{-# LANGUAGE CPP #-}
{-# LANGUAGE DuplicateRecordFields #-}
{-# LANGUAGE LambdaCase #-}
{-# LANGUAGE MultiWayIf #-}

module GHC.Tc.Solver.Equality(
     solveEquality
  ) where


import GHC.Prelude

import {-# SOURCE #-} GHC.Tc.Solver.Solve( solveSimpleWanteds )

import GHC.Tc.Solver.Irred( solveIrred )
import GHC.Tc.Solver.Dict( matchLocalInst, chooseInstance )
import GHC.Tc.Solver.Rewrite
import GHC.Tc.Solver.Monad
import GHC.Tc.Solver.FunDeps( tryEqFunDeps )
import GHC.Tc.Solver.InertSet
import GHC.Tc.Types.Evidence
import GHC.Tc.Types.Constraint
import GHC.Tc.Types.CtLoc
import GHC.Tc.Types.Origin
import GHC.Tc.Utils.Unify
import GHC.Tc.Utils.TcType
import GHC.Tc.Instance.Family ( tcTopNormaliseNewTypeTF_maybe )
import qualified GHC.Tc.Utils.Monad    as TcM

import GHC.Core.Type
import GHC.Core.Predicate
import GHC.Core.Class
import GHC.Core.DataCon ( dataConName )
import GHC.Core.TyCon
import GHC.Core.TyCo.Rep   -- cleverly decomposes types, good for completeness checking
import GHC.Core.Coercion
import GHC.Core.Reduction
import GHC.Core.FamInstEnv ( FamInstEnvs )
import GHC.Core
import GHC.Types.Var
import GHC.Types.Var.Env
import GHC.Types.Var.Set
import GHC.Types.Name.Reader
import GHC.Types.Basic

import GHC.Utils.Outputable
import GHC.Utils.Panic
import GHC.Utils.Misc
import GHC.Utils.Monad

import GHC.Data.Pair
import GHC.Data.Bag
import Control.Monad
import Data.Maybe ( isJust, isNothing )
import Data.List  ( zip4 )

import qualified Data.Semigroup as S
import Data.Bifunctor ( bimap )
import Data.Void( Void )

{- *********************************************************************
*                                                                      *
*        Equalities
*                                                                      *
************************************************************************

Note [Canonicalising equalities]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In order to canonicalise an equality, we look at the structure of the
two types at hand, looking for similarities. A difficulty is that the
types may look dissimilar before rewriting but similar after rewriting.
However, we don't just want to jump in and rewrite right away, because
this might be wasted effort. So, after looking for similarities and failing,
we rewrite and then try again. Of course, we don't want to loop, so we
track whether or not we've already rewritten.

It is conceivable to do a better job at tracking whether or not a type
is rewritten, but this is left as future work. (Mar '15)

