: * -> *).
(forall a b. (a -> b) -> f a -> f b)
-> (forall a b. a -> f b -> f a) -> Functor f
$cfmap :: forall a a b. (a -> b) -> PuResult a a -> PuResult a b
fmap :: forall a b. (a -> b) -> PuResult a a -> PuResult a b
$c<$ :: forall a a b. a -> PuResult a b -> PuResult a a
<$ :: forall a b. a -> PuResult a b -> PuResult a a
Functor, (forall m. Monoid m => PuResult a m -> m)
-> (forall m a. Monoid m => (a -> m) -> PuResult a a -> m)
-> (forall m a. Monoid m => (a -> m) -> PuResult a a -> m)
-> (forall a b. (a -> b -> b) -> b -> PuResult a a -> b)
-> (forall a b. (a -> b -> b) -> b -> PuResult a a -> b)
-> (forall b a. (b -> a -> b) -> b -> PuResult a a -> b)
-> (forall b a. (b -> a -> b) -> b -> PuResult a a -> b)
-> (forall a. (a -> a -> a) -> PuResult a a -> a)
-> (forall a. (a -> a -> a) -> PuResult a a -> a)
-> (forall a. PuResult a a -> [a])
-> (forall a. PuResult a a -> Bool)
-> (forall a. PuResult a a -> Int)
-> (forall a. Eq a => a -> PuResult a a -> Bool)
-> (forall a. Ord a => PuResult a a -> a)
-> (forall a. Ord a => PuResult a a -> a)
-> (forall a. Num a => PuResult a a -> a)
-> (forall a. Num a => PuResult a a -> a)
-> Foldable (PuResult a)
forall a. Eq a => a -> PuResult a a -> Bool
forall a. Num a => PuResult a a -> a
forall a. Ord a => PuResult a a -> a
forall m. Monoid m => PuResult a m -> m
forall a. PuResult a a -> Bool
forall a. PuResult a a -> Int
forall a. PuResult a a -> [a]
forall a. (a -> a -> a) -> PuResult a a -> a
forall a a. Eq a => a -> PuResult a a -> Bool
forall a a. Num a => PuResult a a -> a
forall a a. Ord a => PuResult a a -> a
forall a m. Monoid m => PuResult a m -> m
forall m a. Monoid m => (a -> m) -> PuResult a a -> m
forall a a. PuResult a a -> Bool
forall a a. PuResult a a -> Int
forall a a. PuResult a a -> [a]
forall b a. (b -> a -> b) -> b -> PuResult a a -> b
forall a b. (a -> b -> b) -> b -> PuResult a a -> b
forall a a. (a -> a -> a) -> PuResult a a -> a
forall a m a. Monoid m => (a -> m) -> PuResult a a -> m
forall a b a. (b -> a -> b) -> b -> PuResult a a -> b
forall a a b. (a -> b -> b) -> b -> PuResult a a -> b
forall (t :: * -> *).
(forall m. Monoid m => t m -> m)
-> (forall m a. Monoid m => (a -> m) -> t a -> m)
-> (forall m a. Monoid m => (a -> m) -> t a -> m)
-> (forall a b. (a -> b -> b) -> b -> t a -> b)
-> (forall a b. (a -> b -> b) -> b -> t a -> b)
-> (forall b a. (b -> a -> b) -> b -> t a -> b)
-> (forall b a. (b -> a -> b) -> b -> t a -> b)
-> (forall a. (a -> a -> a) -> t a -> a)
-> (forall a. (a -> a -> a) -> t a -> a)
-> (forall a. t a -> [a])
-> (forall a. t a -> Bool)
-> (forall a. t a -> Int)
-> (forall a. Eq a => a -> t a -> Bool)
-> (forall a. Ord a => t a -> a)
-> (forall a. Ord a => t a -> a)
-> (forall a. Num a => t a -> a)
-> (forall a. Num a => t a -> a)
-> Foldable t
$cfold :: forall a m. Monoid m => PuResult a m -> m
fold :: forall m. Monoid m => PuResult a m -> m
$cfoldMap :: forall a m a. Monoid m => (a -> m) -> PuResult a a -> m
foldMap :: forall m a. Monoid m => (a -> m) -> PuResult a a -> m
$cfoldMap' :: forall a m a. Monoid m => (a -> m) -> PuResult a a -> m
foldMap' :: forall m a. Monoid m => (a -> m) -> PuResult a a -> m
$cfoldr :: forall a a b. (a -> b -> b) -> b -> PuResult a a -> b
foldr :: forall a b. (a -> b -> b) -> b -> PuResult a a -> b
$cfoldr' :: forall a a b. (a -> b -> b) -> b -> PuResult a a -> b
foldr' :: forall a b. (a -> b -> b) -> b -> PuResult a a -> b
$cfoldl :: forall a b a. (b -> a -> b) -> b -> PuResult a a -> b
foldl :: forall b a. (b -> a -> b) -> b -> PuResult a a -> b
$cfoldl' :: forall a b a. (b -> a -> b) -> b -> PuResult a a -> b
foldl' :: forall b a. (b -> a -> b) -> b -> PuResult a a -> b
$cfoldr1 :: forall a a. (a -> a -> a) -> PuResult a a -> a
foldr1 :: forall a. (a -> a -> a) -> PuResult a a -> a
$cfoldl1 :: forall a a. (a -> a -> a) -> PuResult a a -> a
foldl1 :: forall a. (a -> a -> a) -> PuResult a a -> a
$ctoList :: forall a a. PuResult a a -> [a]
toList :: forall a. PuResult a a -> [a]
$cnull :: forall a a. PuResult a a -> Bool
null :: forall a. PuResult a a -> Bool
$clength :: forall a a. PuResult a a -> Int
length :: forall a. PuResult a a -> Int
$celem :: forall a a. Eq a => a -> PuResult a a -> Bool
elem :: forall a. Eq a => a -> PuResult a a -> Bool
$cmaximum :: forall a a. Ord a => PuResult a a -> a
maximum :: forall a. Ord a => PuResult a a -> a
$cminimum :: forall a a. Ord a => PuResult a a -> a
minimum :: forall a. Ord a => PuResult a a -> a
$csum :: forall a a. Num a => PuResult a a -> a
sum :: forall a. Num a => PuResult a a -> a
$cproduct :: forall a a. Num a => PuResult a a -> a
product :: forall a. Num a => PuResult a a -> a
Foldable, Functor (PuResult a)
Foldable (PuResult a)
(Functor (PuResult a), Foldable (PuResult a)) =>
(forall (f :: * -> *) a b.
 Applicative f =>
 (a -> f b) -> PuResult a a -> f (PuResult a b))
-> (forall (f :: * -> *) a.
    Applicative f =>
    PuResult a (f a) -> f (PuResult a a))
-> (forall (m :: * -> *) a b.
    Monad m =>
    (a -> m b) -> PuResult a a -> m (PuResult a b))
-> (forall (m :: * -> *) a.
    Monad m =>
    PuResult a (m a) -> m (PuResult a a))
-> Traversable (PuResult a)
forall a. Functor (PuResult a)
forall a. Foldable (PuResult a)
forall a (m :: * -> *) a.
Monad m =>
PuResult a (m a) -> m (PuResult a a)
forall a (f :: * -> *) a.
Applicative f =>
PuResult a (f a) -> f (PuResult a a)
forall a (m :: * -> *) a b.
Monad m =>
(a -> m b) -> PuResult a a -> m (PuResult a b)
forall a (f :: * -> *) a b.
Applicative f =>
(a -> f b) -> PuResult a a -> f (PuResult a b)
forall (t :: * -> *).
(Functor t, Foldable t) =>
(forall (f :: * -> *) a b.
 Applicative f =>
 (a -> f b) -> t a -> f (t b))
-> (forall (f :: * -> *) a. Applicative f => t (f a) -> f (t a))
-> (forall (m :: * -> *) a b.
    Monad m =>
    (a -> m b) -> t a -> m (t b))
-> (forall (m :: * -> *) a. Monad m => t (m a) -> m (t a))
-> Traversable t
forall (m :: * -> *) a.
Monad m =>
PuResult a (m a) -> m (PuResult a a)
forall (f :: * -> *) a.
Applicative f =>
PuResult a (f a) -> f (PuResult a a)
forall (m :: * -> *) a b.
Monad m =>
(a -> m b) -> PuResult a a -> m (PuResult a b)
forall (f :: * -> *) a b.
Applicative f =>
(a -> f b) -> PuResult a a -> f (PuResult a b)
$ctraverse :: forall a (f :: * -> *) a b.
Applicative f =>
(a -> f b) -> PuResult a a -> f (PuResult a b)
traverse :: forall (f :: * -> *) a b.
Applicative f =>
(a -> f b) -> PuResult a a -> f (PuResult a b)
$csequenceA :: forall a (f :: * -> *) a.
Applicative f =>
PuResult a (f a) -> f (PuResult a a)
sequenceA :: forall (f :: * -> *) a.
Applicative f =>
PuResult a (f a) -> f (PuResult a a)
$cmapM :: forall a (m :: * -> *) a b.
Monad m =>
(a -> m b) -> PuResult a a -> m (PuResult a b)
mapM :: forall (m :: * -> *) a b.
Monad m =>
(a -> m b) -> PuResult a a -> m (PuResult a b)
$csequence :: forall a (m :: * -> *) a.
Monad m =>
PuResult a (m a) -> m (PuResult a a)
sequence :: forall (m :: * -> *) a.
Monad m =>
PuResult a (m a) -> m (PuResult a a)
Traversable)

instance Applicative (PuResult a) where
  pure :: forall a. a -> PuResult a a
pure a
x = Bag a -> a -> PuResult a a
forall a b. Bag a -> b -> PuResult a b
PuOK Bag a
forall a. Bag a
emptyBag a
x
  PuFail CheckTyEqResult
p1 <*> :: forall a b. PuResult a (a -> b) -> PuResult a a -> PuResult a b
<*> PuFail CheckTyEqResult
p2 = CheckTyEqResult -> PuResult a b
forall a b. CheckTyEqResult -> PuResult a b
PuFail (CheckTyEqResult
p1 CheckTyEqResult -> CheckTyEqResult -> CheckTyEqResult
forall a. Semigroup a => a -> a -> a
S.<> CheckTyEqResult
p2)
  PuFail CheckTyEqResult
p1 <*> PuOK {}   = CheckTyEqResult -> PuResult a b
forall a b. CheckTyEqResult -> PuResult a b
PuFail CheckTyEqResult
p1
  PuOK {}   <*> PuFail CheckTyEqResult
p2 = CheckTyEqResult -> PuResult a b
forall a b. CheckTyEqResult -> PuResult a b
PuFail CheckTyEqResult
p2
  PuOK Bag a
c1 a -> b
f <*> PuOK Bag a
c2 a
x = Bag a -> b -> PuResult a b
forall a b. Bag a -> b -> PuResult a b
PuOK (Bag a
c1 Bag a -> Bag a -> Bag a
forall a. Bag a -> Bag a -> Bag a
`unionBags` Bag a
c2) (a -> b
f a
x)