Note [Decomposing FunTy]
~~~~~~~~~~~~~~~~~~~~~~~~
can_eq_nc' may attempt to decompose a FunTy that is un-zonked.  This
means that we may very well have a FunTy containing a type of some
unknown kind. For instance, we may have,

    FunTy (a :: k) Int

Where k is a unification variable. So the calls to splitRuntimeRep_maybe may
fail (returning Nothing).  In that case we'll fall through, zonk, and try again.
Zonking should fill the variable k, meaning that decomposition will succeed the
second time around.

Also note that we require the FunTyFlag to match.  This will stop
us decomposing
   (Int -> Bool)  ~  (Show a => blah)
It's as if we treat (->) and (=>) as different type constructors, which
indeed they are!
-}

solveEquality :: CtEvidence -> EqRel -> Type -> Type
              -> SolverStage Void
solveEquality :: CtEvidence -> EqRel -> PredType -> PredType -> SolverStage Void
solveEquality CtEvidence
ev EqRel
eq_rel PredType
ty1 PredType
ty2
  = do { Pair ty1' ty2' <- CtEvidence
-> EqRel -> PredType -> PredType -> SolverStage (Pair PredType)
zonkEqTypes CtEvidence
ev EqRel
eq_rel PredType
ty1 PredType
ty2
       ; mb_canon <- canonicaliseEquality ev eq_rel ty1' ty2'

       ; case mb_canon of {

            -- An IrredCt equality may be insoluble; but maybe not!
            -- E.g.  m a ~R# m b  is not canonical, but may be
            --       solved by a quantified constraint (T15290)
            -- See Note [Looking up primitive equalities in quantified constraints]
            Left IrredCt
irred_ct -> do { IrredCt -> SolverStage ()
tryQCsIrredEqCt IrredCt
irred_ct
                                ; IrredCt -> SolverStage Void
solveIrred IrredCt
irred_ct } ;

            Right EqCt
eq_ct   -> do { EqCt -> SolverStage ()
tryInertEqs  EqCt
eq_ct
                                ; EqCt -> SolverStage ()
tryEqFunDeps EqCt
eq_ct
                                ; EqCt -> SolverStage ()
tryQCsEqCt   EqCt
eq_ct
                                ; TcS () -> SolverStage ()
forall a. TcS a -> SolverStage a
simpleStage (EqCt -> TcS ()
updInertEqs EqCt
eq_ct)
                                ; CtEvidence -> String -> SolverStage Void
forall a. CtEvidence -> String -> SolverStage a
stopWithStage (EqCt -> CtEvidence
eqCtEvidence EqCt
eq_ct) String
"Kept inert EqCt" } } }

updInertEqs :: EqCt -> TcS ()
updInertEqs :: EqCt -> TcS ()
updInertEqs EqCt
eq_ct
  = do { KickOutSpec -> CtFlavourRole -> TcS ()
kickOutRewritable (CanEqLHS -> KickOutSpec
KOAfterAdding (EqCt -> CanEqLHS
eqCtLHS EqCt
eq_ct)) (EqCt -> CtFlavourRole
eqCtFlavourRole EqCt
eq_ct)
       ; tc_lvl <- TcS TcLevel
getTcLevel
       ; updInertCans (addEqToCans tc_lvl eq_ct) }


{- *********************************************************************
*                                                                      *
*           zonkEqTypes
*                                                                      *
********************************************************************* -}

---------------------------------
-- | Compare types for equality, while zonking as necessary. Gives up
-- as soon as it finds that two types are not equal.
-- This is quite handy when some unification has made two
-- types in an inert Wanted to be equal. We can discover the equality without
-- rewriting, which is sometimes very expensive (in the case of type functions).
-- In particular, this function makes a ~20% improvement in test case
-- perf/compiler/T5030.
--
-- Returns either the (partially zonked) types in the case of
-- inequality, or the one type in the case of equality. canEqReflexive is
-- a good next step in the 'Right' case. Returning 'Left' is always safe.
--
-- NB: This does *not* look through type synonyms. In fact, it treats type
-- synonyms as rigid constructors. In the future, it might be convenient
-- to look at only those arguments of type synonyms that actually appear
-- in the synonym RHS. But we're not there yet.
zonkEqTypes :: CtEvidence -> EqRel -> TcType -> TcType -> SolverStage (Pair TcType)
zonkEqTypes :: CtEvidence
-> EqRel -> PredType -> PredType -> SolverStage (Pair PredType)
zonkEqTypes CtEvidence
ev EqRel
eq_rel PredType
ty1 PredType
ty2
  = TcS (StopOrContinue (Pair PredType)) -> SolverStage (Pair PredType)
TcS (StopOrContinue (Pair PredType)) -> SolverStage (Pair PredType)
forall a. TcS (StopOrContinue a) -> SolverStage a
Stage (TcS (StopOrContinue (Pair PredType))
 -> SolverStage (Pair PredType))
-> TcS (StopOrContinue (Pair PredType))
-> SolverStage (Pair PredType)
forall a b. (a -> b) -> a -> b
$ do { res <- PredType -> PredType -> TcS (Either (Pair PredType) PredType)
go PredType
ty1 PredType
ty2
               ; case res of
                    Left Pair PredType
pair -> Pair PredType -> TcS (StopOrContinue (Pair PredType))
forall a. a -> TcS (StopOrContinue a)
continueWith Pair PredType
pair
                    Right PredType
ty  -> CtEvidence
-> EqRel -> PredType -> TcS (StopOrContinue (Pair PredType))
forall a. CtEvidence -> EqRel -> PredType -> TcS (StopOrContinue a)
canEqReflexive CtEvidence
ev EqRel
eq_rel PredType
ty }
  where
    go :: TcType -> TcType -> TcS (Either (Pair TcType) TcType)
    go :: PredType -> PredType -> TcS (Either (Pair PredType) PredType)
go (TyVarTy TyVar
tv1) (TyVarTy TyVar
tv2) = TyVar -> TyVar -> TcS (Either (Pair PredType) PredType)
tyvar_tyvar TyVar
tv1 TyVar
tv2
    go (TyVarTy TyVar
tv1) PredType
ty2           = SwapFlag
-> TyVar -> PredType -> TcS (Either (Pair PredType) PredType)
tyvar SwapFlag
NotSwapped TyVar
tv1 PredType
ty2
    go PredType
ty1 (TyVarTy TyVar
tv2)           = SwapFlag
-> TyVar -> PredType -> TcS (Either (Pair PredType) PredType)
tyvar SwapFlag
IsSwapped  TyVar
tv2 PredType
ty1

    -- We handle FunTys explicitly here despite the fact that they could also be
    -- treated as an application. Why? Well, for one it's cheaper to just look
    -- at two types (the argument and result types) than four (the argument,
    -- result, and their RuntimeReps). Also, we haven't completely zonked yet,
    -- so we may run into an unzonked type variable while trying to compute the
    -- RuntimeReps of the argument and result types. This can be observed in
    -- testcase tc269.
    go (FunTy FunTyFlag
af1 PredType
w1 PredType
arg1 PredType
res1) (FunTy FunTyFlag
af2 PredType
w2 PredType
arg2 PredType
res2)
      | FunTyFlag
af1 FunTyFlag -> FunTyFlag -> Bool
forall a. Eq a => a -> a -> Bool
== FunTyFlag
af2
      , HasCallStack => PredType -> PredType -> Bool
PredType -> PredType -> Bool
eqType PredType
w1 PredType
w2
      = do { res_a <- PredType -> PredType -> TcS (Either (Pair PredType) PredType)
go PredType
arg1 PredType
arg2
           ; res_b <- go res1 res2
           ; return $ combine_rev (FunTy af1 w1) res_b res_a }

    go ty1 :: PredType
ty1@(FunTy {}) PredType
ty2 = PredType -> PredType -> TcS (Either (Pair PredType) PredType)
forall {m :: * -> *} {a} {b}.
Monad m =>
a -> a -> m (Either (Pair a) b)
bale_out PredType
ty1 PredType
ty2
    go PredType
ty1 ty2 :: PredType
ty2@(FunTy {}) = PredType -> PredType -> TcS (Either (Pair PredType) PredType)
forall {m :: * -> *} {a} {b}.
Monad m =>
a -> a -> m (Either (Pair a) b)
bale_out PredType
ty1 PredType
ty2

    go PredType
ty1 PredType
ty2
      | Just (TyCon
tc1, [PredType]
tys1) <- HasDebugCallStack => PredType -> Maybe (TyCon, [PredType])
PredType -> Maybe (TyCon, [PredType])
splitTyConAppNoView_maybe PredType
ty1
      , Just (TyCon
tc2, [PredType]
tys2) <- HasDebugCallStack => PredType -> Maybe (TyCon, [PredType])
PredType -> Maybe (TyCon, [PredType])
splitTyConAppNoView_maybe PredType
ty2
      = if TyCon
tc1 TyCon -> TyCon -> Bool
forall a. Eq a => a -> a -> Bool
== TyCon
tc2 Bool -> Bool -> Bool
&& [PredType]
tys1 [PredType] -> [PredType] -> Bool
forall a b. [a] -> [b] -> Bool
`equalLength` [PredType]
tys2
          -- Crucial to check for equal-length args, because
          -- we cannot assume that the two args to 'go' have
          -- the same kind.  E.g go (Proxy *      (Maybe Int))
          --                        (Proxy (*->*) Maybe)
          -- We'll call (go (Maybe Int) Maybe)
          -- See #13083
        then TyCon
-> [PredType]
-> [PredType]
-> TcS (Either (Pair PredType) PredType)
tycon TyCon
tc1 [PredType]
tys1 [PredType]
tys2
        else PredType -> PredType -> TcS (Either (Pair PredType) PredType)
forall {m :: * -> *} {a} {b}.
Monad m =>
a -> a -> m (Either (Pair a) b)
bale_out PredType
ty1 PredType
ty2

    -- If you are temppted to add a case for AppTy/AppTy, be careful
    -- See Note [zonkEqTypes and the PKTI]

    go ty1 :: PredType
ty1@(LitTy TyLit
lit1) (LitTy TyLit
lit2)
      | TyLit
lit1 TyLit -> TyLit -> Bool
forall a. Eq a => a -> a -> Bool
== TyLit
lit2
      = Either (Pair PredType) PredType
-> TcS (Either (Pair PredType) PredType)
forall a. a -> TcS a
forall (m :: * -> *) a. Monad m => a -> m a
return (PredType -> Either (Pair PredType) PredType
forall a b. b -> Either a b
Right PredType
ty1)

    go PredType
ty1 PredType
ty2 = PredType -> PredType -> TcS (Either (Pair PredType) PredType)
forall {m :: * -> *} {a} {b}.
Monad m =>
a -> a -> m (Either (Pair a) b)
bale_out PredType
ty1 PredType
ty2
      -- We don't handle more complex forms here

    bale_out :: a -> a -> m (Either (Pair a) b)
bale_out a
ty1 a
ty2 = Either (Pair a) b -> m (Either (Pair a) b)
forall a. a -> m a
forall (m :: * -> *) a. Monad m => a -> m a
return (Either (Pair a) b -> m (Either (Pair a) b))
-> Either (Pair a) b -> m (Either (Pair a) b)
forall a b. (a -> b) -> a -> b
$ Pair a -> Either (Pair a) b
forall a b. a -> Either a b
Left (a -> a -> Pair a
forall a. a -> a -> Pair a
Pair a
ty1 a
ty2)

    tyvar :: SwapFlag -> TcTyVar -> TcType
          -> TcS (Either (Pair TcType) TcType)
      -- Try to do as little as possible, as anything we do here is redundant
      -- with rewriting. In particular, no need to zonk kinds. That's why
      -- we don't use the already-defined zonking functions
    tyvar :: SwapFlag
-> TyVar -> PredType -> TcS (Either (Pair PredType) PredType)
tyvar SwapFlag
swapped TyVar
tv PredType
ty
      = case TyVar -> TcTyVarDetails
tcTyVarDetails TyVar
tv of
          MetaTv { mtv_ref :: TcTyVarDetails -> IORef MetaDetails
mtv_ref = IORef MetaDetails
ref }
            -> do { cts <- IORef MetaDetails -> TcS MetaDetails
forall a. TcRef a -> TcS a
readTcRef IORef MetaDetails
ref
                  ; case cts of
                      MetaDetails
Flexi        -> TcS (Either (Pair PredType) PredType)
give_up
                      Indirect PredType
ty' -> do { TyVar -> PredType -> TcS ()
forall {a} {a}. (Outputable a, Outputable a) => a -> a -> TcS ()
trace_indirect TyVar
tv PredType
ty'
                                         ; SwapFlag
-> (PredType -> PredType -> TcS (Either (Pair PredType) PredType))
-> PredType
-> PredType
-> TcS (Either (Pair PredType) PredType)
forall a b. SwapFlag -> (a -> a -> b) -> a -> a -> b
unSwap SwapFlag
swapped PredType -> PredType -> TcS (Either (Pair PredType) PredType)
go PredType
ty' PredType
ty } }
          TcTyVarDetails
_ -> TcS (Either (Pair PredType) PredType)
give_up
      where
        give_up :: TcS (Either (Pair PredType) PredType)
give_up = Either (Pair PredType) PredType
-> TcS (Either (Pair PredType) PredType)
forall a. a -> TcS a
forall (m :: * -> *) a. Monad m => a -> m a
return (Either (Pair PredType) PredType
 -> TcS (Either (Pair PredType) PredType))
-> Either (Pair PredType) PredType
-> TcS (Either (Pair PredType) PredType)
forall a b. (a -> b) -> a -> b
$ Pair PredType -> Either (Pair PredType) PredType
Pair PredType -> Either (Pair PredType) PredType
forall a b. a -> Either a b
Left (Pair PredType -> Either (Pair PredType) PredType)
-> Pair PredType -> Either (Pair PredType) PredType
forall a b. (a -> b) -> a -> b
$ SwapFlag
-> (PredType -> PredType -> Pair PredType)
-> PredType
-> PredType
-> Pair PredType
forall a b. SwapFlag -> (a -> a -> b) -> a -> a -> b
unSwap SwapFlag
swapped PredType -> PredType -> Pair PredType
PredType -> PredType -> Pair PredType
forall a. a -> a -> Pair a
Pair (TyVar -> PredType
mkTyVarTy TyVar
tv) PredType
ty

    tyvar_tyvar :: TyVar -> TyVar -> TcS (Either (Pair PredType) PredType)
tyvar_tyvar TyVar
tv1 TyVar
tv2
      | TyVar
tv1 TyVar -> TyVar -> Bool
forall a. Eq a => a -> a -> Bool
== TyVar
tv2 = Either (Pair PredType) PredType
-> TcS (Either (Pair PredType) PredType)
forall a. a -> TcS a
forall (m :: * -> *) a. Monad m => a -> m a
return (PredType -> Either (Pair PredType) PredType
forall a b. b -> Either a b
Right (TyVar -> PredType
mkTyVarTy TyVar
tv1))
      | Bool
otherwise  = do { (ty1', progress1) <- TyVar -> TcS (PredType, Bool)
quick_zonk TyVar
tv1
                        ; (ty2', progress2) <- quick_zonk tv2
                        ; if progress1 || progress2
                          then go ty1' ty2'
                          else return $ Left (Pair (TyVarTy tv1) (TyVarTy tv2)) }

    trace_indirect :: a -> a -> TcS ()
trace_indirect a
tv a
ty
       = String -> SDoc -> TcS ()
traceTcS String
"Following filled tyvar (zonk_eq_types)"
                  (a -> SDoc
forall a. Outputable a => a -> SDoc
ppr a
tv SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> SDoc
forall doc. IsLine doc => doc
equals SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> a -> SDoc
forall a. Outputable a => a -> SDoc
ppr a
ty)

    quick_zonk :: TyVar -> TcS (PredType, Bool)
quick_zonk TyVar
tv = case TyVar -> TcTyVarDetails
tcTyVarDetails TyVar
tv of
      MetaTv { mtv_ref :: TcTyVarDetails -> IORef MetaDetails
mtv_ref = IORef MetaDetails
ref }
        -> do { cts <- IORef MetaDetails -> TcS MetaDetails
forall a. TcRef a -> TcS a
readTcRef IORef MetaDetails
ref
              ; case cts of
                  MetaDetails
Flexi        -> (PredType, Bool) -> TcS (PredType, Bool)
forall a. a -> TcS a
forall (m :: * -> *) a. Monad m => a -> m a
return (TyVar -> PredType
TyVarTy TyVar
tv, Bool
False)
                  Indirect PredType
ty' -> do { TyVar -> PredType -> TcS ()
forall {a} {a}. (Outputable a, Outputable a) => a -> a -> TcS ()
trace_indirect TyVar
tv PredType
ty'
                                     ; (PredType, Bool) -> TcS (PredType, Bool)
forall a. a -> TcS a
forall (m :: * -> *) a. Monad m => a -> m a
return (PredType
ty', Bool
True) } }
      TcTyVarDetails
_ -> (PredType, Bool) -> TcS (PredType, Bool)
forall a. a -> TcS a
forall (m :: * -> *) a. Monad m => a -> m a
return (TyVar -> PredType
TyVarTy TyVar
tv, Bool
False)

      -- This happens for type families, too. But recall that failure
      -- here just means to try harder, so it's OK if the type function
      -- isn't injective.
    tycon :: TyCon -> [TcType] -> [TcType]
          -> TcS (Either (Pair TcType) TcType)
    tycon :: TyCon
-> [PredType]
-> [PredType]
-> TcS (Either (Pair PredType) PredType)
tycon TyCon
tc [PredType]
tys1 [PredType]
tys2
      = do { results <- (PredType -> PredType -> TcS (Either (Pair PredType) PredType))
-> [PredType]
-> [PredType]
-> TcS [Either (Pair PredType) PredType]
forall (m :: * -> *) a b c.
Applicative m =>
(a -> b -> m c) -> [a] -> [b] -> m [c]
zipWithM PredType -> PredType -> TcS (Either (Pair PredType) PredType)
go [PredType]
tys1 [PredType]
tys2
           ; return $ case combine_results results of
               Left Pair [PredType]
tys  -> Pair PredType -> Either (Pair PredType) PredType
forall a b. a -> Either a b
Left (TyCon -> [PredType] -> PredType
mkTyConApp TyCon
tc ([PredType] -> PredType) -> Pair [PredType] -> Pair PredType
forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
<$> Pair [PredType]
tys)
               Right [PredType]
tys -> PredType -> Either (Pair PredType) PredType
forall a b. b -> Either a b
Right (TyCon -> [PredType] -> PredType
mkTyConApp TyCon
tc [PredType]
tys) }

    combine_results :: [Either (Pair TcType) TcType]
                    -> Either (Pair [TcType]) [TcType]
    combine_results :: [Either (Pair PredType) PredType]
-> Either (Pair [PredType]) [PredType]
combine_results = (Pair [PredType] -> Pair [PredType])
-> ([PredType] -> [PredType])
-> Either (Pair [PredType]) [PredType]
-> Either (Pair [PredType]) [PredType]
forall a b c d. (a -> b) -> (c -> d) -> Either a c -> Either b d
forall (p :: * -> * -> *) a b c d.
Bifunctor p =>
(a -> b) -> (c -> d) -> p a c -> p b d
bimap (([PredType] -> [PredType]) -> Pair [PredType] -> Pair [PredType]
forall a b. (a -> b) -> Pair a -> Pair b
forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
fmap [PredType] -> [PredType]
forall a. [a] -> [a]
reverse) [PredType] -> [PredType]
forall a. [a] -> [a]
reverse (Either (Pair [PredType]) [PredType]
 -> Either (Pair [PredType]) [PredType])
-> ([Either (Pair PredType) PredType]
    -> Either (Pair [PredType]) [PredType])
-> [Either (Pair PredType) PredType]
-> Either (Pair [PredType]) [PredType]
forall b c a. (b -> c) -> (a -> b) -> a -> c
.
                      (Either (Pair [PredType]) [PredType]
 -> Either (Pair PredType) PredType
 -> Either (Pair [PredType]) [PredType])
-> Either (Pair [PredType]) [PredType]
-> [Either (Pair PredType) PredType]
-> Either (Pair [PredType]) [PredType]
forall b a. (b -> a -> b) -> b -> [a] -> b
forall (t :: * -> *) b a.
Foldable t =>
(b -> a -> b) -> b -> t a -> b
foldl' ((PredType -> [PredType] -> [PredType])
-> Either (Pair [PredType]) [PredType]
-> Either (Pair PredType) PredType
-> Either (Pair [PredType]) [PredType]
forall a b c.
(a -> b -> c)
-> Either (Pair b) b -> Either (Pair a) a -> Either (Pair c) c
combine_rev (:)) ([PredType] -> Either (Pair [PredType]) [PredType]
forall a b. b -> Either a b
Right [])

      -- combine (in reverse) a new result onto an already-combined result
    combine_rev :: (a -> b -> c)
                -> Either (Pair b) b
                -> Either (Pair a) a
                -> Either (Pair c) c
    combine_rev :: forall a b c.
(a -> b -> c)
-> Either (Pair b) b -> Either (Pair a) a -> Either (Pair c) c
combine_rev a -> b -> c
f (Left Pair b
list) (Left Pair a
elt) = Pair c -> Either (Pair c) c
forall a b. a -> Either a b
Left (a -> b -> c
f (a -> b -> c) -> Pair a -> Pair (b -> c)
forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
<$> Pair a
elt     Pair (b -> c) -> Pair b -> Pair c
forall a b. Pair (a -> b) -> Pair a -> Pair b
forall (f :: * -> *) a b. Applicative f => f (a -> b) -> f a -> f b
<*> Pair b
list)
    combine_rev a -> b -> c
f (Left Pair b
list) (Right a
ty) = Pair c -> Either (Pair c) c
forall a b. a -> Either a b
Left (a -> b -> c
f (a -> b -> c) -> Pair a -> Pair (b -> c)
forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
<$> a -> Pair a
forall a. a -> Pair a
forall (f :: * -> *) a. Applicative f => a -> f a
pure a
ty Pair (b -> c) -> Pair b -> Pair c
forall a b. Pair (a -> b) -> Pair a -> Pair b
forall (f :: * -> *) a b. Applicative f => f (a -> b) -> f a -> f b
<*> Pair b
list)
    combine_rev a -> b -> c
f (Right b
tys) (Left Pair a
elt) = Pair c -> Either (Pair c) c
forall a b. a -> Either a b
Left (a -> b -> c
f (a -> b -> c) -> Pair a -> Pair (b -> c)
forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
<$> Pair a
elt     Pair (b -> c) -> Pair b -> Pair c
forall a b. Pair (a -> b) -> Pair a -> Pair b
forall (f :: * -> *) a b. Applicative f => f (a -> b) -> f a -> f b
<*> b -> Pair b
forall a. a -> Pair a
forall (f :: * -> *) a. Applicative f => a -> f a
pure b
tys)
    combine_rev a -> b -> c
f (Right b
tys) (Right a
ty) = c -> Either (Pair c) c
forall a b. b -> Either a b
Right (a -> b -> c
f a
ty b
tys)


{- Note [zonkEqTypes and the PKTI]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Because `zonkEqTypes` does /partial/ zonking, we need to be very careful
to maintain the Purely Kinded Type Invariant: see GHC.Tc.Gen/HsType
HsNote [The Purely Kinded Type Invariant (PKTI)].

In #26256 we try to solve this equality constraint:
   Int :-> Maybe Char ~# k0 Int (m0 Char)
where m0 and k0 are unification variables, and
   m0 :: Type -> Type
It happens that m0 was already unified
   m0 := (w0 :: kappa)
where kappa is another unification variable that is also already unified:
   kappa := Type->Type.
So the original type satisifed the PKTI, but a partially-zonked form
   k0 Int (w0 Char)
does not!! (This a bit reminiscent of Note [mkAppTyM].)

The solution I have adopted is simply to make `zonkEqTypes` bale out on `AppTy`.
After all, it's only supposed to be a quick hack to see if two types are already
equal; if we bale out we'll just get into the "proper" canonicaliser.

The only tricky thing about this approach is that it relies on /omitting/
code -- for the AppTy/AppTy case!  Hence this Note
-}

{- *********************************************************************
*                                                                      *
*           canonicaliseEquality
*                                                                      *
********************************************************************* -}

canonicaliseEquality
   :: CtEvidence -> EqRel
   -> Type -> Type     -- LHS and RHS
   -> SolverStage (Either IrredCt EqCt)
-- Nota Bene: `ty1` and `ty2` may be more-zonked than `ev`
-- This matters when calling mkSelCo, in canDecomposableTyConAppOK and
--   canDecomposableFunTy, when we need the precondition of mkSelCo to
--   hold.

canonicaliseEquality :: CtEvidence
-> EqRel
-> PredType
-> PredType
-> SolverStage (Either IrredCt EqCt)
canonicaliseEquality CtEvidence
ev EqRel
eq_rel PredType
ty1 PredType
ty2
  = TcS (StopOrContinue (Either IrredCt EqCt))
-> SolverStage (Either IrredCt EqCt)
TcS (StopOrContinue (Either IrredCt EqCt))
-> SolverStage (Either IrredCt EqCt)
forall a. TcS (StopOrContinue a) -> SolverStage a
Stage (TcS (StopOrContinue (Either IrredCt EqCt))
 -> SolverStage (Either IrredCt EqCt))
-> TcS (StopOrContinue (Either IrredCt EqCt))
-> SolverStage (Either IrredCt EqCt)
forall a b. (a -> b) -> a -> b
$ do { String -> SDoc -> TcS ()
traceTcS String
"canonicaliseEquality" (SDoc -> TcS ()) -> SDoc -> TcS ()
forall a b. (a -> b) -> a -> b
$
                 [SDoc] -> SDoc
forall doc. IsDoc doc => [doc] -> doc
vcat [ CtEvidence -> SDoc
forall a. Outputable a => a -> SDoc
ppr CtEvidence
ev, EqRel -> SDoc
forall a. Outputable a => a -> SDoc
ppr EqRel
eq_rel, PredType -> SDoc
forall a. Outputable a => a -> SDoc
ppr PredType
ty1, PredType -> SDoc
forall a. Outputable a => a -> SDoc
ppr PredType
ty2 ]
               ; rdr_env   <- TcS GlobalRdrEnv
getGlobalRdrEnvTcS
               ; fam_insts <- getFamInstEnvs
               ; can_eq_nc False rdr_env fam_insts ev eq_rel ty1 ty1 ty2 ty2 }

can_eq_nc
   :: Bool           -- True => both input types are rewritten
   -> GlobalRdrEnv   -- needed to see which newtypes are in scope
   -> FamInstEnvs    -- needed to unwrap data instances
   -> CtEvidence
   -> EqRel
   -> Type -> Type    -- LHS, after and before type-synonym expansion, resp
   -> Type -> Type    -- RHS, after and before type-synonym expansion, resp
   -> TcS (StopOrContinue (Either IrredCt EqCt))

-- See Note [Unifying type synonyms] in GHC.Core.Unify
can_eq_nc :: Bool
-> GlobalRdrEnv
-> (FamInstEnv, FamInstEnv)
-> CtEvidence
-> EqRel
-> PredType
-> PredType
-> PredType
-> PredType
-> TcS (StopOrContinue (Either IrredCt EqCt))
can_eq_nc Bool
_flat GlobalRdrEnv
_rdr_env (FamInstEnv, FamInstEnv)
_envs CtEvidence
ev EqRel
eq_rel ty1 :: PredType
ty1@(TyConApp TyCon
tc1 []) PredType
_ps_ty1 (TyConApp TyCon
tc2 []) PredType
_ps_ty2
  | TyCon
tc1 TyCon -> TyCon -> Bool
forall a. Eq a => a -> a -> Bool
== TyCon
tc2
  = CtEvidence
-> EqRel -> PredType -> TcS (StopOrContinue (Either IrredCt EqCt))
forall a. CtEvidence -> EqRel -> PredType -> TcS (StopOrContinue a)
canEqReflexive CtEvidence
ev EqRel
eq_rel PredType
ty1

-- Expand synonyms first; see Note [Type synonyms and canonicalization]
can_eq_nc Bool
rewritten GlobalRdrEnv
rdr_env (FamInstEnv, FamInstEnv)
envs CtEvidence
ev EqRel
eq_rel PredType
ty1 PredType
ps_ty1 PredType
ty2 PredType
ps_ty2
  | Just PredType
ty1' <- PredType -> Maybe PredType
coreView PredType
ty1 = Bool
-> GlobalRdrEnv
-> (FamInstEnv, FamInstEnv)
-> CtEvidence
-> EqRel
-> PredType
-> PredType
-> PredType
-> PredType
-> TcS (StopOrContinue (Either IrredCt EqCt))
can_eq_nc Bool
rewritten GlobalRdrEnv
rdr_env (FamInstEnv, FamInstEnv)
envs CtEvidence
ev EqRel
eq_rel PredType
ty1' PredType
ps_ty1 PredType
ty2  PredType
ps_ty2
  | Just PredType
ty2' <- PredType -> Maybe PredType
coreView PredType
ty2 = Bool
-> GlobalRdrEnv
-> (FamInstEnv, FamInstEnv)
-> CtEvidence
-> EqRel
-> PredType
-> PredType
-> PredType
-> PredType
-> TcS (StopOrContinue (Either IrredCt EqCt))
can_eq_nc Bool
rewritten GlobalRdrEnv
rdr_env (FamInstEnv, FamInstEnv)
envs CtEvidence
ev EqRel
eq_rel PredType
ty1  PredType
ps_ty1 PredType
ty2' PredType
ps_ty2

-- need to check for reflexivity in the ReprEq case.
-- See Note [Eager reflexivity check]
-- Check only when rewritten because the zonk_eq_types check in canEqNC takes
-- care of the non-rewritten case.
can_eq_nc Bool
True GlobalRdrEnv
_rdr_env (FamInstEnv, FamInstEnv)
_envs CtEvidence
ev EqRel
ReprEq PredType
ty1 PredType
_ PredType
ty2 PredType
_
  | PredType
ty1 HasDebugCallStack => PredType -> PredType -> Bool
PredType -> PredType -> Bool
`tcEqType` PredType
ty2
  = CtEvidence
-> EqRel -> PredType -> TcS (StopOrContinue (Either IrredCt EqCt))
forall a. CtEvidence -> EqRel -> PredType -> TcS (StopOrContinue a)
canEqReflexive CtEvidence
ev EqRel
ReprEq PredType
ty1