instance (Outputable a, Outputable b) => Outputable (PuResult a b) where
  ppr :: PuResult a b -> SDoc
ppr (PuFail CheckTyEqResult
prob) = String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"PuFail" SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> (CheckTyEqResult -> SDoc
forall a. Outputable a => a -> SDoc
ppr CheckTyEqResult
prob)
  ppr (PuOK Bag a
cts b
x)  = String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"PuOK" SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc
braces
                        ([SDoc] -> SDoc
forall doc. IsDoc doc => [doc] -> doc
vcat [ String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"redn:" SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> b -> SDoc
forall a. Outputable a => a -> SDoc
ppr b
x
                              , String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"cts:" SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> Bag a -> SDoc
forall a. Outputable a => a -> SDoc
ppr Bag a
cts ])

pprPur :: PuResult a b -> SDoc
-- For debugging
pprPur :: forall a b. PuResult a b -> SDoc
pprPur (PuFail CheckTyEqResult
prob) = String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"PuFail:" SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<> CheckTyEqResult -> SDoc
forall a. Outputable a => a -> SDoc
ppr CheckTyEqResult
prob
pprPur (PuOK {})     = String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"PuOK"

okCheckRefl :: TcType -> TcM (PuResult a Reduction)
okCheckRefl :: forall a. TcType -> TcM (PuResult a Reduction)
okCheckRefl TcType
ty = PuResult a Reduction
-> IOEnv (Env TcGblEnv TcLclEnv) (PuResult a Reduction)
forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
forall (m :: * -> *) a. Monad m => a -> m a
return (Bag a -> Reduction -> PuResult a Reduction
forall a b. Bag a -> b -> PuResult a b
PuOK Bag a
forall a. Bag a
emptyBag (Role -> TcType -> Reduction
mkReflRedn Role
Nominal TcType
ty))

failCheckWith :: CheckTyEqResult -> TcM (PuResult a b)
failCheckWith :: forall a b. CheckTyEqResult -> TcM (PuResult a b)
failCheckWith CheckTyEqResult
p = PuResult a b -> IOEnv (Env TcGblEnv TcLclEnv) (PuResult a b)
forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
forall (m :: * -> *) a. Monad m => a -> m a
return (CheckTyEqResult -> PuResult a b
forall a b. CheckTyEqResult -> PuResult a b
PuFail CheckTyEqResult
p)

mapCheck :: (x -> TcM (PuResult a Reduction))
         -> [x]
         -> TcM (PuResult a Reductions)
mapCheck :: forall x a.
(x -> TcM (PuResult a Reduction))
-> [x] -> TcM (PuResult a Reductions)
mapCheck x -> TcM (PuResult a Reduction)
f [x]
xs
  = do { (ress :: [PuResult a Reduction]) <- (x -> TcM (PuResult a Reduction))
-> [x] -> IOEnv (Env TcGblEnv TcLclEnv) [PuResult a Reduction]
forall (t :: * -> *) (m :: * -> *) a b.
(Traversable t, Monad m) =>
(a -> m b) -> t a -> m (t b)
forall (m :: * -> *) a b. Monad m => (a -> m b) -> [a] -> m [b]
mapM x -> TcM (PuResult a Reduction)
f [x]
xs
       ; return (unzipRedns <$> sequenceA ress) }
         -- sequenceA :: [PuResult a Reduction] -> PuResult a [Reduction]
         -- unzipRedns :: [Reduction] -> Reductions

-----------------------------
-- | Options describing how to deal with a type equality
-- in the pure unifier. See 'checkTyEqRhs'
data TyEqFlags a
  = TEF { forall a. TyEqFlags a -> TEFTask
tef_task     :: TEFTask
           -- ^ LHS structure, and which checks to perform on the RHS
        , forall a. TyEqFlags a -> TyEqFamApp a
tef_fam_app  :: TyEqFamApp a
            -- ^ How to deal with type family applications
        }

-- | The structure of the LHS and which checks to perform in 'checkTyEqRhs',
-- for an equality @lhs ~# rhs@.
--
-- See Note [TEFTask].
data TEFTask
  -- | LHS is a type family application; we are not unifying.
  = TEFTyFam
    { TEFTask -> CheckTyEqProblem
tefTyFam_occursCheck :: CheckTyEqProblem
       -- ^ The 'CheckTyEqProblem' to report for occurs-check failures
       -- (soluble or insoluble)
    , TEFTask -> TyCon
tefTyFam_tyCon :: TyCon
    , TEFTask -> [TcType]
tefTyFam_args  :: [Type]
    }
  -- | LHS is a 'TyVar'.
  | TEFTyVar
    { TEFTask -> Maybe (Name, CheckTyEqProblem)
tefTyVar_occursCheck    :: Maybe (Name, CheckTyEqProblem)
       -- ^ Occurs check: LHS 'TyVar' 'Name',
       -- and which 'CheckTyEqProblem' to report for occurs-check failures
       -- (soluble or insoluble)
    , TEFTask -> Maybe (TcLevel, LevelCheck)
tefTyVar_levelCheck     :: Maybe (TcLevel, LevelCheck)
          -- ^ Level check: LHS 'TyVar' 'TcLevel',
          -- and which 'LevelCheck' to perform
    , TEFTask -> Maybe ConcreteTvOrigin
tefTyVar_concreteCheck  :: Maybe ConcreteTvOrigin
          -- ^ Concreteness check: LHS 'TyVar' 'ConcreteTvOrigin'
          -- to use for the check
    }