-- When working with ReprEq, unwrap newtypes.
-- See Note [Unwrap newtypes first]
-- This must be above the TyVarTy case, in order to guarantee (TyEq:N)
can_eq_nc Bool
_rewritten GlobalRdrEnv
rdr_env (FamInstEnv, FamInstEnv)
envs CtEvidence
ev EqRel
eq_rel PredType
ty1 PredType
ps_ty1 PredType
ty2 PredType
ps_ty2
  | EqRel
ReprEq <- EqRel
eq_rel
  , Just ((Bag GlobalRdrElt, TcCoercion), PredType)
stuff1 <- (FamInstEnv, FamInstEnv)
-> GlobalRdrEnv
-> PredType
-> Maybe ((Bag GlobalRdrElt, TcCoercion), PredType)
tcTopNormaliseNewTypeTF_maybe (FamInstEnv, FamInstEnv)
envs GlobalRdrEnv
rdr_env PredType
ty1
  = GlobalRdrEnv
-> (FamInstEnv, FamInstEnv)
-> CtEvidence
-> SwapFlag
-> PredType
-> ((Bag GlobalRdrElt, TcCoercion), PredType)
-> PredType
-> PredType
-> TcS (StopOrContinue (Either IrredCt EqCt))
can_eq_newtype_nc GlobalRdrEnv
rdr_env (FamInstEnv, FamInstEnv)
envs CtEvidence
ev SwapFlag
NotSwapped PredType
ty1 ((Bag GlobalRdrElt, TcCoercion), PredType)
stuff1 PredType
ty2 PredType
ps_ty2

  | EqRel
ReprEq <- EqRel
eq_rel
  , Just ((Bag GlobalRdrElt, TcCoercion), PredType)
stuff2 <- (FamInstEnv, FamInstEnv)
-> GlobalRdrEnv
-> PredType
-> Maybe ((Bag GlobalRdrElt, TcCoercion), PredType)
tcTopNormaliseNewTypeTF_maybe (FamInstEnv, FamInstEnv)
envs GlobalRdrEnv
rdr_env PredType
ty2
  = GlobalRdrEnv
-> (FamInstEnv, FamInstEnv)
-> CtEvidence
-> SwapFlag
-> PredType
-> ((Bag GlobalRdrElt, TcCoercion), PredType)
-> PredType
-> PredType
-> TcS (StopOrContinue (Either IrredCt EqCt))
can_eq_newtype_nc GlobalRdrEnv
rdr_env (FamInstEnv, FamInstEnv)
envs CtEvidence
ev SwapFlag
IsSwapped PredType
ty2 ((Bag GlobalRdrElt, TcCoercion), PredType)
stuff2 PredType
ty1 PredType
ps_ty1

-- Then, get rid of casts
can_eq_nc Bool
rewritten GlobalRdrEnv
rdr_env (FamInstEnv, FamInstEnv)
envs CtEvidence
ev EqRel
eq_rel (CastTy PredType
ty1 TcCoercion
co1) PredType
_ PredType
ty2 PredType
ps_ty2
  | Maybe CanEqLHS -> Bool
forall a. Maybe a -> Bool
isNothing (PredType -> Maybe CanEqLHS
canEqLHS_maybe PredType
ty2)
  = -- See (EIK3) in Note [Equalities with heterogeneous kinds]
    Bool
-> GlobalRdrEnv
-> (FamInstEnv, FamInstEnv)
-> CtEvidence
-> EqRel
-> SwapFlag
-> PredType
-> TcCoercion
-> PredType
-> PredType
-> TcS (StopOrContinue (Either IrredCt EqCt))
canEqCast Bool
rewritten GlobalRdrEnv
rdr_env (FamInstEnv, FamInstEnv)
envs CtEvidence
ev EqRel
eq_rel SwapFlag
NotSwapped PredType
ty1 TcCoercion
co1 PredType
ty2 PredType
ps_ty2
can_eq_nc Bool
rewritten GlobalRdrEnv
rdr_env (FamInstEnv, FamInstEnv)
envs CtEvidence
ev EqRel
eq_rel PredType
ty1 PredType
ps_ty1 (CastTy PredType
ty2 TcCoercion
co2) PredType
_
  | Maybe CanEqLHS -> Bool
forall a. Maybe a -> Bool
isNothing (PredType -> Maybe CanEqLHS
canEqLHS_maybe PredType
ty1)
  = -- See (EIK3) in Note [Equalities with heterogeneous kinds]
    Bool
-> GlobalRdrEnv
-> (FamInstEnv, FamInstEnv)
-> CtEvidence
-> EqRel
-> SwapFlag
-> PredType
-> TcCoercion
-> PredType
-> PredType
-> TcS (StopOrContinue (Either IrredCt EqCt))
canEqCast Bool
rewritten GlobalRdrEnv
rdr_env (FamInstEnv, FamInstEnv)
envs CtEvidence
ev EqRel
eq_rel SwapFlag
IsSwapped PredType
ty2 TcCoercion
co2 PredType
ty1 PredType
ps_ty1

----------------------
-- Otherwise try to decompose
----------------------

-- Literals
can_eq_nc Bool
_rewritten GlobalRdrEnv
_rdr_env (FamInstEnv, FamInstEnv)
_envs CtEvidence
ev EqRel
eq_rel ty1 :: PredType
ty1@(LitTy TyLit
l1) PredType
_ (LitTy TyLit
l2) PredType
_
 | TyLit
l1 TyLit -> TyLit -> Bool
forall a. Eq a => a -> a -> Bool
== TyLit
l2
  = do { CtEvidence -> CoercionPlusHoles -> TcS ()
setEqIfWanted CtEvidence
ev (EqRel -> PredType -> CoercionPlusHoles
mkReflCPH EqRel
eq_rel PredType
ty1)
       ; CtEvidence -> String -> TcS (StopOrContinue (Either IrredCt EqCt))
forall a. CtEvidence -> String -> TcS (StopOrContinue a)
stopWith CtEvidence
ev String
"Equal LitTy" }

-- Decompose FunTy: (s -> t) and (c => t)
-- NB: don't decompose (Int -> blah) ~ (Show a => blah)
can_eq_nc Bool
_rewritten GlobalRdrEnv
_rdr_env (FamInstEnv, FamInstEnv)
_envs CtEvidence
ev EqRel
eq_rel
           ty1 :: PredType
ty1@(FunTy { ft_mult :: PredType -> PredType
ft_mult = PredType
am1, ft_af :: PredType -> FunTyFlag
ft_af = FunTyFlag
af1, ft_arg :: PredType -> PredType
ft_arg = PredType
ty1a, ft_res :: PredType -> PredType
ft_res = PredType
ty1b }) PredType
_ps_ty1
           ty2 :: PredType
ty2@(FunTy { ft_mult :: PredType -> PredType
ft_mult = PredType
am2, ft_af :: PredType -> FunTyFlag
ft_af = FunTyFlag
af2, ft_arg :: PredType -> PredType
ft_arg = PredType
ty2a, ft_res :: PredType -> PredType
ft_res = PredType
ty2b }) PredType
_ps_ty2
  | FunTyFlag
af1 FunTyFlag -> FunTyFlag -> Bool
forall a. Eq a => a -> a -> Bool
== FunTyFlag
af2  -- See Note [Decomposing FunTy]
  = CtEvidence
-> EqRel
-> FunTyFlag
-> (PredType, PredType, PredType, PredType)
-> (PredType, PredType, PredType, PredType)
-> TcS (StopOrContinue (Either IrredCt EqCt))
forall a.
CtEvidence
-> EqRel
-> FunTyFlag
-> (PredType, PredType, PredType, PredType)
-> (PredType, PredType, PredType, PredType)
-> TcS (StopOrContinue a)
canDecomposableFunTy CtEvidence
ev EqRel
eq_rel FunTyFlag
af1 (PredType
ty1,PredType
am1,PredType
ty1a,PredType
ty1b) (PredType
ty2,PredType
am2,PredType
ty2a,PredType
ty2b)

-- Decompose type constructor applications
-- NB: we have expanded type synonyms already
can_eq_nc Bool
rewritten GlobalRdrEnv
_rdr_env (FamInstEnv, FamInstEnv)
_envs CtEvidence
ev EqRel
eq_rel PredType
ty1 PredType
_ PredType
ty2 PredType
_
  | Just (TyCon
tc1, [PredType]
tys1) <- HasDebugCallStack => PredType -> Maybe (TyCon, [PredType])
PredType -> Maybe (TyCon, [PredType])
tcSplitTyConApp_maybe PredType
ty1
  , Just (TyCon
tc2, [PredType]
tys2) <- HasDebugCallStack => PredType -> Maybe (TyCon, [PredType])
PredType -> Maybe (TyCon, [PredType])
tcSplitTyConApp_maybe PredType
ty2
   -- tcSplitTyConApp_maybe: we want to catch e.g. Maybe Int ~ (Int -> Int)
   -- here for better error messages rather than decomposing into AppTys;
   -- hence not using a direct match on TyConApp

  , Bool -> Bool
not (TyCon -> Bool
isTypeFamilyTyCon TyCon
tc1 Bool -> Bool -> Bool
|| TyCon -> Bool
isTypeFamilyTyCon TyCon
tc2)
    -- A type family at the top of LHS or RHS: we want to fall through
    -- to the canonical-LHS cases (look for canEqLHS_maybe)

  -- See (TC1) in Note [Canonicalising TyCon/TyCon equalities]
  , let role :: Role
role            = EqRel -> Role
eqRelRole EqRel
eq_rel
        both_generative :: Bool
both_generative = TyCon -> Role -> Bool
isGenerativeTyCon TyCon
tc1 Role
role Bool -> Bool -> Bool
&& TyCon -> Role -> Bool
isGenerativeTyCon TyCon
tc2 Role
role
  , Bool
rewritten Bool -> Bool -> Bool
|| Bool
both_generative
  = CtEvidence
-> EqRel
-> Bool
-> (PredType, TyCon, [PredType])
-> (PredType, TyCon, [PredType])
-> TcS (StopOrContinue (Either IrredCt EqCt))
canTyConApp CtEvidence
ev EqRel
eq_rel Bool
both_generative (PredType
ty1,TyCon
tc1,[PredType]
tys1) (PredType
ty2,TyCon
tc2,[PredType]
tys2)

can_eq_nc Bool
_rewritten GlobalRdrEnv
_rdr_env (FamInstEnv, FamInstEnv)
_envs CtEvidence
ev EqRel
eq_rel
           s1 :: PredType
s1@ForAllTy{} PredType
_
           s2 :: PredType
s2@ForAllTy{} PredType
_
  = CtEvidence
-> EqRel
-> PredType
-> PredType
-> TcS (StopOrContinue (Either IrredCt EqCt))
can_eq_nc_forall CtEvidence
ev EqRel
eq_rel PredType
s1 PredType
s2

-- See Note [Canonicalising type applications] about why we require rewritten types
-- Use tcSplitAppTy, not matching on AppTy, to catch oversaturated type families
-- NB: Only decompose AppTy for nominal equality.
--     See Note [Decomposing AppTy equalities]
can_eq_nc Bool
True GlobalRdrEnv
_rdr_env (FamInstEnv, FamInstEnv)
_envs CtEvidence
ev EqRel
NomEq PredType
ty1 PredType
_ PredType
ty2 PredType
_
  | Just (PredType
t1, PredType
s1) <- PredType -> Maybe (PredType, PredType)
tcSplitAppTy_maybe PredType
ty1
  , Just (PredType
t2, PredType
s2) <- PredType -> Maybe (PredType, PredType)
tcSplitAppTy_maybe PredType
ty2
  = CtEvidence
-> PredType
-> PredType
-> PredType
-> PredType
-> TcS (StopOrContinue (Either IrredCt EqCt))
can_eq_app CtEvidence
ev PredType
t1 PredType
s1 PredType
t2 PredType
s2

-------------------
-- Can't decompose.
-------------------

-- No similarity in type structure detected. Rewrite and try again.
can_eq_nc Bool
False GlobalRdrEnv
rdr_env (FamInstEnv, FamInstEnv)
envs CtEvidence
ev EqRel
eq_rel PredType
_ PredType
ps_ty1 PredType
_ PredType
ps_ty2
  = -- Rewrite the two types and try again
    do { (redn1@(Reduction _ xi1), rewriters1) <- CtEvidence -> PredType -> TcS (Reduction, CoHoleSet)
rewrite CtEvidence
ev PredType
ps_ty1
       ; (redn2@(Reduction _ xi2), rewriters2) <- rewrite ev ps_ty2
       ; new_ev <- rewriteEqEvidence ev NotSwapped redn1 redn2
                                     (rewriters1 S.<> rewriters2)
       ; traceTcS "can_eq_nc: go round again" (ppr new_ev $$ ppr xi1 $$ ppr xi2)
       ; can_eq_nc True rdr_env envs new_ev eq_rel xi1 xi1 xi2 xi2 }

----------------------------
-- Look for a canonical LHS.
-- Only rewritten types end up below here.
----------------------------

-- NB: pattern match on rewritten=True: we want only rewritten types sent to canEqLHS
-- This means we've rewritten any variables and reduced any type family redexes
-- See also Note [No top-level newtypes on RHS of representational equalities]

can_eq_nc Bool
True GlobalRdrEnv
_rdr_env (FamInstEnv, FamInstEnv)
_envs CtEvidence
ev EqRel
eq_rel PredType
ty1 PredType
ps_ty1 PredType
ty2 PredType
ps_ty2
  | Just CanEqLHS
can_eq_lhs1 <- PredType -> Maybe CanEqLHS
canEqLHS_maybe PredType
ty1
  = do { String -> SDoc -> TcS ()
traceTcS String
"can_eq1" (PredType -> SDoc
forall a. Outputable a => a -> SDoc
ppr PredType
ty1 SDoc -> SDoc -> SDoc
forall doc. IsDoc doc => doc -> doc -> doc
$$ PredType -> SDoc
forall a. Outputable a => a -> SDoc
ppr PredType
ty2)
       ; CtEvidence
-> EqRel
-> SwapFlag
-> CanEqLHS
-> PredType
-> PredType
-> PredType
-> TcS (StopOrContinue (Either IrredCt EqCt))
canEqCanLHS CtEvidence
ev EqRel
eq_rel SwapFlag
NotSwapped CanEqLHS
can_eq_lhs1 PredType
ps_ty1 PredType
ty2 PredType
ps_ty2 }

  | Just CanEqLHS
can_eq_lhs2 <- PredType -> Maybe CanEqLHS
canEqLHS_maybe PredType
ty2
  = do { String -> SDoc -> TcS ()
traceTcS String
"can_eq2" (PredType -> SDoc
forall a. Outputable a => a -> SDoc
ppr PredType
ty1 SDoc -> SDoc -> SDoc
forall doc. IsDoc doc => doc -> doc -> doc
$$ PredType -> SDoc
forall a. Outputable a => a -> SDoc
ppr PredType
ty2)
       ; CtEvidence
-> EqRel
-> SwapFlag
-> CanEqLHS
-> PredType
-> PredType
-> PredType
-> TcS (StopOrContinue (Either IrredCt EqCt))
canEqCanLHS CtEvidence
ev EqRel
eq_rel SwapFlag
IsSwapped CanEqLHS
can_eq_lhs2 PredType
ps_ty2 PredType
ty1 PredType
ps_ty1 }

     -- If the type is TyConApp tc1 args1, then args1 really can't be less
     -- than tyConArity tc1. It could be *more* than tyConArity, but then we
     -- should have handled the case as an AppTy. That case only fires if
     -- _both_ sides of the equality are AppTy-like... but if one side is
     -- AppTy-like and the other isn't (and it also isn't a variable or
     -- saturated type family application, both of which are handled by
     -- can_eq_nc), we're in a failure mode and can just fall through.

----------------------------
-- Fall-through. Give up.
----------------------------

-- We've rewritten and the types don't match. Give up.
can_eq_nc Bool
True GlobalRdrEnv
_rdr_env (FamInstEnv, FamInstEnv)
_envs CtEvidence
ev EqRel
eq_rel PredType
_ PredType
ps_ty1 PredType
_ PredType
ps_ty2
  = do { String -> SDoc -> TcS ()
traceTcS String
"can_eq_nc catch-all case" (PredType -> SDoc
forall a. Outputable a => a -> SDoc
ppr PredType
ps_ty1 SDoc -> SDoc -> SDoc
forall doc. IsDoc doc => doc -> doc -> doc
$$ PredType -> SDoc
forall a. Outputable a => a -> SDoc
ppr PredType
ps_ty2)
       ; case EqRel
eq_rel of -- See Note [Unsolved equalities]
            EqRel
ReprEq -> CtIrredReason
-> CtEvidence -> TcS (StopOrContinue (Either IrredCt EqCt))
forall a.
CtIrredReason
-> CtEvidence -> TcS (StopOrContinue (Either IrredCt a))
finishCanWithIrred CtIrredReason
ReprEqReason CtEvidence
ev
            EqRel
NomEq  -> CtIrredReason
-> CtEvidence -> TcS (StopOrContinue (Either IrredCt EqCt))
forall a.
CtIrredReason
-> CtEvidence -> TcS (StopOrContinue (Either IrredCt a))
finishCanWithIrred CtIrredReason
ShapeMismatchReason CtEvidence
ev }
          -- No need to call canEqSoftFailure/canEqHardFailure because they
          -- rewrite, and the types involved here are already rewritten


{- Note [Unsolved equalities]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
If we have an unsolved equality like
  (a b ~R# Int)
that is not necessarily insoluble!  Maybe 'a' will turn out to be a newtype.
So we want to make it a potentially-soluble Irred not an insoluble one.
Missing this point is what caused #15431
-}

---------------------------------
can_eq_nc_forall :: CtEvidence -> EqRel
                 -> Type -> Type    -- LHS and RHS
                 -> TcS (StopOrContinue (Either IrredCt EqCt))
-- See Note [Solving forall equalities]

can_eq_nc_forall :: CtEvidence
-> EqRel
-> PredType
-> PredType
-> TcS (StopOrContinue (Either IrredCt EqCt))
can_eq_nc_forall CtEvidence
ev EqRel
eq_rel PredType
s1 PredType
s2
 | CtWanted (WantedCt { ctev_dest :: WantedCtEvidence -> TcEvDest
ctev_dest = TcEvDest
orig_dest }) <- CtEvidence
ev
 = do { let ([ForAllTyBinder]
bndrs1, PredType
phi1, [ForAllTyBinder]
bndrs2, PredType
phi2) = PredType
-> PredType
-> ([ForAllTyBinder], PredType, [ForAllTyBinder], PredType)
split_foralls PredType
s1 PredType
s2
            flags1 :: [ForAllTyFlag]
flags1 = [ForAllTyBinder] -> [ForAllTyFlag]
forall tv argf. [VarBndr tv argf] -> [argf]
binderFlags [ForAllTyBinder]
bndrs1
            flags2 :: [ForAllTyFlag]
flags2 = [ForAllTyBinder] -> [ForAllTyFlag]
forall tv argf. [VarBndr tv argf] -> [argf]
binderFlags [ForAllTyBinder]
bndrs2

      ; if EqRel
eq_rel EqRel -> EqRel -> Bool
forall a. Eq a => a -> a -> Bool
== EqRel
NomEq Bool -> Bool -> Bool
&& Bool -> Bool
not ((ForAllTyFlag -> ForAllTyFlag -> Bool)
-> [ForAllTyFlag] -> [ForAllTyFlag] -> Bool
forall a b. (a -> b -> Bool) -> [a] -> [b] -> Bool
all2 ForAllTyFlag -> ForAllTyFlag -> Bool
eqForAllVis [ForAllTyFlag]
flags1 [ForAllTyFlag]
flags2) -- Note [ForAllTy and type equality]
        then do { String -> SDoc -> TcS ()
traceTcS String
"Forall failure: visibility-mismatch" (SDoc -> TcS ()) -> SDoc -> TcS ()
forall a b. (a -> b) -> a -> b
$
                     [SDoc] -> SDoc
forall doc. IsDoc doc => [doc] -> doc
vcat [ PredType -> SDoc
forall a. Outputable a => a -> SDoc
ppr PredType
s1, PredType -> SDoc
forall a. Outputable a => a -> SDoc
ppr PredType
s2, [ForAllTyBinder] -> SDoc
forall a. Outputable a => a -> SDoc
ppr [ForAllTyBinder]
bndrs1, [ForAllTyBinder] -> SDoc
forall a. Outputable a => a -> SDoc
ppr [ForAllTyBinder]
bndrs2
                          , [ForAllTyFlag] -> SDoc
forall a. Outputable a => a -> SDoc
ppr [ForAllTyFlag]
flags1, [ForAllTyFlag] -> SDoc
forall a. Outputable a => a -> SDoc
ppr [ForAllTyFlag]
flags2 ]
                ; CtEvidence
-> PredType
-> PredType
-> TcS (StopOrContinue (Either IrredCt EqCt))
forall a.
CtEvidence
-> PredType -> PredType -> TcS (StopOrContinue (Either IrredCt a))
canEqHardFailure CtEvidence
ev PredType
s1 PredType
s2 }

        else
   do { let free_tvs :: TcTyVarSet
free_tvs       = [PredType] -> TcTyVarSet
tyCoVarsOfTypes [PredType
s1,PredType
s2]
            empty_subst1 :: Subst
empty_subst1   = InScopeSet -> Subst
mkEmptySubst (InScopeSet -> Subst) -> InScopeSet -> Subst
forall a b. (a -> b) -> a -> b
$ TcTyVarSet -> InScopeSet
mkInScopeSet TcTyVarSet
free_tvs
            skol_info_anon :: SkolemInfoAnon
skol_info_anon = PredType -> SkolemInfoAnon
UnifyForAllSkol PredType
phi1
      ; skol_info <- SkolemInfoAnon -> TcS SkolemInfo
forall (m :: * -> *). MonadIO m => SkolemInfoAnon -> m SkolemInfo
mkSkolemInfo SkolemInfoAnon
skol_info_anon
      ; (subst1, skol_tvs) <- tcInstSkolTyVarsX skol_info empty_subst1 $
                              binderVars bndrs1

      ; let phi1' = HasDebugCallStack => Subst -> PredType -> PredType
Subst -> PredType -> PredType
substTy Subst
subst1 PredType
phi1

            -- Unify the kinds, extend the substitution
            go :: UnifyEnv -> [TcTyVar] -> Subst
               -> [TyVarBinder] -> [TyVarBinder] -> TcM.TcM TcCoercion
            go UnifyEnv
uenv (TyVar
skol_tv:[TyVar]
skol_tvs) Subst
subst2 (ForAllTyBinder
bndr1:[ForAllTyBinder]
bndrs1) (ForAllTyBinder
bndr2:[ForAllTyBinder]
bndrs2)
              = do { let tv2 :: TyVar
tv2  = ForAllTyBinder -> TyVar
forall tv argf. VarBndr tv argf -> tv
binderVar ForAllTyBinder
bndr2
                         vis1 :: ForAllTyFlag
vis1 = ForAllTyBinder -> ForAllTyFlag
forall tv argf. VarBndr tv argf -> argf
binderFlag ForAllTyBinder
bndr1
                         vis2 :: ForAllTyFlag
vis2 = ForAllTyBinder -> ForAllTyFlag
forall tv argf. VarBndr tv argf -> argf
binderFlag ForAllTyBinder
bndr2

                   -- Unify the kinds, at Nominal role
                   -- See (SF1) in Note [Solving forall equalities]
                   ; kind_co <- UnifyEnv -> PredType -> PredType -> TcM TcCoercion
uType (UnifyEnv
uenv UnifyEnv -> Role -> UnifyEnv
`setUEnvRole` Role
Nominal)
                                      (TyVar -> PredType
tyVarKind TyVar
skol_tv)
                                      (HasDebugCallStack => Subst -> PredType -> PredType
Subst -> PredType -> PredType
substTy Subst
subst2 (TyVar -> PredType
tyVarKind TyVar
tv2))

                   ; let subst2' = Subst -> TyVar -> PredType -> Subst
extendTvSubstAndInScope Subst
subst2 TyVar
tv2
                                       (PredType -> TcCoercion -> PredType
mkCastTy (TyVar -> PredType
mkTyVarTy TyVar
skol_tv) TcCoercion
kind_co)
                         -- skol_tv is already in the in-scope set, but the
                         -- free vars of kind_co are not; hence "...AndInScope"
                   ; co <- go uenv skol_tvs subst2' bndrs1 bndrs2

                   ; return (mkNakedForAllCo skol_tv vis1 vis2 kind_co co)}
                     -- mkNaked.. because these types are not zonked, and the
                     -- assertions in mkForAllCo may fail without that zonking

            -- Done: unify phi1 ~ phi2
            go UnifyEnv
uenv [] Subst
subst2 [ForAllTyBinder]
bndrs1 [ForAllTyBinder]
bndrs2
              = Bool -> TcM TcCoercion -> TcM TcCoercion
forall a. HasCallStack => Bool -> a -> a
assert ([ForAllTyBinder] -> Bool
forall a. [a] -> Bool
forall (t :: * -> *) a. Foldable t => t a -> Bool
null [ForAllTyBinder]
bndrs1 Bool -> Bool -> Bool
&& [ForAllTyBinder] -> Bool
forall a. [a] -> Bool
forall (t :: * -> *) a. Foldable t => t a -> Bool
null [ForAllTyBinder]
bndrs2) (TcM TcCoercion -> TcM TcCoercion)
-> TcM TcCoercion -> TcM TcCoercion
forall a b. (a -> b) -> a -> b
$
                UnifyEnv -> PredType -> PredType -> TcM TcCoercion
uType UnifyEnv
uenv PredType
phi1' (Subst -> PredType -> PredType
substTyUnchecked Subst
subst2 PredType
phi2)

            go UnifyEnv
_ [TyVar]
_ Subst
_ [ForAllTyBinder]
_ [ForAllTyBinder]
_ = String -> TcM TcCoercion
forall a. HasCallStack => String -> a
panic String
"can_eq_nc_forall"  -- case (s:ss) []

            init_subst2 = InScopeSet -> Subst
mkEmptySubst (Subst -> InScopeSet
substInScopeSet Subst
subst1)

      ; traceTcS "Generating wanteds" (ppr s1 $$ ppr s2)

      -- Generate and solve the constraints that live in the body of the implication
      -- See (SF5) and (SF6) in Note [Solving forall equalities]
      ; nested_ev_binds_var <- newNoTcEvBinds
      ; (unifs, (all_co, solved))
             <- reportFineGrainUnifications $
                do { -- Generate constraints
                     (tclvl, (all_co, wanteds))
                          <- pushLevelNoWorkList (ppr skol_info) $
                             wrapUnifier ev (eqRelRole eq_rel)   $ \UnifyEnv
uenv ->
                             UnifyEnv
-> [TyVar]
-> Subst
-> [ForAllTyBinder]
-> [ForAllTyBinder]
-> TcM TcCoercion
go UnifyEnv
uenv [TyVar]
skol_tvs Subst
init_subst2 [ForAllTyBinder]
bndrs1 [ForAllTyBinder]
bndrs2

                   ; traceTcS "Trying to solve the implication" (ppr s1 $$ ppr s2 $$ ppr wanteds)

                   -- Solve the `wanteds` in a nested context.
                   ; residual_wanted <- nestImplicTcS skol_info_anon nested_ev_binds_var tclvl $
                                        solveSimpleWanteds wanteds

                   ; return (all_co, isSolvedWC residual_wanted) }

      -- Kick out any inerts constraints that mention unified type variables
      ; kickOutAfterUnification unifs

      ; if solved
        then do {  -- Record any used used Given coercions (in `evb_tcvs`) so that
                   -- they are kept alive by `neededEvVars`. Admittedly they are free in `all_co`,
                   -- but only if we zonk it, which `neededEvVars` does not do (see test T7196).
                  ev_binds_var <- getTcEvBindsVar
                ; updTcEvBinds ev_binds_var nested_ev_binds_var

                ; setWantedEq orig_dest (CPH { cph_co = all_co, cph_holes = emptyCoHoleSet })
                     -- emptyCoHoleSet: fully solved, so all_co has no holes
                ; stopWith ev "Polytype equality: solved" }

        else canEqSoftFailure IrredShapeReason ev s1 s2 } }

 | Bool
otherwise
 = do { String -> SDoc -> TcS ()
traceTcS String
"Omitting decomposition of given polytype equality" (SDoc -> TcS ()) -> SDoc -> TcS ()
forall a b. (a -> b) -> a -> b
$
        PredType -> PredType -> SDoc
pprEq PredType
s1 PredType
s2    -- See Note [Do not decompose Given polytype equalities]
      ; CtEvidence -> String -> TcS (StopOrContinue (Either IrredCt EqCt))
forall a. CtEvidence -> String -> TcS (StopOrContinue a)
stopWith CtEvidence
ev String
"Discard given polytype equality" }

 where
    split_foralls :: TcType -> TcType
                  -> ( [ForAllTyBinder], TcType
                     , [ForAllTyBinder], TcType)
    -- Split matching foralls; stop when the foralls don't match
    -- See #22537.  See (SF3) in Note [Solving forall equalities]
    -- Postcondition: the two lists of binders returned have the same length
    split_foralls :: PredType
-> PredType
-> ([ForAllTyBinder], PredType, [ForAllTyBinder], PredType)
split_foralls PredType
s1 PredType
s2
      | Just (ForAllTyBinder
bndr1, PredType
s1') <- PredType -> Maybe (ForAllTyBinder, PredType)
splitForAllForAllTyBinder_maybe PredType
s1
      , Just (ForAllTyBinder
bndr2, PredType
s2') <- PredType -> Maybe (ForAllTyBinder, PredType)
splitForAllForAllTyBinder_maybe PredType
s2
      = let !([ForAllTyBinder]
bndrs1, PredType
phi1, [ForAllTyBinder]
bndrs2, PredType
phi2) = PredType
-> PredType
-> ([ForAllTyBinder], PredType, [ForAllTyBinder], PredType)
split_foralls PredType
s1' PredType
s2'
        in (ForAllTyBinder
bndr1ForAllTyBinder -> [ForAllTyBinder] -> [ForAllTyBinder]
forall a. a -> [a] -> [a]
:[ForAllTyBinder]
bndrs1, PredType
phi1, ForAllTyBinder
bndr2ForAllTyBinder -> [ForAllTyBinder] -> [ForAllTyBinder]
forall a. a -> [a] -> [a]
:[ForAllTyBinder]
bndrs2, PredType
phi2)
    split_foralls PredType
s1 PredType
s2 = ([], PredType
s1, [], PredType
s2)


{- Note [Solving forall equalities]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
To solve an equality between foralls
   [W] (forall a. t1) ~ (forall b. t2)
the basic plan is simple: behave rather as if we were solving the
implication constraint
   [W] forall a. { t1 ~ (t2[a/b]) }

The evidence we produce is a ForAllCo; see the typing rule for
ForAllCo in Note [ForAllCo] in GHC.Tc.TyCo.Rep.

There are lots of wrinkles of course:

(SF1) We must check the kinds match (at Nominal role).  So from
      [W] (forall (a:ka). t1) ~ (forall (b:kb). t2)
   we actually generate the implication
      [W] forall (a:ka). { ka ~N kb,  t1 ~ t2[a/b] }
   These kind equalities are generated by the `go` loop in `can_eq_nc_forall`.
   Why Nominal role? See the typing rule for ForAllCo.

(SF2) At Nominal role we must check that the visiblities match.
  For example
     [W] (forall a. a -> a) ~N  (forall b -> b -> b)
  should fail.  At /Representational/ role we allow this; see the
  typing rule for ForAllCo, mentioned above.

(SF3) Consider this (#22537)
     newtype P a = MkP (forall c. (a,c))
     [W] (forall a. P a) ~R (forall a b. (a,b))
  The number of foralls does not line up.  But if we just unwrap the outer
  forall a, we'll get
     [W] P a ~R forall b. (a,b)
  Now unwrap the newtype
     [W] (forall c. (a,c)) ~R (forall b. (a,b))
  and all is good.

  Conclusion: Don't fail if the number of foralls does not line up.  Instead,
  handle as many binders as are visibly apparent on both sides, and then keep
  going with unification. See `split_foralls` in `can_eq_nc_forall`.  In the
  above example, at Representational role, the unifier will proceed to unwrap
  the newtype on the RHS and we'll end up back in can_eq_nc_forall.

(SF4) Remember also that we might have forall z (a:z). blah
  so in that `go` loop, we must proceed one binder at a time (#13879)

(SF5) Rather than manually gather the constraints needed in the body of the
   implication, we use `uType`.  That way we can solve some of them on the fly,
   especially Refl ones.  We use the `wrapUnifier` wrapper for `uType`,
   because we want to /gather/ the equality constraint (to put in the implication)