{- Note [TEFTask]
~~~~~~~~~~~~~~~~~
When we call the pure unifier, e.g. through 'checkTyEqRhs', we can decide what
kind of checks the unifier performs via the 'TyEqFlags' argument. In particular,
when the LHS type in a unification is a type variable, we might want to perform
different checks; this is achieved using the 'TEFTyVar' constructor to 'TEFTask':

  1. LHS is a skolem tyvar, or an untouchable meta-tyvar.
     We are not unifying; we only want to perform occurs-checks.

      TEFTyVar
        { tefTyVar_occursCheck   = Just ...
        , tefTyVar_levelCheck    = Nothing
        , tefTyVar_concreteCheck = Nothing
        }

  2. LHS is a touchable meta-tyvar.
     We are unifying; we want to perform an occurs check, a level check,
     and a concreteness check (when the meta-tyvar is a ConcreteTv).

      TEFTyVar
        { tefTyVar_occursCheck   = Just ...
        , tefTyVar_levelCheck    = Just ...
        , tefTyVar_concreteCheck = isConcreteTyVar_maybe lhs_tv
        }

  3. LHS is a fresh ConcreteTv meta-tyvar (see call to 'checkTyEqRhs' in
     'makeTypeConcrete'). We are unifying; we only want to perform
     a concreteness check.

      TEFTyVar
        { tefTyVar_occursCheck   = Nothing
        , tefTyVar_levelCheck    = Nothing
        , tefTyVar_concreteCheck = Just ...
        }
-}

-- | Create a "not unifying" 'TEFTask' from a 'CanEqLHS'.
--
-- See use-case (1) in Note [TEFTask].
notUnifying_TEFTask :: CheckTyEqProblem -> CanEqLHS -> TEFTask
notUnifying_TEFTask :: CheckTyEqProblem -> CanEqLHS -> TEFTask
notUnifying_TEFTask CheckTyEqProblem
occ_prob = \case
  TyFamLHS TyCon
tc [TcType]
tys ->
    CheckTyEqProblem -> TyCon -> [TcType] -> TEFTask
TEFTyFam CheckTyEqProblem
occ_prob TyCon
tc [TcType]
tys
  TyVarLHS TcTyVar
tv ->
    TEFTyVar
      { tefTyVar_occursCheck :: Maybe (Name, CheckTyEqProblem)
tefTyVar_occursCheck   = (Name, CheckTyEqProblem) -> Maybe (Name, CheckTyEqProblem)
forall a. a -> Maybe a
Just (TcTyVar -> Name
tyVarName TcTyVar
tv, CheckTyEqProblem
occ_prob)
      , tefTyVar_levelCheck :: Maybe (TcLevel, LevelCheck)
tefTyVar_levelCheck    = Maybe (TcLevel, LevelCheck)
forall a. Maybe a
Nothing
      , tefTyVar_concreteCheck :: Maybe ConcreteTvOrigin
tefTyVar_concreteCheck = Maybe ConcreteTvOrigin
forall a. Maybe a
Nothing
      }
    -- We need an occurs-check here, but no level check.
    -- See Note [Promotion and level-checking] wrinkle (W1)

-- | Create "unifying" 'TEFTask' from a 'TyVarLHS'.
--
-- Invariant: the argument 'TyVar' is a 'MetaTv'.
unifyingLHSMetaTyVar_TEFTask :: TyVar -> LevelCheck -> TEFTask
unifyingLHSMetaTyVar_TEFTask :: TcTyVar -> LevelCheck -> TEFTask
unifyingLHSMetaTyVar_TEFTask TcTyVar
lhs_tv LevelCheck
lc =
  TEFTyVar
    { tefTyVar_occursCheck :: Maybe (Name, CheckTyEqProblem)
tefTyVar_occursCheck   = (Name, CheckTyEqProblem) -> Maybe (Name, CheckTyEqProblem)
forall a. a -> Maybe a
Just (TcTyVar -> Name
tyVarName TcTyVar
lhs_tv, CheckTyEqProblem
cteInsolubleOccurs)
    , tefTyVar_levelCheck :: Maybe (TcLevel, LevelCheck)
tefTyVar_levelCheck    = (TcLevel, LevelCheck) -> Maybe (TcLevel, LevelCheck)
forall a. a -> Maybe a
Just (TcTyVar -> TcLevel
tcTyVarLevel TcTyVar
lhs_tv, LevelCheck
lc)
    , tefTyVar_concreteCheck :: Maybe ConcreteTvOrigin
tefTyVar_concreteCheck = TcTyVar -> Maybe ConcreteTvOrigin
isConcreteTyVar_maybe TcTyVar
lhs_tv
    }

-- | Do we want to perform a concreteness check in 'checkTyEqRhs'?
tefTaskConcrete_maybe :: TEFTask -> Maybe ConcreteTvOrigin
tefTaskConcrete_maybe :: TEFTask -> Maybe ConcreteTvOrigin
tefTaskConcrete_maybe (TEFTyFam {}) = Maybe ConcreteTvOrigin
forall a. Maybe a
Nothing
tefTaskConcrete_maybe (TEFTyVar { tefTyVar_concreteCheck :: TEFTask -> Maybe ConcreteTvOrigin
tefTyVar_concreteCheck = Maybe ConcreteTvOrigin
conc }) = Maybe ConcreteTvOrigin
conc

instance Outputable TEFTask where
  ppr :: TEFTask -> SDoc
ppr = \case
    TEFTyFam CheckTyEqProblem
occ TyCon
tc [TcType]
tys ->
      String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"TEFTyFam" SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> CheckTyEqProblem -> SDoc
forall a. Outputable a => a -> SDoc
ppr CheckTyEqProblem
occ SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> TcType -> SDoc
forall a. Outputable a => a -> SDoc
ppr (TyCon -> [TcType] -> TcType
mkTyConApp TyCon
tc [TcType]
tys)
    TEFTyVar Maybe (Name, CheckTyEqProblem)
mb_occ Maybe (TcLevel, LevelCheck)
mb_lc Maybe ConcreteTvOrigin
mb_conc ->
      String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"TEFTyVar" SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> [SDoc] -> SDoc
forall doc. IsLine doc => [doc] -> doc
hcat (SDoc -> [SDoc] -> [SDoc]
forall doc. IsLine doc => doc -> [doc] -> [doc]
punctuate SDoc
forall doc. IsLine doc => doc
comma [SDoc]
fields)
      where
        fields :: [SDoc]
fields = [ String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"OccursCheck:" SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> Name -> SDoc
forall a. Outputable a => a -> SDoc
ppr Name
tv | (Name
tv, CheckTyEqProblem
_) <- Maybe (Name, CheckTyEqProblem) -> [(Name, CheckTyEqProblem)]
forall a. Maybe a -> [a]
maybeToList Maybe (Name, CheckTyEqProblem)
mb_occ ]
                   [SDoc] -> [SDoc] -> [SDoc]
forall a. [a] -> [a] -> [a]
++
                 [ String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"LevelCheck:" SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> (TcLevel, LevelCheck) -> SDoc
forall a. Outputable a => a -> SDoc
ppr (TcLevel, LevelCheck)
lc | (TcLevel, LevelCheck)
lc <- Maybe (TcLevel, LevelCheck) -> [(TcLevel, LevelCheck)]
forall a. Maybe a -> [a]
maybeToList Maybe (TcLevel, LevelCheck)
mb_lc ]
                   [SDoc] -> [SDoc] -> [SDoc]
forall a. [a] -> [a] -> [a]
++
                 [ String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"ConcreteCheck" | Maybe ConcreteTvOrigin -> Bool
forall a. Maybe a -> Bool
isJust Maybe ConcreteTvOrigin
mb_conc ]

-- | What to do when encountering a type-family application while processing
-- a type equality in the pure unifier.
--
-- See Note [Family applications in canonical constraints]
data TyEqFamApp a
  = TEFA_Fail                    -- ^ Always fail
  | TEFA_Recurse                 -- ^ Just recurse
  | TEFA_Break (FamAppBreaker a) -- ^ Recurse, but replace with cycle breaker if fails,
                                 --   using the FamAppBreaker

-- | What level check to perform, in a call to the pure unifier?
data LevelCheck
  = LC_Check      -- ^ Do a level check between the LHS tyvar and the occurrence tyvar.
                  --
                  -- Fail if the level check fails.

  | LC_Promote Bool
      -- ^ Do a level check between the LHS tyvar and the occurrence tyvar.
      --
      -- If the level check fails, and the occurrence is a unification
      -- variable, promote it.
      --
      --   - False <=> don't promote under type families (the common case)
      --   -  True <=> promote even under type families
      --             (see Note [Defaulting equalities] in GHC.Tc.Solver)

instance Outputable (TyEqFlags a) where
  ppr :: TyEqFlags a -> SDoc
ppr (TEF { TyEqFamApp a
TEFTask
tef_task :: forall a. TyEqFlags a -> TEFTask
tef_fam_app :: forall a. TyEqFlags a -> TyEqFamApp a
tef_task :: TEFTask
tef_fam_app :: TyEqFamApp a
.. }) = String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"TEF" SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc
braces (
                        [SDoc] -> SDoc
forall doc. IsDoc doc => [doc] -> doc
vcat [ String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"tef_task =" SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> TEFTask -> SDoc
forall a. Outputable a => a -> SDoc
ppr TEFTask
tef_task
                             , String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"tef_fam_app =" SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> TyEqFamApp a -> SDoc
forall a. Outputable a => a -> SDoc
ppr TyEqFamApp a
tef_fam_app ])

instance Outputable (TyEqFamApp a) where
  ppr :: TyEqFamApp a -> SDoc
ppr TyEqFamApp a
TEFA_Fail       = String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"TEFA_Fail"
  ppr TyEqFamApp a
TEFA_Recurse    = String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"TEFA_Recurse"
  ppr (TEFA_Break {}) = String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"TEFA_Break"

instance Outputable LevelCheck where
  ppr :: LevelCheck -> SDoc
ppr LevelCheck
LC_Check       = String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"LC_Check"
  ppr (LC_Promote Bool
b) = String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"LC_Promote" SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<> Bool -> SDoc -> SDoc
forall doc. IsOutput doc => Bool -> doc -> doc
ppWhen Bool
b (String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"(deep)")

-- | Adjust the 'TyEqFlags' when going undter a type family:
--
--  1. Only the outer family application gets the loop-breaker treatment
--  2. Weaken level checks for tyvar promotion. For example, in @[W] alpha[2] ~ Maybe (F beta[3])@,
--     do not promote @beta[3]@, instead promote @(F beta[3])@.
--  3. Occurs checks become potentially soluble (after additional type family
--     reductions).
famAppArgFlags :: TyEqFlags a -> TyEqFlags a
famAppArgFlags :: forall a. TyEqFlags a -> TyEqFlags a
famAppArgFlags flags :: TyEqFlags a
flags@(TEF { tef_task :: forall a. TyEqFlags a -> TEFTask
tef_task = TEFTask
task })
  = TyEqFlags a
flags { tef_fam_app  = TEFA_Recurse -- (1)
          , tef_task     = fam_app_task task
          }
  where
    fam_app_task :: TEFTask -> TEFTask
    fam_app_task :: TEFTask -> TEFTask
fam_app_task TEFTask
task = case TEFTask
task of
      TEFTyFam {} ->
        TEFTask
task
          { tefTyFam_occursCheck = cteSolubleOccurs -- (3)
          }
      TEFTyVar { tefTyVar_occursCheck :: TEFTask -> Maybe (Name, CheckTyEqProblem)
tefTyVar_occursCheck = Maybe (Name, CheckTyEqProblem)
mb_occ, tefTyVar_levelCheck :: TEFTask -> Maybe (TcLevel, LevelCheck)
tefTyVar_levelCheck = Maybe (TcLevel, LevelCheck)
mb_lc } ->
        TEFTask
task
          { tefTyVar_occursCheck =
              fmap (\ (Name
tv, CheckTyEqProblem
_old_occ_prob) -> (Name
tv, CheckTyEqProblem
cteSolubleOccurs)) mb_occ -- (3)
          , tefTyVar_levelCheck =
              fmap (\ (TcLevel
lvl, LevelCheck
lc) -> (TcLevel
lvl, LevelCheck -> LevelCheck
zap_lc LevelCheck
lc)) mb_lc -- (2)
          }
    zap_lc :: LevelCheck -> LevelCheck
zap_lc = \case
      LC_Promote Bool
deeply
        | Bool -> Bool
not Bool
deeply
        -> LevelCheck
LC_Check
      LevelCheck
lc -> LevelCheck
lc

type FamAppBreaker a = TcType -> TcM (PuResult a Reduction)
     -- Given a family-application ty, return a Reduction :: ty ~ cvb
     -- where 'cbv' is a fresh loop-breaker tyvar (for Given), or
     -- just a fresh TauTv (for Wanted)

checkTyEqRhs :: forall a. TyEqFlags a
                       -> TcType           -- Already zonked
                       -> TcM (PuResult a Reduction)
checkTyEqRhs :: forall a. TyEqFlags a -> TcType -> TcM (PuResult a Reduction)
checkTyEqRhs TyEqFlags a
flags TcType
ty
  = case TcType
ty of
      LitTy {}        -> TcType -> TcM (PuResult a Reduction)
forall a. TcType -> TcM (PuResult a Reduction)
okCheckRefl TcType
ty
      TyConApp TyCon
tc [TcType]
tys -> TyEqFlags a
-> TcType -> TyCon -> [TcType] -> TcM (PuResult a Reduction)
forall a.
TyEqFlags a
-> TcType -> TyCon -> [TcType] -> TcM (PuResult a Reduction)
checkTyConApp TyEqFlags a
flags TcType
ty TyCon
tc [TcType]
tys
      TyVarTy TcTyVar
tv      -> TyEqFlags a -> TcTyVar -> TcM (PuResult a Reduction)
forall a. TyEqFlags a -> TcTyVar -> TcM (PuResult a Reduction)
checkTyVar TyEqFlags a
flags TcTyVar
tv
        -- Don't worry about foralls inside the kind; see Note [Checking for foralls]
        -- Nor can we expand synonyms; see Note [Occurrence checking: look inside kinds]
        --                             in GHC.Core.FVs

      FunTy {ft_af :: TcType -> FunTyFlag
ft_af = FunTyFlag
af, ft_mult :: TcType -> TcType
ft_mult = TcType
w, ft_arg :: TcType -> TcType
ft_arg = TcType
a, ft_res :: TcType -> TcType
ft_res = TcType
r}
       | FunTyFlag -> Bool
isInvisibleFunArg FunTyFlag
af  -- e.g.  Num a => blah
       -> CheckTyEqResult -> TcM (PuResult a Reduction)
forall a b. CheckTyEqResult -> TcM (PuResult a b)
failCheckWith CheckTyEqResult
impredicativeProblem -- Not allowed (TyEq:F)
       | Bool
otherwise
       -> do { w_res <- TyEqFlags a -> TcType -> TcM (PuResult a Reduction)
forall a. TyEqFlags a -> TcType -> TcM (PuResult a Reduction)
checkTyEqRhs TyEqFlags a
flags TcType
w
             ; a_res <- checkTyEqRhs flags a
             ; r_res <- checkTyEqRhs flags r
             ; return (mkFunRedn Nominal af <$> w_res <*> a_res <*> r_res) }

      AppTy TcType
fun TcType
arg -> do { fun_res <- TyEqFlags a -> TcType -> TcM (PuResult a Reduction)
forall a. TyEqFlags a -> TcType -> TcM (PuResult a Reduction)
checkTyEqRhs TyEqFlags a
flags TcType
fun
                          ; arg_res <- checkTyEqRhs flags arg
                          ; return (mkAppRedn <$> fun_res <*> arg_res) }

      CastTy TcType
ty Coercion
co  -> do { ty_res <- TyEqFlags a -> TcType -> TcM (PuResult a Reduction)
forall a. TyEqFlags a -> TcType -> TcM (PuResult a Reduction)
checkTyEqRhs TyEqFlags a
flags TcType
ty
                          ; co_res <- checkCo flags co
                          ; return (mkCastRedn1 Nominal ty <$> co_res <*> ty_res) }

      CoercionTy Coercion
co -> do { co_res <- TyEqFlags a -> Coercion -> TcM (PuResult a Coercion)
forall a. TyEqFlags a -> Coercion -> TcM (PuResult a Coercion)
checkCo TyEqFlags a
flags Coercion
co
                          ; return (mkReflCoRedn Nominal <$> co_res) }

      ForAllTy {}   -> CheckTyEqResult -> TcM (PuResult a Reduction)
forall a b. CheckTyEqResult -> TcM (PuResult a b)
failCheckWith CheckTyEqResult
impredicativeProblem -- Not allowed (TyEq:F)

-------------------
checkCo :: TyEqFlags a -> Coercion -> TcM (PuResult a Coercion)
-- See Note [checkCo]
checkCo :: forall a. TyEqFlags a -> Coercion -> TcM (PuResult a Coercion)
checkCo (TEF { tef_task :: forall a. TyEqFlags a -> TEFTask
tef_task = TEFTask
task }) Coercion
co =
  case TEFTask
task of
    TEFTyFam {} ->
      -- NB: 'TEFTyFam' case means we are not unifying.
      PuResult a Coercion -> TcM (PuResult a Coercion)
forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
forall (m :: * -> *) a. Monad m => a -> m a
return (Coercion -> PuResult a Coercion
forall a. a -> PuResult a a
forall (f :: * -> *) a. Applicative f => a -> f a
pure Coercion
co)
    TEFTyVar
      { tefTyVar_concreteCheck :: TEFTask -> Maybe ConcreteTvOrigin
tefTyVar_concreteCheck = Maybe ConcreteTvOrigin
mb_conc
      , tefTyVar_levelCheck :: TEFTask -> Maybe (TcLevel, LevelCheck)
tefTyVar_levelCheck    = Maybe (TcLevel, LevelCheck)
mb_lc
      , tefTyVar_occursCheck :: TEFTask -> Maybe (Name, CheckTyEqProblem)
tefTyVar_occursCheck   = Maybe (Name, CheckTyEqProblem)
mb_occ
      }
        -- Coercions cannot appear in concrete types.
        --
        -- See Note [Concrete types] in GHC.Tc.Utils.Concrete.
        | Just {} <- Maybe ConcreteTvOrigin
mb_conc
        -> CheckTyEqResult -> TcM (PuResult a Coercion)
forall a b. CheckTyEqResult -> TcM (PuResult a b)
failCheckWith (CheckTyEqProblem -> CheckTyEqResult
cteProblem CheckTyEqProblem
cteConcrete)

        -- Check for coercion holes, if unifying.
        -- See (COERCION-HOLE) in Note [Unification preconditions]
        | Just {} <- Maybe (TcLevel, LevelCheck)
mb_lc -- equivalent to "we are unifying"; see Note [TEFTask]
        , Coercion -> Bool
hasCoercionHoleCo Coercion
co
        -> CheckTyEqResult -> TcM (PuResult a Coercion)
forall a b. CheckTyEqResult -> TcM (PuResult a b)
failCheckWith (CheckTyEqProblem -> CheckTyEqResult
cteProblem CheckTyEqProblem
cteCoercionHole)

        -- Occurs check (can promote)
        | Just (Name
lhs_tv, CheckTyEqProblem
occ_prob) <- Maybe (Name, CheckTyEqProblem)
mb_occ
        , Just (TcLevel
lhs_tv_lvl, LC_Promote {}) <- Maybe (TcLevel, LevelCheck)
mb_lc
        -> do { reason <- CheckTyEqProblem
-> Name -> TcLevel -> VarSet -> TcM CheckTyEqResult
checkPromoteFreeVars CheckTyEqProblem
occ_prob Name
lhs_tv TcLevel
lhs_tv_lvl (Coercion -> VarSet
tyCoVarsOfCo Coercion
co)
              ; if cterHasNoProblem reason
                then return (pure co)
                else failCheckWith reason }

        -- Occurs check (no promotion)
        | Just (Name
lhs_tv, CheckTyEqProblem
occ_prob) <- Maybe (Name, CheckTyEqProblem)
mb_occ
        , Name -> Unique
nameUnique Name
lhs_tv Unique -> VarSet -> Bool
`elemVarSetByKey` Coercion -> VarSet
tyCoVarsOfCo Coercion
co
        -> CheckTyEqResult -> TcM (PuResult a Coercion)
forall a b. CheckTyEqResult -> TcM (PuResult a b)
failCheckWith (CheckTyEqProblem -> CheckTyEqResult
cteProblem CheckTyEqProblem
occ_prob)

        | Bool
otherwise
        -> PuResult a Coercion -> TcM (PuResult a Coercion)
forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
forall (m :: * -> *) a. Monad m => a -> m a
return (Coercion -> PuResult a Coercion
forall a. a -> PuResult a a
forall (f :: * -> *) a. Applicative f => a -> f a
pure Coercion
co)

{- Note [checkCo]
~~~~~~~~~~~~~~~~~
We don't often care about the contents of coercions, so checking
coercions before making an equality constraint may be surprising.
But there are several cases we need to be wary of:

(1) When we're unifying a variable, we must make sure that the variable
    appears nowhere on the RHS -- even in a coercion. Otherwise, we'll
    create a loop.

(2) We must still make sure that no variable in a coercion is at too
    high a level. But, when unifying, we can promote any variables we encounter.

(3) We do not unify variables with a type with a free coercion hole.
    See (COERCION-HOLE) in Note [Unification preconditions].


Note [Promotion and level-checking]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
"Promotion" happens when we have this:

  [W] w1: alpha[2] ~ Maybe beta[4]

Here we must NOT unify alpha := Maybe beta, because beta may turn out
to stand for a type involving some inner skolem.  Yikes!
Skolem-escape.  So instead we /promote/ beta, like this:

  beta[4] := beta'[2]
  [W] w1: alpha[2] ~ Maybe beta'[2]

Now we can unify alpha := Maybe beta', which might unlock other
constraints.  But if some other constraint wants to unify beta with a
nested skolem, it'll get stuck with a skolem-escape error.

Now consider `w2` where a type family is involved (#22194):

  [W] w2: alpha[2] ~ Maybe (F gamma beta[4])

In `w2`, it may or may not be the case that `beta` is level 2; suppose
we later discover gamma := Int, and type instance F Int _ = Int.
So, instead, we promote the entire funcion call:

  [W] w2': alpha[2] ~ Maybe gamma[2]
  [W] w3:  gamma[2] ~ F gamma beta[4]

Now we can unify alpha := Maybe gamma, which is a Good Thng.

Wrinkle (W1)

There is an important wrinkle: /all this only applies when unifying/.
For example, suppose we have
 [G] a[2] ~ Maybe b[4]
where 'a' is a skolem.  This Given might arise from a GADT match, and
we can absolutely use it to rewrite locally. In fact we must do so:
that is how we exploit local knowledge about the outer skolem a[2].
This applies equally for a Wanted [W] a[2] ~ Maybe b[4]. Using it for
local rewriting is fine. (It's not clear to me that it is /useful/,
but it's fine anyway.)

So we only do the level-check in checkTyVar when /unifying/ not for
skolems (or untouchable unification variables).

Note [Family applications in canonical constraints]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
A constraint with a type family application in the RHS needs special care.

* First, occurs checks.  If we have
     [G] a ~ Maybe (F (Maybe a))
     [W] alpha ~ Maybe (F (Maybe alpha))
  it looks as if we have an occurs check.  But go read
  Note [Type equality cycles] in GHC.Tc.Solver.Equality

  The same considerations apply when the LHS is a type family:
     [G] G a ~ Maybe (F (Maybe (G a)))
     [W] G alpha ~ Maybe (F (Maybe (G alpha)))

* Second, promotion. If we have (#22194)
     [W] alpha[2] ~ Maybe (F beta[4])
  it is wrong to promote beta.  Instead we want to split to
     [W] alpha[2] ~ Maybe gamma[2]
     [W] gamma[2] ~ F beta[4]
  See Note [Promotion and level-checking] above.

* Third, concrete type variables.  If we have
     [W] alpha[conc] ~ Maybe (F tys)
  we want to add an extra variable thus:
     [W] alpha[conc] ~ Maybe gamma[conc]
     [W] gamma[conc] ~ F tys
  Now we can unify alpha, and that might unlock something else.

In all these cases we want to create a fresh type variable, and
emit a new equality connecting it to the type family application.

The `tef_fam_app` field of `TypeEqFlags` says what to do at a type
family application in the RHS of the constraint.  `TEFA_Fail` and
`TEFA_Recurse` are straightforward.  `TEFA_Break` is the clever
one. As you can see in `checkFamApp`, it
  * Checks the arguments, but using `famAppArgFlags` to record that
    we are now "under" a type-family application. It `tef_fam_app` to
    `TEFA_Recurse`.
  * If any of the arguments fail (level-check error, occurs check)
    use the `FamAppBreaker` to create the extra binding.