   rather than /emit/ them into the monad, as `wrapUnifierAndEmit` does.

(SF6) We solve the nested constraints right away.  In a type-correct program,
   this attempt will usually succeed.  But if we don't completely solve it, we
   abandon the attempt and keep the origin forall-equality.

   In the abandon-the-attempt case, we are left with an unsolved
      (forall a. blah) ~ (forall b. blah)

   and it may not be immediately apparent /why/ we couldn't solve it.  But actually
   such errors messages can be /better/.  For example, it is easier to understand
   that (forall a. a->a) ~ (forall b. b->Int) is insoluble than it is to understand a
   message about matching `a` with `Int`, because `a` is a skolem.

   Historical note: In the past we instead generated an `Implication`, storing it in
   a dedicated field `wl_implics` in the work-list to be solved later. But that
   plumbing was tiresome, and now we just try to solve them on the spot, and abandon
   the attempt if we fail.
-}

{- Note [Unwrap newtypes first]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
See also Note [Decomposing newtype equalities]

Consider
  newtype N m a = MkN (m a)
N will get a conservative, Nominal role for its second parameter 'a',
because it appears as an argument to the unknown 'm'. Now consider
  [W] N Maybe a  ~R#  N Maybe b

If we /decompose/, we'll get
  [W] a ~N# b

But if instead we /unwrap/ we'll get
  [W] Maybe a ~R# Maybe b
which in turn gives us
  [W] a ~R# b
which is easier to satisfy.

Conclusion: we must unwrap newtypes before decomposing them. This happens
in `can_eq_newtype_nc`

We did flirt with making the /rewriter/ expand newtypes, rather than
doing it in `can_eq_newtype_nc`.   But with recursive newtypes we want
to be super-careful about expanding!

   newtype A = MkA [A]   -- Recursive!

   f :: A -> [A]
   f = coerce

We have [W] A ~R# [A].  If we rewrite [A], it'll expand to
   [[[[[...]]]]]
and blow the reduction stack.  See Note [Newtypes can blow the stack]
in GHC.Tc.Solver.Rewrite.  But if we expand only the /top level/ of
both sides, we get
   [W] [A] ~R# [A]
which we can, just, solve by reflexivity.

So we simply unwrap, on-demand, at top level, in `can_eq_newtype_nc`.

This is all very delicate. There is a real risk of a loop in the type checker
with recursive newtypes -- but I think we're doomed to do *something*
delicate, as we're really trying to solve for equirecursive type
equality. Bottom line for users: recursive newtypes do not play well with type
inference for representational equality.  See also Section 5.3.1 and 5.3.4 of
"Safe Zero-cost Coercions for Haskell" (JFP 2016).

See also Note [Decomposing newtype equalities].

--- Historical side note ---

We flirted with doing /both/ unwrap-at-top-level /and/ rewrite-deeply;
see #22519.  But that didn't work: see discussion in #22924. Specifically
we got a loop with a minor variation:
   f2 :: a -> [A]
   f2 = coerce

Note [Eager reflexivity check]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Suppose we have

  newtype X = MkX (Int -> X)

and

  [W] X ~R X

Naively, we would start unwrapping X and end up in a loop. Instead,
we do this eager reflexivity check. This is necessary only for representational
equality because the rewriter technology deals with the similar case
(recursive type families) for nominal equality.

Note that this check does not catch all cases, but it will catch the cases
we're most worried about, types like X above that are actually inhabited.

Here's another place where this reflexivity check is key:
Consider trying to prove (f a) ~R (f a). The AppTys in there can't
be decomposed, because representational equality isn't congruent with respect
to AppTy. So, when canonicalising the equality above, we get stuck and
would normally produce a CIrredCan. However, we really do want to
be able to solve (f a) ~R (f a). So, in the representational case only,
we do a reflexivity check.

(This would be sound in the nominal case, but unnecessary, and I [Richard
E.] am worried that it would slow down the common case.)

 Note [Newtypes can blow the stack]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Suppose we have

  newtype X = MkX (Int -> X)
  newtype Y = MkY (Int -> Y)

and now wish to prove

  [W] X ~R Y

This Wanted will loop, expanding out the newtypes ever deeper looking
for a solid match or a solid discrepancy. Indeed, there is something
appropriate to this looping, because X and Y *do* have the same representation,
in the limit -- they're both (Fix ((->) Int)). However, no finitely-sized
coercion will ever witness it. This loop won't actually cause GHC to hang,
though, because we check our depth in `can_eq_newtype_nc`.
-}

------------------------
-- | We're able to unwrap a newtype. Update the bits accordingly.
can_eq_newtype_nc :: GlobalRdrEnv -> FamInstEnvs
                  -> CtEvidence           -- ^ :: ty1 ~ ty2
                  -> SwapFlag
                  -> TcType                                    -- ^ ty1
                  -> ((Bag GlobalRdrElt, TcCoercion), TcType)  -- ^ :: ty1 ~ ty1'
                  -> TcType               -- ^ ty2
                  -> TcType               -- ^ ty2, with type synonyms
                  -> TcS (StopOrContinue (Either IrredCt EqCt))
can_eq_newtype_nc :: GlobalRdrEnv
-> (FamInstEnv, FamInstEnv)
-> CtEvidence
-> SwapFlag
-> PredType
-> ((Bag GlobalRdrElt, TcCoercion), PredType)
-> PredType
-> PredType
-> TcS (StopOrContinue (Either IrredCt EqCt))
can_eq_newtype_nc GlobalRdrEnv
rdr_env (FamInstEnv, FamInstEnv)
envs CtEvidence
ev SwapFlag
swapped PredType
ty1 ((Bag GlobalRdrElt
gres, TcCoercion
co1), PredType
ty1') PredType
ty2 PredType
ps_ty2
  = do { String -> SDoc -> TcS ()
traceTcS String
"can_eq_newtype_nc" (SDoc -> TcS ()) -> SDoc -> TcS ()
forall a b. (a -> b) -> a -> b
$
         [SDoc] -> SDoc
forall doc. IsDoc doc => [doc] -> doc
vcat [ CtEvidence -> SDoc
forall a. Outputable a => a -> SDoc
ppr CtEvidence
ev, SwapFlag -> SDoc
forall a. Outputable a => a -> SDoc
ppr SwapFlag
swapped, TcCoercion -> SDoc
forall a. Outputable a => a -> SDoc
ppr TcCoercion
co1, Bag GlobalRdrElt -> SDoc
forall a. Outputable a => a -> SDoc
ppr Bag GlobalRdrElt
gres, PredType -> SDoc
forall a. Outputable a => a -> SDoc
ppr PredType
ty1', PredType -> SDoc
forall a. Outputable a => a -> SDoc
ppr PredType
ty2 ]

         -- Check for blowing our stack, and increase the depth
         -- See Note [Newtypes can blow the stack]
       ; let loc :: CtLoc
loc = CtEvidence -> CtLoc
ctEvLoc CtEvidence
ev
             ev' :: CtEvidence
ev' = CtEvidence
ev CtEvidence -> CtLoc -> CtEvidence
`setCtEvLoc` CtLoc -> CtLoc
bumpCtLocDepth CtLoc
loc
       ; CtLoc -> PredType -> TcS ()
checkReductionDepth CtLoc
loc PredType
ty1

         -- Next, we record uses of newtype constructors, since coercing
         -- through newtypes is tantamount to using their constructors.
       ; Bag GlobalRdrElt -> TcS ()
recordUsedGREs Bag GlobalRdrElt
gres

       ; let redn1 :: Reduction
redn1 = TcCoercion -> PredType -> Reduction
mkReduction TcCoercion
co1 PredType
ty1'
             redn2 :: Reduction
redn2 = Role -> PredType -> Reduction
mkReflRedn Role
Representational PredType
ps_ty2
       ; new_ev <- CtEvidence
-> SwapFlag
-> Reduction
-> Reduction
-> CoHoleSet
-> TcS CtEvidence
rewriteEqEvidence CtEvidence
ev' SwapFlag
swapped Reduction
redn1 Reduction
redn2
                                     CoHoleSet
emptyCoHoleSet

       ; can_eq_nc False rdr_env envs new_ev ReprEq ty1' ty1' ty2 ps_ty2 }

---------
-- ^ Decompose a type application.
-- All input types must be rewritten. See Note [Canonicalising type applications]
-- Nominal equality only!
can_eq_app :: CtEvidence       -- :: s1 t1 ~N s2 t2
           -> Xi -> Xi         -- s1 t1
           -> Xi -> Xi         -- s2 t2
           -> TcS (StopOrContinue (Either IrredCt EqCt))

-- AppTys only decompose for nominal equality, so this case just leads
-- to an irreducible constraint; see typecheck/should_compile/T10494
-- See Note [Decomposing AppTy equalities]
can_eq_app :: CtEvidence
-> PredType
-> PredType
-> PredType
-> PredType
-> TcS (StopOrContinue (Either IrredCt EqCt))
can_eq_app CtEvidence
ev PredType
s1 PredType
t1 PredType
s2 PredType
t2
  | CtWanted (WantedCt { ctev_dest :: WantedCtEvidence -> TcEvDest
ctev_dest = TcEvDest
dest }) <- CtEvidence
ev
  = do { String -> SDoc -> TcS ()
traceTcS String
"can_eq_app" ([SDoc] -> SDoc
forall doc. IsDoc doc => [doc] -> doc
vcat [ String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"s1:" SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> PredType -> SDoc
forall a. Outputable a => a -> SDoc
ppr PredType
s1, String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"t1:" SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> PredType -> SDoc
forall a. Outputable a => a -> SDoc
ppr PredType
t1
                                     , String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"s2:" SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> PredType -> SDoc
forall a. Outputable a => a -> SDoc
ppr PredType
s2, String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"t2:" SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> PredType -> SDoc
forall a. Outputable a => a -> SDoc
ppr PredType
t2
                                     , String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"vis:" SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> Bool -> SDoc
forall a. Outputable a => a -> SDoc
ppr (PredType -> Bool
isNextArgVisible PredType
s1) ])
       ; co_plus_holes <- CtEvidence
-> Role -> (UnifyEnv -> TcM TcCoercion) -> TcS CoercionPlusHoles
wrapUnifierAndEmit CtEvidence
ev Role
Nominal ((UnifyEnv -> TcM TcCoercion) -> TcS CoercionPlusHoles)
-> (UnifyEnv -> TcM TcCoercion) -> TcS CoercionPlusHoles
forall a b. (a -> b) -> a -> b
$ \UnifyEnv
uenv ->
            -- Unify arguments t1/t2 before function s1/s2, because
            -- the former have smaller kinds, and hence simpler error messages
            -- c.f. GHC.Tc.Utils.Unify.uType (go_app)
            do { let mb_invis :: Maybe InvisibleBit
mb_invis = if PredType -> Bool
isNextArgVisible PredType
s1
                                then Maybe InvisibleBit
forall a. Maybe a
Nothing
                                else InvisibleBit -> Maybe InvisibleBit
forall a. a -> Maybe a
Just InvisibleBit
InvisibleKind
                     arg_env :: UnifyEnv
arg_env = UnifyEnv -> (CtLoc -> CtLoc) -> UnifyEnv
updUEnvLoc UnifyEnv
uenv (Maybe InvisibleBit -> Bool -> CtLoc -> CtLoc
adjustCtLoc Maybe InvisibleBit
mb_invis Bool
False)
               ; co_t <- UnifyEnv -> PredType -> PredType -> TcM TcCoercion
uType UnifyEnv
arg_env PredType
t1 PredType
t2
               ; co_s <- uType uenv s1 s2
               ; return (mkAppCo co_s co_t) }
       ; setWantedEq dest co_plus_holes
       ; stopWith ev "Decomposed [W] AppTy" }

    -- If there is a ForAll/(->) mismatch, the use of the Left coercion
    -- below is ill-typed, potentially leading to a panic in splitTyConApp
    -- Test case: typecheck/should_run/Typeable1
    -- We could also include this mismatch check above (for W and D), but it's slow
    -- and we'll get a better error message not doing it
  | PredType
s1k PredType -> PredType -> Bool
`mismatches` PredType
s2k
  = CtEvidence
-> PredType
-> PredType
-> TcS (StopOrContinue (Either IrredCt EqCt))
forall a.
CtEvidence
-> PredType -> PredType -> TcS (StopOrContinue (Either IrredCt a))
canEqHardFailure CtEvidence
ev (PredType
s1 PredType -> PredType -> PredType
`mkAppTy` PredType
t1) (PredType
s2 PredType -> PredType -> PredType
`mkAppTy` PredType
t2)

  | CtGiven (GivenCt { ctev_evar :: GivenCtEvidence -> TyVar
ctev_evar = TyVar
evar }) <- CtEvidence
ev
  = do { let co :: TcCoercion
co   = TyVar -> TcCoercion
mkCoVarCo TyVar
evar
             co_s :: TcCoercion
co_s = LeftOrRight -> TcCoercion -> TcCoercion
mkLRCo LeftOrRight
CLeft  TcCoercion
co
             co_t :: TcCoercion
co_t = LeftOrRight -> TcCoercion -> TcCoercion
mkLRCo LeftOrRight
CRight TcCoercion
co
       ; ev_s <- CtLoc -> (PredType, EvTerm) -> TcS GivenCtEvidence
newGivenEv CtLoc
loc ( CtEvidence -> PredType -> PredType -> PredType
mkTcEqPredLikeEv CtEvidence
ev PredType
s1 PredType
s2
                                , TcCoercion -> EvTerm
evCoercion TcCoercion
co_s )
       ; ev_t <- newGivenEv loc ( mkTcEqPredLikeEv ev t1 t2
                                , evCoercion co_t )
       ; emitWorkNC [CtGiven ev_t]
       ; startAgainWith (mkNonCanonical $ CtGiven ev_s) }

  where
    loc :: CtLoc
loc = CtEvidence -> CtLoc
ctEvLoc CtEvidence
ev

    s1k :: PredType
s1k = HasDebugCallStack => PredType -> PredType
PredType -> PredType
typeKind PredType
s1
    s2k :: PredType
s2k = HasDebugCallStack => PredType -> PredType
PredType -> PredType
typeKind PredType
s2

    PredType
k1 mismatches :: PredType -> PredType -> Bool
`mismatches` PredType
k2
      =  PredType -> Bool
isForAllTy PredType
k1 Bool -> Bool -> Bool
&& Bool -> Bool
not (PredType -> Bool
isForAllTy PredType
k2)
      Bool -> Bool -> Bool
|| Bool -> Bool
not (PredType -> Bool
isForAllTy PredType
k1) Bool -> Bool -> Bool
&& PredType -> Bool
isForAllTy PredType
k2

-----------------------
-- | Break apart an equality over a casted type
-- looking like   (ty1 |> co1) ~ ty2   (modulo a swap-flag)
canEqCast :: Bool         -- are both types rewritten?
          -> GlobalRdrEnv -> FamInstEnvs
          -> CtEvidence
          -> EqRel
          -> SwapFlag
          -> TcType -> Coercion   -- LHS (res. RHS), ty1 |> co1
          -> TcType -> TcType     -- RHS (res. LHS), ty2 both normal and pretty
          -> TcS (StopOrContinue (Either IrredCt EqCt))
canEqCast :: Bool
-> GlobalRdrEnv
-> (FamInstEnv, FamInstEnv)
-> CtEvidence
-> EqRel
-> SwapFlag
-> PredType
-> TcCoercion
-> PredType
-> PredType
-> TcS (StopOrContinue (Either IrredCt EqCt))
canEqCast Bool
rewritten GlobalRdrEnv
rdr_env (FamInstEnv, FamInstEnv)
envs CtEvidence
ev EqRel
eq_rel SwapFlag
swapped PredType
ty1 TcCoercion
co1 PredType
ty2 PredType
ps_ty2
  = do { String -> SDoc -> TcS ()
traceTcS String
"Decomposing cast" ([SDoc] -> SDoc
forall doc. IsDoc doc => [doc] -> doc
vcat [ CtEvidence -> SDoc
forall a. Outputable a => a -> SDoc
ppr CtEvidence
ev
                                           , PredType -> SDoc
forall a. Outputable a => a -> SDoc
ppr PredType
ty1 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
<+> TcCoercion -> SDoc
forall a. Outputable a => a -> SDoc
ppr TcCoercion
co1
                                           , PredType -> SDoc
forall a. Outputable a => a -> SDoc
ppr PredType
ps_ty2 ])
       ; new_ev <- CtEvidence
-> SwapFlag
-> Reduction
-> Reduction
-> CoHoleSet
-> TcS CtEvidence
rewriteEqEvidence CtEvidence
ev SwapFlag
swapped
                      (Role -> PredType -> TcCoercion -> Reduction
mkGReflLeftRedn Role
role PredType
ty1 TcCoercion
co1)
                      (Role -> PredType -> Reduction
mkReflRedn Role
role PredType
ps_ty2)
                      CoHoleSet
emptyCoHoleSet
       ; can_eq_nc rewritten rdr_env envs new_ev eq_rel ty1 ty1 ty2 ps_ty2 }
  where
    role :: Role
role = EqRel -> Role
eqRelRole EqRel
eq_rel

------------------------
canTyConApp :: CtEvidence -> EqRel
            -> Bool  -- True <=> both TyCons are generative
            -> (Type,TyCon,[TcType])
            -> (Type,TyCon,[TcType])
            -> TcS (StopOrContinue (Either IrredCt EqCt))
-- See Note [Decomposing TyConApp equalities]
-- Neither tc1 nor tc2 is a saturated funTyCon, nor a type family
-- But they can be data families.
canTyConApp :: CtEvidence
-> EqRel
-> Bool
-> (PredType, TyCon, [PredType])
-> (PredType, TyCon, [PredType])
-> TcS (StopOrContinue (Either IrredCt EqCt))
canTyConApp CtEvidence
ev EqRel
eq_rel Bool
both_generative (PredType
ty1,TyCon
tc1,[PredType]
tys1) (PredType
ty2,TyCon
tc2,[PredType]
tys2)
  | TyCon
tc1 TyCon -> TyCon -> Bool
forall a. Eq a => a -> a -> Bool
== TyCon
tc2
  , [PredType]
tys1 [PredType] -> [PredType] -> Bool
forall a b. [a] -> [b] -> Bool
`equalLength` [PredType]
tys2
  = do { inerts <- TcS InertSet
getInertSet
       ; if can_decompose inerts
         then canDecomposableTyConAppOK ev eq_rel tc1 (ty1,tys1) (ty2,tys2)
         else assert (eq_rel == ReprEq) $
              canEqSoftFailure ReprEqReason ev ty1 ty2 }

  -- See Note [Skolem abstract data] in GHC.Core.Tycon
  | TyCon -> Bool
tyConSkolem TyCon
tc1 Bool -> Bool -> Bool
|| TyCon -> Bool
tyConSkolem TyCon
tc2
  = do { String -> SDoc -> TcS ()
traceTcS String
"canTyConApp: skolem abstract" (TyCon -> SDoc
forall a. Outputable a => a -> SDoc
ppr TyCon
tc1 SDoc -> SDoc -> SDoc
forall doc. IsDoc doc => doc -> doc -> doc
$$ TyCon -> SDoc
forall a. Outputable a => a -> SDoc
ppr TyCon
tc2)
       ; CtIrredReason
-> CtEvidence -> TcS (StopOrContinue (Either IrredCt EqCt))
forall a.
CtIrredReason
-> CtEvidence -> TcS (StopOrContinue (Either IrredCt a))
finishCanWithIrred CtIrredReason
AbstractTyConReason CtEvidence
ev }

  -- Different TyCons
  | EqRel
NomEq <- EqRel
eq_rel
  = CtEvidence
-> PredType
-> PredType
-> TcS (StopOrContinue (Either IrredCt EqCt))
forall a.
CtEvidence
-> PredType -> PredType -> TcS (StopOrContinue (Either IrredCt a))
canEqHardFailure CtEvidence
ev PredType
ty1 PredType
ty2

  -- Different TyCons, eq_rel = ReprEq
  -- See (TC2) and (TC3) in
  -- Note [Canonicalising TyCon/TyCon equalities]
  | Bool
both_generative
  = CtEvidence
-> PredType
-> PredType
-> TcS (StopOrContinue (Either IrredCt EqCt))
forall a.
CtEvidence
-> PredType -> PredType -> TcS (StopOrContinue (Either IrredCt a))
canEqHardFailure CtEvidence
ev PredType
ty1 PredType
ty2

  | Bool
otherwise
  = CtIrredReason
-> CtEvidence
-> PredType
-> PredType
-> TcS (StopOrContinue (Either IrredCt EqCt))
forall a.
CtIrredReason
-> CtEvidence
-> PredType
-> PredType
-> TcS (StopOrContinue (Either IrredCt a))
canEqSoftFailure CtIrredReason
ReprEqReason CtEvidence
ev PredType
ty1 PredType
ty2
  where
     -- See Note [Decomposing TyConApp equalities]
     -- and Note [Decomposing newtype equalities]
    can_decompose :: InertSet -> Bool
can_decompose InertSet
inerts
      =  TyCon -> Role -> Bool
isInjectiveTyCon TyCon
tc1 (EqRel -> Role
eqRelRole EqRel
eq_rel)
      Bool -> Bool -> Bool
|| (Bool -> Bool -> Bool
forall a. HasCallStack => Bool -> a -> a
assert (EqRel
eq_rel EqRel -> EqRel -> Bool
forall a. Eq a => a -> a -> Bool
== EqRel
ReprEq) (Bool -> Bool) -> Bool -> Bool
forall a b. (a -> b) -> a -> b
$
          -- assert: isInjectiveTyCon is always True for Nominal except
          --   for type synonyms/families, neither of which happen here
          -- Moreover isInjectiveTyCon is True for Representational
          --   for algebraic data types.  So we are down to newtypes
          --   and data families.
          CtEvidence -> CtFlavour
ctEvFlavour CtEvidence
ev CtFlavour -> CtFlavour -> Bool
forall a. Eq a => a -> a -> Bool
== CtFlavour
Wanted Bool -> Bool -> Bool
&& TyCon -> InertSet -> Bool
noGivenNewtypeReprEqs TyCon
tc1 InertSet
inerts)
             -- See Note [Decomposing newtype equalities] (EX2)

{- Note [Canonicalising TyCon/TyCon equalities]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider

  type family TF a where TF Char = Bool
  data family DF a
  newtype instance DF Bool = MkDF Int

Suppose we are canonicalising [W] Int ~R# DF (TF a).  Then

(TC1) We might have an inert Given (a ~# Char), so if we rewrote the wanted
      (i.e. went around again in `can_eq_nc` with `rewritten`=True, we'd get
         [W] Int ~R# DF Bool
      and then the `tcTopNormaliseNewTypeTF_maybe` call would fire and
      we'd unwrap the newtype.  So we must do that "go round again" bit.
      Hence the complicated guard (rewritten || both_generative) in `can_eq_nc`.

(TC2) If we can't rewrite `a` yet, we'll finish with an unsolved
         [W] Int ~R# DF (TF a)
      in the inert set. But we must use canEqSoftFailure, not canEqHardFailure,
      because it might be solved "later" when we learn more about `a`.
      Hence the use of `both_generative` in `canTyConApp`.

(TC3) Here's another example:
         [G] Age ~R# Int
      where Age's constructor is not in scope. We don't want to report
      an "inaccessible code" error in the context of this Given!  So again
      we want `canEqSoftFailure`.

      For example, see typecheck/should_compile/T10493, repeated here:
        import Data.Ord (Down)  -- no constructor
        foo :: Coercible (Down Int) Int => Down Int -> Int
        foo = coerce

      That should compile, but only because we use canEqSoftFailure and
      not canEqHardFailure.

Note [Fast path when decomposing TyConApps]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
If we see (T s1 t1 ~ T s2 t2), then we can just decompose to
  (s1 ~ s2, t1 ~ t2)
and push those back into the work list.  But if
  s1 = K k1    s2 = K k2
then we will just decompose s1~s2, and it might be better to
do so on the spot.  An important special case is where s1=s2,
and we get just Refl.

So canDecomposableTyConAppOK uses wrapUnifierAndEmit etc to short-cut
that work.  See also Note [Work-list ordering].

Note [Decomposing TyConApp equalities]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Suppose we have
        [G/W] T ty1 ~r T ty2
Can we decompose it, and replace it by
        [G/W] ty1 ~r' ty2
and if so what role is r'?  (In this Note, all the "~" are primitive
equalities "~#", but I have dropped the noisy "#" symbols.)  Lots of
background in the paper "Safe zero-cost coercions for Haskell".

This Note covers the topic for
  * Datatypes
  * Newtypes
  * Data families
For the rest:
  * Type synonyms: are always expanded
  * Type families: not decomposed
  * AppTy:         see Note [Decomposing AppTy equalities].

---- Roles of the decomposed constraints ----
For a start, the role r' will always be defined like this:
  * If r=N then r' = N
  * If r=R then r' = role of T's first argument

For example:
   data TR a = MkTR a       -- Role of T's first arg is Representational
   data TN a = MkTN (F a)   -- Role of T's first arg is Nominal

The function tyConRolesX :: Role -> TyCon -> [Role] gets the argument
role r' for a TyCon T at role r.  E.g.
   tyConRolesX Nominal          TR = [Nominal]
   tyConRolesX Representational TR = [Representational]

---- Soundness and completeness ----
For Givens, for /soundness/ of decomposition we need, forall ty1,ty2:
    T ty1 ~r T ty2   ===>    ty1 ~r' ty2
Here "===>" means "implies".  That is, given evidence for (co1 : T ty1 ~r T co2)
we can produce evidence for (co2 : ty1 ~r' ty2).  But in the solver we
/replace/ co1 with co2 in the inert set, and we don't want to lose any proofs
thereby. So for /completeness/ of decomposition we also need the reverse:
    ty1 ~r' ty2   ===>    T ty1 ~r T ty2

For Wanteds, for /soundness/ of decomposition we need:
    ty1 ~r' ty2   ===>    T ty1 ~r T ty2
because if we do decompose we'll get evidence (co2 : ty1 ~r' ty2) and
from that we want to derive evidence (T co2 : T ty1 ~r T ty2).
For /completeness/ of decomposition we need the reverse implication too,
else we may decompose to a new proof obligation that is stronger than
the one we started with.  See Note [Decomposing newtype equalities].

---- Injectivity ----
When do these bi-implications hold? In one direction it is easy.
We /always/ have
    ty1 ~r'  ty2   ===>    T ty1 ~r T ty2
This is the CO_TYCONAPP rule of the paper (Fig 5); see also the
TyConAppCo case of GHC.Core.Lint.lintCoercion.

In the other direction, we have
    T ty1 ~r T ty2   ==>   ty1 ~r' ty2  if T is /injective at role r/
This is the very /definition/ of injectivity: injectivity means result
is the same => arguments are the same, modulo the role shift.
See comments on GHC.Core.TyCon.isInjectiveTyCon.  This is also
the CO_NTH rule in Fig 5 of the paper, except in the paper only
newtypes are non-injective at representation role, so the rule says
"H is not a newtype".

Injectivity is a bit subtle:
                 Nominal   Representational
   Datatype        YES        YES
   Newtype         YES        NO{1}
   Data family     YES        NO{2}

{1} Consider newtype N a = MkN (F a)   -- Arg has Nominal role
    Is it true that (N t1) ~R (N t2)   ==>   t1 ~N t2  ?
    No, absolutely not.  E.g.
       type instance F Int = Int; type instance F Bool = Char
       Then (N Int) ~R (N Bool), by unwrapping, but we don't want Int~Char!

    See Note [Decomposing newtype equalities]

{2} We must treat data families precisely like newtypes, because of the
    possibility of newtype instances. See also
    Note [Decomposing newtype equalities]. See #10534 and
    test case typecheck/should_fail/T10534.

---- Takeaway summary -----
For sound and complete decomposition, we simply need injectivity;
that is for isInjectiveTyCon to be true:

* At Nominal role, isInjectiveTyCon is True for all the TyCons we are
  considering in this Note: datatypes, newtypes, and data families.

* For Givens, injectivity is necessary for soundness; completeness has no
  side conditions.

* For Wanteds, soundness has no side conditions; but injectivity is needed
  for completeness. See Note [Decomposing newtype equalities]

This is implemented in `can_decompose` in `canTyConApp`; it looks at
injectivity, just as specified above.

Note [Work-list ordering]
~~~~~~~~~~~~~~~~~~~~~~~~~
Consider decomposing a TyCon equality