Note that this always cycle-breaks the /outermost/ family application.
If we have  [W] alpha ~ Maybe (F (G alpha))
* We'll use checkFamApp on `(F (G alpha))`
* It will recurse into `(G alpha)` with TEFA_Recurse, but not cycle-break it
* The occurs check will fire when we hit `alpha`
* `checkFamApp` on `(F (G alpha))` will see the failure and invoke
  the `FamAppBreaker`.
-}

-------------------
checkTyConApp :: TyEqFlags a
              -> TcType -> TyCon -> [TcType]
              -> TcM (PuResult a Reduction)
checkTyConApp :: forall a.
TyEqFlags a
-> TcType -> TyCon -> [TcType] -> TcM (PuResult a Reduction)
checkTyConApp flags :: TyEqFlags a
flags@(TEF { tef_task :: forall a. TyEqFlags a -> TEFTask
tef_task = TEFTask
task })
              TcType
tc_app TyCon
tc [TcType]
tys
  | TyCon -> Bool
isTypeFamilyTyCon TyCon
tc
  , let arity :: Int
arity = TyCon -> Int
tyConArity TyCon
tc
  = if [TcType]
tys [TcType] -> Int -> Bool
forall a. [a] -> Int -> Bool
`lengthIs` Int
arity
    then TyEqFlags a
-> TcType -> TyCon -> [TcType] -> TcM (PuResult a Reduction)
forall a.
TyEqFlags a
-> TcType -> TyCon -> [TcType] -> TcM (PuResult a Reduction)
checkFamApp TyEqFlags a
flags TcType
tc_app TyCon
tc [TcType]
tys  -- Common case
    else do { let ([TcType]
fun_args, [TcType]
extra_args) = Int -> [TcType] -> ([TcType], [TcType])
forall a. Int -> [a] -> ([a], [a])
splitAt (TyCon -> Int
tyConArity TyCon
tc) [TcType]
tys
                  fun_app :: TcType
fun_app                = TyCon -> [TcType] -> TcType
mkTyConApp TyCon
tc [TcType]
fun_args
            ; fun_res   <- TyEqFlags a
-> TcType -> TyCon -> [TcType] -> TcM (PuResult a Reduction)
forall a.
TyEqFlags a
-> TcType -> TyCon -> [TcType] -> TcM (PuResult a Reduction)
checkFamApp TyEqFlags a
flags TcType
fun_app TyCon
tc [TcType]
fun_args
            ; extra_res <- mapCheck (checkTyEqRhs flags) extra_args
            ; traceTc "Over-sat" (ppr tc <+> ppr tys $$ ppr arity $$ pprPur fun_res $$ pprPur extra_res)
            ; return (mkAppRedns <$> fun_res <*> extra_res) }

  | Just TcType
ty' <- TcType -> Maybe TcType
rewriterView TcType
tc_app
       -- e.g. S a  where  type S a = F [a]
       --             or   type S a = Int
       -- See Note [Forgetful synonyms in checkTyConApp]
  = TyEqFlags a -> TcType -> TcM (PuResult a Reduction)
forall a. TyEqFlags a -> TcType -> TcM (PuResult a Reduction)
checkTyEqRhs TyEqFlags a
flags TcType
ty'

  | Bool -> Bool
not (TyCon -> Bool
isTauTyCon TyCon
tc)
  = CheckTyEqResult -> TcM (PuResult a Reduction)
forall a b. CheckTyEqResult -> TcM (PuResult a b)
failCheckWith CheckTyEqResult
impredicativeProblem

  | Just {} <- TEFTask -> Maybe ConcreteTvOrigin
tefTaskConcrete_maybe TEFTask
task
  , Bool -> Bool
not (TyCon -> Bool
isConcreteTyCon TyCon
tc)
  = CheckTyEqResult -> TcM (PuResult a Reduction)
forall a b. CheckTyEqResult -> TcM (PuResult a b)
failCheckWith (CheckTyEqProblem -> CheckTyEqResult
cteProblem CheckTyEqProblem
cteConcrete)

  | Bool
otherwise  -- Recurse on arguments
  = TyEqFlags a -> TyCon -> [TcType] -> TcM (PuResult a Reduction)
forall a.
TyEqFlags a -> TyCon -> [TcType] -> TcM (PuResult a Reduction)
recurseIntoTyConApp TyEqFlags a
flags TyCon
tc [TcType]
tys

recurseIntoTyConApp :: TyEqFlags a -> TyCon -> [TcType] -> TcM (PuResult a Reduction)
recurseIntoTyConApp :: forall a.
TyEqFlags a -> TyCon -> [TcType] -> TcM (PuResult a Reduction)
recurseIntoTyConApp TyEqFlags a
flags TyCon
tc [TcType]
tys
  = do { tys_res <- (TcType -> TcM (PuResult a Reduction))
-> [TcType] -> TcM (PuResult a Reductions)
forall x a.
(x -> TcM (PuResult a Reduction))
-> [x] -> TcM (PuResult a Reductions)
mapCheck (TyEqFlags a -> TcType -> TcM (PuResult a Reduction)
forall a. TyEqFlags a -> TcType -> TcM (PuResult a Reduction)
checkTyEqRhs TyEqFlags a
flags) [TcType]
tys
       ; return (mkTyConAppRedn Nominal tc <$> tys_res) }

-------------------
checkFamApp :: TyEqFlags a
            -> TcType -> TyCon -> [TcType]  -- Saturated family application
            -> TcM (PuResult a Reduction)
-- See Note [Family applications in canonical constraints]
checkFamApp :: forall a.
TyEqFlags a
-> TcType -> TyCon -> [TcType] -> TcM (PuResult a Reduction)
checkFamApp flags :: TyEqFlags a
flags@(TEF { tef_task :: forall a. TyEqFlags a -> TEFTask
tef_task = TEFTask
task, tef_fam_app :: forall a. TyEqFlags a -> TyEqFamApp a
tef_fam_app = TyEqFamApp a
fam_app_flag })
            TcType
fam_app TyCon
tc [TcType]
tys
  = case TyEqFamApp a
fam_app_flag of
      TyEqFamApp a
TEFA_Fail -> CheckTyEqResult -> TcM (PuResult a Reduction)
forall a b. CheckTyEqResult -> TcM (PuResult a b)
failCheckWith (CheckTyEqProblem -> CheckTyEqResult
cteProblem CheckTyEqProblem
cteTypeFamily)

      -- Occurs check: F ty ~ ...(F ty)...
      TyEqFamApp a
_ | TEFTyFam CheckTyEqProblem
occ_prob TyCon
lhs_tc [TcType]
lhs_tys <- TEFTask
task
        , TyCon -> [TcType] -> TyCon -> [TcType] -> Bool
tcEqTyConApps TyCon
lhs_tc [TcType]
lhs_tys TyCon
tc [TcType]
tys
        -> case TyEqFamApp a
fam_app_flag of
             TyEqFamApp a
TEFA_Recurse       -> CheckTyEqResult -> TcM (PuResult a Reduction)
forall a b. CheckTyEqResult -> TcM (PuResult a b)
failCheckWith (CheckTyEqProblem -> CheckTyEqResult
cteProblem CheckTyEqProblem
occ_prob)
             TEFA_Break FamAppBreaker a
breaker -> FamAppBreaker a
breaker TcType
fam_app

      TyEqFamApp a
_ | Just {} <- TEFTask -> Maybe ConcreteTvOrigin
tefTaskConcrete_maybe TEFTask
task
        -> case TyEqFamApp a
fam_app_flag of
             TyEqFamApp a
TEFA_Recurse       -> CheckTyEqResult -> TcM (PuResult a Reduction)
forall a b. CheckTyEqResult -> TcM (PuResult a b)
failCheckWith (CheckTyEqProblem -> CheckTyEqResult
cteProblem CheckTyEqProblem
cteConcrete)
             TEFA_Break FamAppBreaker a
breaker -> FamAppBreaker a
breaker TcType
fam_app

      TyEqFamApp a
TEFA_Recurse
        -> do { tys_res <- FamAppBreaker a -> [TcType] -> TcM (PuResult a Reductions)
forall x a.
(x -> TcM (PuResult a Reduction))
-> [x] -> TcM (PuResult a Reductions)
mapCheck (TyEqFlags a -> FamAppBreaker a
forall a. TyEqFlags a -> TcType -> TcM (PuResult a Reduction)
checkTyEqRhs TyEqFlags a
arg_flags) [TcType]
tys
              ; traceTc "under" (ppr tc $$ pprPur tys_res $$ ppr flags)
              ; return (mkTyConAppRedn Nominal tc <$> tys_res) }

      -- For TEFA_Break, try recursion; and break if there is a problem
      -- e.g.  alpha[2] ~ Maybe (F beta[2])    No problem: just unify
      --       alpha[2] ~ Maybe (F beta[4])    Level-check problem: break
      -- NB: in the latter case, don't promote beta[4]; hence arg_flags!
      TEFA_Break FamAppBreaker a
breaker
        -> do { tys_res <- FamAppBreaker a -> [TcType] -> TcM (PuResult a Reductions)
forall x a.
(x -> TcM (PuResult a Reduction))
-> [x] -> TcM (PuResult a Reductions)
mapCheck (TyEqFlags a -> FamAppBreaker a
forall a. TyEqFlags a -> TcType -> TcM (PuResult a Reduction)
checkTyEqRhs TyEqFlags a
arg_flags) [TcType]
tys
              ; case tys_res of
                  PuOK Bag a
cts Reductions
redns -> PuResult a Reduction -> TcM (PuResult a Reduction)
forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
forall (m :: * -> *) a. Monad m => a -> m a
return (Bag a -> Reduction -> PuResult a Reduction
forall a b. Bag a -> b -> PuResult a b
PuOK Bag a
cts (Role -> TyCon -> Reductions -> Reduction
mkTyConAppRedn Role
Nominal TyCon
tc Reductions
redns))
                  PuFail {}      -> FamAppBreaker a
breaker TcType
fam_app }
  where
    arg_flags :: TyEqFlags a
arg_flags = TyEqFlags a -> TyEqFlags a
forall a. TyEqFlags a -> TyEqFlags a
famAppArgFlags TyEqFlags a
flags

-------------------

-- | The result of a single check in 'checkTyVar', such as a concreteness check
-- or a level check.
data TyVarCheckResult
  -- | Check succeded; nothing else to do.
  = TyVarCheck_Success
  -- | Check succeeded, but requires an additional promotion.
  --
  -- Invariant: at least one of the fields is not 'Nothing'.
  | TyVarCheck_Promote
       (Maybe TcLevel)
         -- ^ @Just lvl@ <=> 'TyVar' needs to be promoted to @lvl@
       (Maybe ConcreteTvOrigin)
         -- ^ @Just conc_orig@ <=> 'TyVar' needs to be make concrete
  -- | Check failed with some 'CheckTyEqProblem's.
  --
  -- Invariant: the 'CheckTyEqResult' is not 'cteOK'.
  | TyVarCheck_Error CheckTyEqResult

instance Semigroup TyVarCheckResult where
  TyVarCheckResult
TyVarCheck_Success <> :: TyVarCheckResult -> TyVarCheckResult -> TyVarCheckResult
<> TyVarCheckResult
r = TyVarCheckResult
r
  TyVarCheckResult
r <> TyVarCheckResult
TyVarCheck_Success = TyVarCheckResult
r
  TyVarCheck_Error CheckTyEqResult
e1 <> TyVarCheck_Error CheckTyEqResult
e2 =
    CheckTyEqResult -> TyVarCheckResult
TyVarCheck_Error (CheckTyEqResult
e1 CheckTyEqResult -> CheckTyEqResult -> CheckTyEqResult
forall a. Semigroup a => a -> a -> a
S.<> CheckTyEqResult
e2)
  e :: TyVarCheckResult
e@(TyVarCheck_Error {}) <> TyVarCheckResult
_ = TyVarCheckResult
e
  TyVarCheckResult
_ <> e :: TyVarCheckResult
e@(TyVarCheck_Error {}) = TyVarCheckResult
e
  TyVarCheck_Promote Maybe TcLevel
l1 Maybe ConcreteTvOrigin
c1 <> TyVarCheck_Promote Maybe TcLevel
l2 Maybe ConcreteTvOrigin
c2 =
    Maybe TcLevel -> Maybe ConcreteTvOrigin -> TyVarCheckResult
TyVarCheck_Promote
      ((TcLevel -> TcLevel -> TcLevel)
-> Maybe TcLevel -> Maybe TcLevel -> Maybe TcLevel
forall a. (a -> a -> a) -> Maybe a -> Maybe a -> Maybe a
combineMaybe TcLevel -> TcLevel -> TcLevel
minTcLevel Maybe TcLevel
l1 Maybe TcLevel
l2)
      ((ConcreteTvOrigin -> ConcreteTvOrigin -> ConcreteTvOrigin)
-> Maybe ConcreteTvOrigin
-> Maybe ConcreteTvOrigin
-> Maybe ConcreteTvOrigin
forall a. (a -> a -> a) -> Maybe a -> Maybe a -> Maybe a
combineMaybe ConcreteTvOrigin -> ConcreteTvOrigin -> ConcreteTvOrigin
forall a b. a -> b -> a
const Maybe ConcreteTvOrigin
c1 Maybe ConcreteTvOrigin
c2) -- pick one 'ConcreteTvOrigin' arbitrarily

combineMaybe :: (a -> a -> a) -> Maybe a -> Maybe a -> Maybe a
combineMaybe :: forall a. (a -> a -> a) -> Maybe a -> Maybe a -> Maybe a
combineMaybe a -> a -> a
_ Maybe a
Nothing Maybe a
r = Maybe a
r
combineMaybe a -> a -> a
_ Maybe a
r Maybe a
Nothing = Maybe a
r
combineMaybe a -> a -> a
f (Just a
a) (Just a
b) = a -> Maybe a
forall a. a -> Maybe a
Just (a -> a -> a
f a
a a
b)
instance Monoid TyVarCheckResult where
  mempty :: TyVarCheckResult
mempty = TyVarCheckResult
TyVarCheck_Success

checkTyVar :: forall a. TyEqFlags a -> TcTyVar -> TcM (PuResult a Reduction)
checkTyVar :: forall a. TyEqFlags a -> TcTyVar -> TcM (PuResult a Reduction)
checkTyVar flags :: TyEqFlags a
flags@(TEF { tef_task :: forall a. TyEqFlags a -> TEFTask
tef_task = TEFTask
task }) TcTyVar
occ_tv
  = case TEFTask
task of
      TEFTyFam {}                   -> TcM (PuResult a Reduction)
success   -- Nothing to do if the LHS is a type-family
      TEFTyVar Maybe (Name, CheckTyEqProblem)
mb_occ Maybe (TcLevel, LevelCheck)
mb_lc Maybe ConcreteTvOrigin
mb_conc -> Maybe (Name, CheckTyEqProblem)
-> Maybe (TcLevel, LevelCheck)
-> Maybe ConcreteTvOrigin
-> TcM (PuResult a Reduction)
check_tv Maybe (Name, CheckTyEqProblem)
mb_occ Maybe (TcLevel, LevelCheck)
mb_lc Maybe ConcreteTvOrigin
mb_conc
  where
    lvl_occ :: TcLevel
lvl_occ = TcTyVar -> TcLevel
tcTyVarLevel TcTyVar
occ_tv
    success :: TcM (PuResult a Reduction)
success = TcType -> TcM (PuResult a Reduction)
forall a. TcType -> TcM (PuResult a Reduction)
okCheckRefl (TcTyVar -> TcType
mkTyVarTy TcTyVar
occ_tv)

    ---------------------
    check_tv :: Maybe (Name, CheckTyEqProblem)
-> Maybe (TcLevel, LevelCheck)
-> Maybe ConcreteTvOrigin
-> TcM (PuResult a Reduction)
check_tv Maybe (Name, CheckTyEqProblem)
mb_occ Maybe (TcLevel, LevelCheck)
mb_lc Maybe ConcreteTvOrigin
mb_conc
      = do { mb_done <- TcTyVar -> IOEnv (Env TcGblEnv TcLclEnv) (Maybe TcType)
isFilledMetaTyVar_maybe TcTyVar
occ_tv
           ; case mb_done of
               Just {} -> TcM (PuResult a Reduction)
success
               -- Already promoted; job done
               -- Example alpha[2] ~ Maybe (beta[4], beta[4])
               -- We promote the first occurrence, and then encounter it
               -- a second time; we don't want to re-promote it!
               -- Remember, the entire process started with a fully zonked type

               Maybe TcType
Nothing ->
                [TyVarCheckResult] -> TcM (PuResult a Reduction)
do_rhs_checks ([TyVarCheckResult] -> TcM (PuResult a Reduction))
-> [TyVarCheckResult] -> TcM (PuResult a Reduction)
forall a b. (a -> b) -> a -> b
$
                  -- Occurs check
                  [ Name -> CheckTyEqProblem -> TyVarCheckResult
simple_occurs_check Name
tv CheckTyEqProblem
occ_prob | (Name
tv, CheckTyEqProblem
occ_prob) <- Maybe (Name, CheckTyEqProblem) -> [(Name, CheckTyEqProblem)]
forall a. Maybe a -> [a]
maybeToList Maybe (Name, CheckTyEqProblem)
mb_occ ]
                  [TyVarCheckResult] -> [TyVarCheckResult] -> [TyVarCheckResult]
forall a. [a] -> [a] -> [a]
++
                  -- Level check
                  [ (TcLevel, LevelCheck) -> TyVarCheckResult
lvl_check (TcLevel, LevelCheck)
lc | (TcLevel, LevelCheck)
lc <- Maybe (TcLevel, LevelCheck) -> [(TcLevel, LevelCheck)]
forall a. Maybe a -> [a]
maybeToList Maybe (TcLevel, LevelCheck)
mb_lc ]
                  [TyVarCheckResult] -> [TyVarCheckResult] -> [TyVarCheckResult]
forall a. [a] -> [a] -> [a]
++
                  -- Concreteness check
                  [ ConcreteTvOrigin -> TyVarCheckResult
conc_check ConcreteTvOrigin
conc | ConcreteTvOrigin
conc <- Maybe ConcreteTvOrigin -> [ConcreteTvOrigin]
forall a. Maybe a -> [a]
maybeToList Maybe ConcreteTvOrigin
mb_conc ]
           }

    ---------------------
    simple_occurs_check :: Name -> CheckTyEqProblem -> TyVarCheckResult
    simple_occurs_check :: Name -> CheckTyEqProblem -> TyVarCheckResult
simple_occurs_check Name
lhs_tv CheckTyEqProblem
occ_prob
      | Name
lhs_tv Name -> Name -> Bool
forall a. Eq a => a -> a -> Bool
== TcTyVar -> Name
tyVarName TcTyVar
occ_tv Bool -> Bool -> Bool
|| TcType -> Bool
check_kind (TcTyVar -> TcType
tyVarKind TcTyVar
occ_tv)
      = CheckTyEqResult -> TyVarCheckResult
TyVarCheck_Error (CheckTyEqProblem -> CheckTyEqResult
cteProblem CheckTyEqProblem
occ_prob)
      | Bool
otherwise
      = TyVarCheckResult
TyVarCheck_Success
      where (TcType -> Bool
check_kind, Coercion -> Bool
_) = Name -> (TcType -> Bool, Coercion -> Bool)
mkOccFolders Name
lhs_tv
    conc_check :: ConcreteTvOrigin -> TyVarCheckResult
    conc_check :: ConcreteTvOrigin -> TyVarCheckResult
conc_check ConcreteTvOrigin
conc_orig
      | Bool -> Bool
not (TcTyVar -> Bool
isConcreteTyVar TcTyVar
occ_tv)
      = if TcTyVar -> Bool
isMetaTyVar TcTyVar
occ_tv
        then Maybe TcLevel -> Maybe ConcreteTvOrigin -> TyVarCheckResult
TyVarCheck_Promote Maybe TcLevel
forall a. Maybe a
Nothing (ConcreteTvOrigin -> Maybe ConcreteTvOrigin
forall a. a -> Maybe a
Just ConcreteTvOrigin
conc_orig)
        else CheckTyEqResult -> TyVarCheckResult
TyVarCheck_Error (CheckTyEqProblem -> CheckTyEqResult
cteProblem CheckTyEqProblem
cteConcrete)
      | Bool
otherwise
      = TyVarCheckResult
TyVarCheck_Success
    lvl_check :: (TcLevel, LevelCheck) -> TyVarCheckResult
    lvl_check :: (TcLevel, LevelCheck) -> TyVarCheckResult
lvl_check (TcLevel
lhs_tv_lvl, LevelCheck
lc)
      | TcLevel
lvl_occ TcLevel -> TcLevel -> Bool
`strictlyDeeperThan` TcLevel
lhs_tv_lvl
      = case LevelCheck
lc of
           LevelCheck
LC_Check   -> CheckTyEqResult -> TyVarCheckResult
TyVarCheck_Error (CheckTyEqProblem -> CheckTyEqResult
cteProblem CheckTyEqProblem
cteSkolemEscape)
           LC_Promote {}
             | TcTyVar -> Bool
isSkolemTyVar TcTyVar
occ_tv  -> CheckTyEqResult -> TyVarCheckResult
TyVarCheck_Error (CheckTyEqProblem -> CheckTyEqResult
cteProblem CheckTyEqProblem
cteSkolemEscape)
             | Bool
otherwise             -> Maybe TcLevel -> Maybe ConcreteTvOrigin -> TyVarCheckResult
TyVarCheck_Promote (TcLevel -> Maybe TcLevel
forall a. a -> Maybe a
Just TcLevel
lhs_tv_lvl) Maybe ConcreteTvOrigin
forall a. Maybe a
Nothing
      | Bool
otherwise
      = TyVarCheckResult
TyVarCheck_Success

    -- Combine the results of individual checks. See 'TyVarCheckResult'.
    do_rhs_checks :: [TyVarCheckResult] -> TcM (PuResult a Reduction)
    do_rhs_checks :: [TyVarCheckResult] -> TcM (PuResult a Reduction)
do_rhs_checks [TyVarCheckResult]
checks =
      case [TyVarCheckResult] -> TyVarCheckResult
forall a. Monoid a => [a] -> a
mconcat [TyVarCheckResult]
checks of
        TyVarCheckResult
TyVarCheck_Success -> TcM (PuResult a Reduction)
success
        TyVarCheck_Promote Maybe TcLevel
mb_lvl Maybe ConcreteTvOrigin
mb_conc -> Maybe TcLevel
-> Maybe ConcreteTvOrigin -> TcM (PuResult a Reduction)
promote Maybe TcLevel
mb_lvl Maybe ConcreteTvOrigin
mb_conc
        TyVarCheck_Error CheckTyEqResult
cte_prob -> CheckTyEqResult -> TcM (PuResult a Reduction)
forall a b. CheckTyEqResult -> TcM (PuResult a b)
failCheckWith CheckTyEqResult
cte_prob

    ---------------------
    -- occ_tv is definitely a MetaTyVar; we need to promote it/make it concrete
    promote :: Maybe TcLevel -> Maybe ConcreteTvOrigin -> TcM (PuResult a Reduction)
    promote :: Maybe TcLevel
-> Maybe ConcreteTvOrigin -> TcM (PuResult a Reduction)
promote Maybe TcLevel
mb_lhs_tv_lvl Maybe ConcreteTvOrigin
mb_conc
      | MetaTv { mtv_info :: TcTyVarDetails -> MetaInfo
mtv_info = MetaInfo
info_occ, mtv_tclvl :: TcTyVarDetails -> TcLevel
mtv_tclvl = TcLevel
lvl_occ } <- TcTyVar -> TcTyVarDetails
tcTyVarDetails TcTyVar
occ_tv
      = do { let new_info :: MetaInfo
new_info | Just ConcreteTvOrigin
conc <- Maybe ConcreteTvOrigin
mb_conc = ConcreteTvOrigin -> MetaInfo
ConcreteTv ConcreteTvOrigin
conc
                          | Bool
otherwise            = MetaInfo
info_occ
                 new_lvl :: TcLevel
new_lvl =
                    case Maybe TcLevel
mb_lhs_tv_lvl of
                      Maybe TcLevel
Nothing         -> TcLevel
lvl_occ
                      Just TcLevel
lhs_tv_lvl -> TcLevel
lhs_tv_lvl TcLevel -> TcLevel -> TcLevel
`minTcLevel` TcLevel
lvl_occ
                        -- c[conc,3] ~ p[tau,2]: want to clone p:=p'[conc,2]
                        -- c[tau,2]  ~ p[tau,3]: want to clone p:=p'[tau,2]

           -- Check the kind of occ_tv
           --
           -- This is important for several reasons:
           --
           --  1. To ensure there is no occurs check or skolem-escape
           --     in the kind of occ_tv.
           --  2. If the LHS is a concrete type variable and the RHS is an
           --     unfilled meta-tyvar, we need to ensure that the kind of
           --     'occ_tv' is concrete.   Test cases: T23051, T23176.
           ; let occ_kind :: TcType
occ_kind = TcTyVar -> TcType
tyVarKind TcTyVar
occ_tv
           ; kind_result <- TyEqFlags a -> TcType -> TcM (PuResult a Reduction)
forall a. TyEqFlags a -> TcType -> TcM (PuResult a Reduction)
checkTyEqRhs TyEqFlags a
flags TcType
occ_kind
           ; traceTc "checkTyVar: kind check" $
               vcat [ text "occ_tv:" <+> ppr occ_tv <+> dcolon <+> ppr occ_kind
                    , text "checkTyEqRHS result:" <+> pprPur kind_result
                    ]
           ; for kind_result $ \ Reduction
kind_redn ->
        do { let kind_co :: Coercion
kind_co  = Reduction -> Coercion
reductionCoercion Reduction
kind_redn
                 new_kind :: TcType
new_kind = Reduction -> TcType
reductionReducedType Reduction
kind_redn
           ; new_tv_ty <- MetaInfo -> TcLevel -> TcTyVar -> TcM TcType
promote_meta_tyvar MetaInfo
new_info TcLevel
new_lvl (TcTyVar -> TcType -> TcTyVar
setTyVarKind TcTyVar
occ_tv TcType
new_kind)
           ; return $ mkGReflLeftRedn Nominal new_tv_ty (mkSymCo kind_co)
           } }

      | Bool
otherwise = String -> SDoc -> TcM (PuResult a Reduction)
forall a. HasCallStack => String -> SDoc -> a
pprPanic String
"promote" (TcTyVar -> SDoc
forall a. Outputable a => a -> SDoc
ppr TcTyVar
occ_tv)

-------------------------
checkPromoteFreeVars :: CheckTyEqProblem    -- What occurs check problem to report
                     -> Name -> TcLevel
                     -> TyCoVarSet -> TcM CheckTyEqResult
-- Check this set of TyCoVars for
--   (a) occurs check
--   (b) promote if necessary, or report skolem escape
checkPromoteFreeVars :: CheckTyEqProblem
-> Name -> TcLevel -> VarSet -> TcM CheckTyEqResult
checkPromoteFreeVars CheckTyEqProblem
occ_prob Name
lhs_tv TcLevel
lhs_tv_lvl VarSet
vs
  = do { oks <- (TcTyVar -> TcM CheckTyEqResult)
-> [TcTyVar] -> IOEnv (Env TcGblEnv TcLclEnv) [CheckTyEqResult]
forall (t :: * -> *) (m :: * -> *) a b.
(Traversable t, Monad m) =>
(a -> m b) -> t a -> m (t b)
forall (m :: * -> *) a b. Monad m => (a -> m b) -> [a] -> m [b]
mapM TcTyVar -> TcM CheckTyEqResult
do_one (VarSet -> [TcTyVar]
forall elt. UniqSet elt -> [elt]
nonDetEltsUniqSet VarSet
vs)
       ; return (mconcat oks) }
  where
    do_one :: TyCoVar -> TcM CheckTyEqResult
    do_one :: TcTyVar -> TcM CheckTyEqResult
do_one TcTyVar
v | TcTyVar -> Bool
isCoVar TcTyVar
v             = CheckTyEqResult -> TcM CheckTyEqResult
forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
forall (m :: * -> *) a. Monad m => a -> m a
return CheckTyEqResult
cteOK
             | TcTyVar -> Name
tyVarName TcTyVar
v Name -> Name -> Bool
forall a. Eq a => a -> a -> Bool
== Name
lhs_tv = CheckTyEqResult -> TcM CheckTyEqResult
forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
forall (m :: * -> *) a. Monad m => a -> m a
return (CheckTyEqProblem -> CheckTyEqResult
cteProblem CheckTyEqProblem
occ_prob)
             | Bool
no_promotion          = CheckTyEqResult -> TcM CheckTyEqResult
forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
forall (m :: * -> *) a. Monad m => a -> m a
return CheckTyEqResult
cteOK
             | Bool -> Bool
not (TcTyVar -> Bool
isMetaTyVar TcTyVar
v)   = CheckTyEqResult -> TcM CheckTyEqResult
forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
forall (m :: * -> *) a. Monad m => a -> m a
return (CheckTyEqProblem -> CheckTyEqResult
cteProblem CheckTyEqProblem
cteSkolemEscape)
             | Bool
otherwise             = TcTyVar -> TcM CheckTyEqResult
promote_one TcTyVar
v
      where
        no_promotion :: Bool
no_promotion = Bool -> Bool
not (TcTyVar -> TcLevel
tcTyVarLevel TcTyVar
v TcLevel -> TcLevel -> Bool
`strictlyDeeperThan` TcLevel
lhs_tv_lvl)

    -- isCoVar case: coercion variables are not an escape risk
    -- If an implication binds a coercion variable, it'll have equalities,
    -- so the "intervening given equalities" test above will catch it
    -- Coercion holes get filled with coercions, so again no problem.

    promote_one :: TcTyVar -> TcM CheckTyEqResult
promote_one TcTyVar
tv = do { _ <- MetaInfo -> TcLevel -> TcTyVar -> TcM TcType
promote_meta_tyvar MetaInfo
TauTv TcLevel
lhs_tv_lvl TcTyVar
tv
                        ; return cteOK }

promote_meta_tyvar :: MetaInfo -> TcLevel -> TcTyVar -> TcM TcType
promote_meta_tyvar :: MetaInfo -> TcLevel -> TcTyVar -> TcM TcType
promote_meta_tyvar MetaInfo
info TcLevel
dest_lvl TcTyVar
occ_tv
  = do { -- Check whether occ_tv is already unified. The rhs-type
         -- started zonked, but we may have promoted one of its type
         -- variables, and we then encounter it for the second time.
         -- But if so, it'll definitely be another already-checked TyVar
         mb_filled <- TcTyVar -> IOEnv (Env TcGblEnv TcLclEnv) (Maybe TcType)
isFilledMetaTyVar_maybe TcTyVar
occ_tv
       ; case mb_filled of {
           Just TcType
ty -> TcType -> TcM TcType
forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
forall (m :: * -> *) a. Monad m => a -> m a
return TcType
ty ;
           Maybe TcType
Nothing ->

    -- OK, not done already, so clone/promote it
    do { new_tv <- MetaInfo -> TcLevel -> TcTyVar -> TcM TcTyVar
cloneMetaTyVarWithInfo MetaInfo
info TcLevel
dest_lvl TcTyVar
occ_tv
       ; liftZonkM $ writeMetaTyVar occ_tv (mkTyVarTy new_tv)
       ; traceTc "promoteTyVar" (ppr occ_tv <+> text "-->" <+> ppr new_tv)
       ; return (mkTyVarTy new_tv) } } }



-------------------------
touchabilityAndShapeTest :: TcLevel -> TcTyVar -> TcType -> Bool
-- This is the key test for untouchability:
-- See Note [Unification preconditions] in GHC.Tc.Utils.Unify
-- and Note [Solve by unification] in GHC.Tc.Solver.Equality
-- True <=> touchability and shape are OK
touchabilityAndShapeTest :: TcLevel -> TcTyVar -> TcType -> Bool
touchabilityAndShapeTest TcLevel
given_eq_lvl TcTyVar
tv TcType
rhs
  | MetaTv { mtv_info :: TcTyVarDetails -> MetaInfo
mtv_info = MetaInfo
info, mtv_tclvl :: TcTyVarDetails -> TcLevel
mtv_tclvl = TcLevel
tv_lvl } <- TcTyVar -> TcTyVarDetails
tcTyVarDetails TcTyVar
tv
  , TcLevel
tv_lvl TcLevel -> TcLevel -> Bool
`deeperThanOrSame` TcLevel
given_eq_lvl
  , MetaInfo -> TcType -> Bool
checkTopShape MetaInfo
info TcType
rhs
  = Bool
True
  | Bool
otherwise
  = Bool
False

-------------------------
-- | checkTopShape checks (TYVAR-TV)
-- Note [Unification preconditions]; returns True if these conditions
-- are satisfied. But see the Note for other preconditions, too.
checkTopShape :: MetaInfo -> TcType -> Bool
checkTopShape :: MetaInfo -> TcType -> Bool
checkTopShape MetaInfo
info TcType
xi
  = case MetaInfo
info of
      MetaInfo
TyVarTv ->
        case TcType -> Maybe TcTyVar
getTyVar_maybe TcType
xi of   -- Looks through type synonyms
           Maybe TcTyVar
Nothing -> Bool
False
           Just TcTyVar
tv -> case TcTyVar -> TcTyVarDetails
tcTyVarDetails TcTyVar
tv of -- (TYVAR-TV) wrinkle
                        SkolemTv {} -> Bool
True
                        TcTyVarDetails
RuntimeUnk  -> Bool
True
                        MetaTv { mtv_info :: TcTyVarDetails -> MetaInfo
mtv_info = MetaInfo
TyVarTv } -> Bool
True
                        TcTyVarDetails
_                             -> Bool
False
      MetaInfo
CycleBreakerTv -> Bool
False  -- We never unify these
      MetaInfo
_ -> Bool
True

--------------------------------------------------------------------------------
-- Making a type concrete.

-- | Try to turn the provided type into a concrete type, by ensuring
-- unfilled metavariables are appropriately marked as concrete.
--
-- Returns a coercion whose RHS is a zonked type which is "as concrete as possible",
-- and a collection of Wanted equality constraints that are necessary to make
-- the type concrete.
--
-- For example, for an input @TYPE a[sk]@ we will return a coercion with RHS
-- @TYPE gamma[conc]@ together with the Wanted equality constraint @a ~# gamma@.
--
-- INVARIANT: the RHS type of the returned coercion is equal to the input type,
-- up to zonking and the returned Wanted equality constraints.
--
-- INVARIANT: if this function returns an empty list of constraints
-- then the RHS type of the returned coercion is concrete,
-- in the sense of Note [Concrete types].
makeTypeConcrete :: FastString -> ConcreteTvOrigin
                 -> TcType -> TcM (TcCoercion, Cts)
makeTypeConcrete :: FastString -> ConcreteTvOrigin -> TcType -> TcM (Coercion, Bag Ct)
makeTypeConcrete FastString
occ_fs ConcreteTvOrigin
conc_orig TcType
ty =
  do { String -> SDoc -> TcM ()
traceTc String
"makeTypeConcrete {" (SDoc -> TcM ()) -> SDoc -> TcM ()
forall a b. (a -> b) -> a -> b
$
        [SDoc] -> SDoc
forall doc. IsDoc doc => [doc] -> doc
vcat [ String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"ty:" SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> TcType -> SDoc
forall a. Outputable a => a -> SDoc
ppr TcType
ty ]

     -- To make a type 'ty' concrete, we query what would happen were we
     -- to try unifying
     --
     --   alpha[conc] ~# ty
     --
     -- for a fresh concrete metavariable 'alpha'.
     --
     -- We do this by calling 'checkTyEqRhs' with suitable 'TyEqFlags'.
     -- NB: we don't actually need to create a fresh concrete metavariable
     -- in order to call 'checkTyEqRhs'.
     ; let ty_eq_flags :: TyEqFlags ()
ty_eq_flags =
             TEF { tef_task :: TEFTask
tef_task =
                    TEFTyVar
                      { tefTyVar_occursCheck :: Maybe (Name, CheckTyEqProblem)
tefTyVar_occursCheck   = Maybe (Name, CheckTyEqProblem)
forall a. Maybe a
Nothing
                          -- LHS is a fresh meta-tyvar: no occurs check needed
                      , tefTyVar_levelCheck :: Maybe (TcLevel, LevelCheck)
tefTyVar_levelCheck    = Maybe (TcLevel, LevelCheck)
forall a. Maybe a
Nothing
                      , tefTyVar_concreteCheck :: Maybe ConcreteTvOrigin
tefTyVar_concreteCheck = ConcreteTvOrigin -> Maybe ConcreteTvOrigin
forall a. a -> Maybe a
Just ConcreteTvOrigin
conc_orig
                      }
                 , tef_fam_app :: TyEqFamApp ()
tef_fam_app = TyEqFamApp ()
forall a. TyEqFamApp a
TEFA_Fail
                 }

     -- NB: 'checkTyEqRhs' expects a fully zonked type as input.
     ; ty' <- ZonkM TcType -> TcM TcType
forall a. ZonkM a -> TcM a
liftZonkM (ZonkM TcType -> TcM TcType) -> ZonkM TcType -> TcM TcType
forall a b. (a -> b) -> a -> b
$ TcType -> ZonkM TcType
zonkTcType TcType
ty
     ; pu_res <- checkTyEqRhs @() ty_eq_flags ty'
     -- NB: 'checkTyEqRhs' will also check the kind, thus upholding the
     -- invariant that the kind of a concrete type must also be a concrete type.

     ; (cts, final_co) <-
         case pu_res of
           PuOK Bag ()
_ Reduction
redn ->
            do { String -> SDoc -> TcM ()
traceTc String
"makeTypeConcrete: unifier success" (SDoc -> TcM ()) -> SDoc -> TcM ()
forall a b. (a -> b) -> a -> b
$
                   [SDoc] -> SDoc
forall doc. IsDoc doc => [doc] -> doc
vcat [ String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"ty:" SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> TcType -> SDoc
forall a. Outputable a => a -> SDoc
ppr TcType
ty SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> SDoc
dcolon SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> TcType -> SDoc
forall a. Outputable a => a -> SDoc
ppr (HasDebugCallStack => TcType -> TcType
TcType -> TcType
typeKind TcType
ty)
                        , String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"redn:" SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> Reduction -> SDoc
forall a. Outputable a => a -> SDoc
ppr Reduction
redn
                        ]
               ; (Bag Ct, Coercion)
-> IOEnv (Env TcGblEnv TcLclEnv) (Bag Ct, Coercion)
forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
forall (m :: * -> *) a. Monad m => a -> m a
return (Bag Ct
forall a. Bag a
emptyBag, Coercion -> Coercion
mkSymCo (Coercion -> Coercion) -> Coercion -> Coercion
forall a b. (a -> b) -> a -> b
$ Reduction -> Coercion
reductionCoercion Reduction
redn)
                  -- NB: the unifier returns a 'Reduction' with the concrete
                  -- type on the left, but we want a coercion with it on the
                  -- right; so we use 'mkSymCo'.
               }
           PuFail CheckTyEqResult
_prob ->
             do { String -> SDoc -> TcM ()
traceTc String
"makeTypeConcrete: unifier failure" (SDoc -> TcM ()) -> SDoc -> TcM ()
forall a b. (a -> b) -> a -> b
$
                   [SDoc] -> SDoc
forall doc. IsDoc doc => [doc] -> doc
vcat [ String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"ty:" SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> TcType -> SDoc
forall a. Outputable a => a -> SDoc
ppr TcType
ty SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> SDoc
dcolon SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> TcType -> SDoc
forall a. Outputable a => a -> SDoc
ppr (HasDebugCallStack => TcType -> TcType
TcType -> TcType
typeKind TcType
ty)
                        , String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"problem:" SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> CheckTyEqResult -> SDoc
forall a. Outputable a => a -> SDoc
ppr CheckTyEqResult
_prob
                        ]
                  -- We failed to make 'ty' concrete. In order to continue
                  -- typechecking, we proceed as follows:
                  --
                  --   - create a new concrete metavariable alpha[conc]
                  --   - emit the equality @ty ~# alpha[conc]@.
                  --
                  -- This equality will eventually get reported as insoluble
                  -- to the user.

                -- The kind of a concrete metavariable must itself be concrete,
                -- so we need to do a concreteness check on the kind first.
                ; let ki :: TcType
ki = HasDebugCallStack => TcType -> TcType
TcType -> TcType
typeKind TcType
ty
                ; (kind_co, kind_cts) <-
                    if TcType -> Bool
isConcreteType TcType
ki
                    then (Coercion, Bag Ct) -> TcM (Coercion, Bag Ct)
forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
forall (m :: * -> *) a. Monad m => a -> m a
return (TcType -> Coercion
mkNomReflCo TcType
ki, Bag Ct
forall a. Bag a
emptyBag)
                    else FastString -> ConcreteTvOrigin -> TcType -> TcM (Coercion, Bag Ct)
makeTypeConcrete FastString
occ_fs ConcreteTvOrigin
conc_orig TcType
ki

                -- Now create the new concrete metavariable.
                ; conc_tv <- newConcreteTyVar conc_orig occ_fs (coercionRKind kind_co)
                ; let conc_ty = TcTyVar -> TcType
mkTyVarTy TcTyVar
conc_tv
                      pty = Role -> TcType -> TcType -> TcType
mkEqPredRole Role
Nominal TcType
ty' TcType
conc_ty
                ; hole <- newCoercionHoleO orig pty
                ; loc <- getCtLocM orig (Just KindLevel)
                ; let ct = CtEvidence -> Ct
mkNonCanonical
                         (CtEvidence -> Ct) -> CtEvidence -> Ct
forall a b. (a -> b) -> a -> b
$ CtWanted { ctev_pred :: TcType
ctev_pred      = TcType
pty
                                    , ctev_dest :: TcEvDest
ctev_dest      = CoercionHole -> TcEvDest
HoleDest CoercionHole
hole
                                    , ctev_loc :: CtLoc
ctev_loc       = CtLoc
loc
                                    , ctev_rewriters :: RewriterSet
ctev_rewriters = RewriterSet
emptyRewriterSet }
                ; return (kind_cts S.<> unitBag ct, HoleCo hole)
                }

     ; traceTc "makeTypeConcrete }" $
        vcat [ text "ty :" <+> _ppr_ty ty
             , text "ty':" <+> _ppr_ty ty'
             , text "final_co:" <+> _ppr_co final_co ]

     ; return (final_co, cts)
     }
  where

    _ppr_ty :: TcType -> SDoc
_ppr_ty TcType
ty = TcType -> SDoc
forall a. Outputable a => a -> SDoc
ppr TcType
ty SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> SDoc
dcolon SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> TcType -> SDoc
forall a. Outputable a => a -> SDoc
ppr (HasDebugCallStack => TcType -> TcType
TcType -> TcType
typeKind TcType
ty)
    _ppr_co :: Coercion -> SDoc
_ppr_co Coercion
co = Coercion -> SDoc
forall a. Outputable a => a -> SDoc
ppr Coercion
co SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> SDoc
dcolon SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc
parens (TcType -> SDoc
_ppr_ty (HasDebugCallStack => Coercion -> TcType
Coercion -> TcType
coercionLKind Coercion
co)) SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"~#" SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc
parens (TcType -> SDoc
_ppr_ty (HasDebugCallStack => Coercion -> TcType
Coercion -> TcType
coercionRKind Coercion
co))

    orig :: CtOrigin
    orig :: CtOrigin
orig = case ConcreteTvOrigin
conc_orig of
      ConcreteFRR FixedRuntimeRepOrigin
frr_orig -> FixedRuntimeRepOrigin -> CtOrigin
FRROrigin FixedRuntimeRepOrigin
frr_orig