    (0) [W] T k_fresh (t1::k_fresh) ~ T k1 (t2::k1)

This gives rise to 2 equalities in the solver worklist

    (1) [W] k_fresh ~ k1
    (2) [W] t1::k_fresh ~ t2::k1

We would like to solve (1) before looking at (2), so that we don't end
up in the complexities of canEqCanLHSHetero.  To do this:

* `canDecomposableTyConAppOK` calls `uType` on the arguments
  /left-to-right/.  See the call to zipWith4M in that function.

* `uType` keeps the bag of emitted constraints in the same
  left-to-right order.  See the use of `snocBag` in `uType_defer`.

* `wrapUnifierAndEmit` adds the bag of deferred constraints from
  `do_unifications` to the work-list using `extendWorkListChildEqs`.

* `extendWorkListChildEqs` and `selectWorkItem` together arrange that the
  list of constraints given to `extendWorkListChildEqs` is processed in
  left-to-right order.

This is not a very big deal.  It reduces the number of solver steps
in the test RaeJobTalk from 1830 to 1815, a 1% reduction.  But still,
it doesn't cost anything either.

Note [Decomposing newtype equalities]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
This Note also applies to data families, which we treat like
newtype in case of 'newtype instance'.

As Note [Decomposing TyConApp equalities] describes, if N is injective
at role r, we can do this decomposition?
   [G/W] (N ty1) ~r (N ty2)    to     [G/W]  ty1 ~r' ty2

For a Given with r=R, the answer is a solid NO: newtypes are not injective at
representational role, and we must not decompose, or we lose soundness.
Example is wrinkle {1} in Note [Decomposing TyConApp equalities].

For a Wanted with r=R, since newtypes are not injective at representational
role, decomposition is sound, but we may lose completeness.  Nevertheless,
if the newtype is abstract (so can't be unwrapped) we can only solve
the equality by (a) using a Given or (b) decomposition.  If (a) is impossible
(e.g. no Givens) then (b) is safe albeit potentially incomplete.

There are two ways in which decomposing (N ty1) ~r (N ty2) could be incomplete:

* Incompleteness example (EX1): unwrap first
      newtype Nt a = MkNt (Id a)
      type family Id a where Id a = a

      [W] Nt Int ~R Nt Age

  Because of its use of a type family, Nt's parameter will get inferred to
  have a nominal role. Thus, decomposing the wanted will yield [W] Int ~N Age,
  which is unsatisfiable. Unwrapping, though, leads to a solution.

  CONCLUSION: always unwrap newtypes before attempting to decompose
  them.  This is done in can_eq_nc.  Of course, we can't unwrap if the data
  constructor isn't in scope.  See Note [Unwrap newtypes first].

* Incompleteness example (EX2): prioritise Nominal equalities. See #24887
      data family D a
      data instance D Int  = MkD1 (D Char)
      data instance D Bool = MkD2 (D Char)
  Now suppose we have
      [W] g1: D Int ~R# D a
      [W] g2: a ~# Bool
  If we solve g2 first, giving a:=Bool, then we can solve g1 easily:
      D Int ~R# D Char ~R# D Bool
  by newtype unwrapping.

  BUT: if we instead attempt to solve g1 first, we can unwrap the LHS (only)
  leaving     [W] D Char ~#R D Bool
  If we decompose now, we'll get (Char ~R# Bool), which is insoluble.

  CONCLUSION: prioritise nominal equalites in the work list.
  See Note [Prioritise equalities] in GHC.Tc.Solver.InertSet.

* Incompleteness example (EX3): check available Givens
      newtype Nt a = Mk Bool         -- NB: a is not used in the RHS,
      type role Nt representational  -- but the user gives it an R role anyway

      [G] Nt t1 ~R Nt t2
      [W] Nt alpha ~R Nt beta

  We *don't* want to decompose to [W] alpha ~R beta, because it's possible
  that alpha and beta aren't representationally equal.  And if we figure
  out (elsewhere) that alpha:=t1 and beta:=t2, we can solve the Wanted
  from the Given.  This is somewhat similar to the question of overlapping
  Givens for class constraints: see Note [Instance and Given overlap] in
  GHC.Tc.Solver.Dict.

  CONCLUSION: don't decompose [W] N s ~R N t, if there are any Given
  equalities that could later solve it.

  But what precisely does it mean to say "any Given equalities that could
  later solve it"?  It's tricky!

  * In #22924 we had
       [G] f a ~R# a     [W] Const (f a) a ~R# Const a a
    where Const is an abstract newtype.  If we decomposed the newtype, we
    could solve.  Not-decomposing on the grounds that (f a ~R# a) might turn
    into (Const (f a) a ~R# Const a a) seems a bit silly.

  * In #22331 we had
       [G] N a ~R# N b   [W] N b ~R# N a
    (where N is abstract so we can't unwrap). Here we really /don't/ want to
    decompose, because the /only/ way to solve the Wanted is from that Given
    (with a Sym).

  * In #22519 we had
       [G] a <= b     [W] IO Age ~R# IO Int

    (where IO is abstract so we can't unwrap, and newtype Age = Int; and (<=)
    is a type-level comparison on Nats).  Here we /must/ decompose, despite the
    existence of an Irred Given, or we will simply be stuck.  (Side note: We
    flirted with deep-rewriting of newtypes (see discussion on #22519 and
    !9623) but that turned out not to solve #22924, and also makes type
    inference loop more often on recursive newtypes.)

  * In #26020 we had a /quantified/ constraint
        forall x. Coercible (N t1) (N t2)
    and (roughly) [W] N t1 ~R# N t2
    That quantified constraint can solve the Wanted, so don't decompose!

  The currently-implemented compromise is this:
       We decompose [W] N s ~R# N t unless there is
       - an Irred [G] N s' ~ N t'
       - a quantified [G] forall ... => N s' ~ N t'
       that is, a Given equality with both sides headed with N.
  See the call to `noGivenNewtypeReprEqs` in `canTyConApp`.

  This is not perfect.  In principle a Given like [G] (a b) ~ (c d), or
  even just [G] c, could later turn into N s ~ N t.  But since the free
  vars of a Given are skolems, or at least untouchable unification
  variables, this is extremely unlikely to happen.

  Another worry: there could, just, be a CDictCan with some
  un-expanded equality superclasses; but only in some very obscure
  recursive-superclass situations.

   Yet another approach (!) is described in
   Note [Decomposing newtypes a bit more aggressively].

Remember: decomposing Wanteds is always /sound/. This Note is
only about /completeness/.

Note [Decomposing newtypes a bit more aggressively]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
IMPORTANT: the ideas in this Note are *not* implemented. Instead, the
current approach is detailed in Note [Decomposing newtype equalities]
and Note [Unwrap newtypes first].
For more details about the ideas in this Note see
  * GHC propoosal: https://github.com/ghc-proposals/ghc-proposals/pull/549
  * issue #22441
  * discussion on !9282.

Consider [G] c, [W] (IO Int) ~R (IO Age)
where IO is abstract, and
   newtype Age = MkAge Int   -- Not abstract
With the above rules, if there any Given Irreds,
the Wanted is insoluble because we can't decompose it.  But in fact,
if we look at the defn of IO, roughly,
    newtype IO a = State# -> (State#, a)
we can see that decomposing [W] (IO Int) ~R (IO Age) to
    [W] Int ~R Age
definitely does not lose completeness. Why not? Because the role of
IO's argment is representational.  Hence:

  DecomposeNewtypeIdea:
     decompose [W] (N s1 .. sn) ~R (N t1 .. tn)
     if the roles of all N's arguments are representational

If N's arguments really /are/ representational this will not lose
completeness.  Here "really are representational" means "if you expand
all newtypes in N's RHS, we'd infer a representational role for each
of N's type variables in that expansion".  See Note [Role inference]
in GHC.Tc.TyCl.Utils.

But the user might /override/ a phantom role with an explicit role
annotation, and then we could (obscurely) get incompleteness.
Consider

   module A( silly, T ) where
     newtype T a = MkT Int
     type role T representational  -- Override phantom role

     silly :: Coercion (T Int) (T Bool)
     silly = Coercion  -- Typechecks by unwrapping the newtype

     data Coercion a b where  -- Actually defined in Data.Type.Coercion
       Coercion :: Coercible a b => Coercion a b

   module B where
     import A
     f :: T Int -> T Bool
     f = case silly of Coercion -> coerce

Here the `coerce` gives [W] (T Int) ~R (T Bool) which, if we decompose,
we'll get stuck with (Int ~R Bool).  Instead we want to use the
[G] (T Int) ~R (T Bool), which will be in the Irreds.

Summary: we could adopt (DecomposeNewtypeIdea), at the cost of a very
obscure incompleteness (above).  But no one is reporting a problem from
the lack of decompostion, so we'll just leave it for now.  This long
Note is just to record the thinking for our future selves.

Note [Decomposing AppTy equalities]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
For AppTy all the same questions arise as in
Note [Decomposing TyConApp equalities]. We have

    s1 ~r s2,  t1 ~N t2   ==>   s1 t1 ~r s2 t2       (rule CO_APP)
    s1 t1 ~N s2 t2        ==>   s1 ~N s2,  t1 ~N t2  (CO_LEFT, CO_RIGHT)

In the first of these, why do we need Nominal equality in (t1 ~N t2)?
See {2} below.

For sound and complete solving, we need both directions to decompose. So:
* At nominal role, all is well: we have both directions.
* At representational role, decomposition of Givens is unsound (see {1} below),
  and decomposition of Wanteds is incomplete.

Here is an example of the incompleteness for Wanteds:

    [G] g1 :: a ~R b
    [W] w1 :: Maybe b ~R alpha a
    [W] w2 :: alpha ~N Maybe

Suppose we see w1 before w2. If we decompose, using AppCo to prove w1, we get

    w1 := AppCo w3 w4
    [W] w3 :: Maybe ~R alpha
    [W] w4 :: b ~N a

Note that w4 is *nominal*. A nominal role here is necessary because AppCo
requires a nominal role on its second argument. (See {2} for an example of
why.) Now we are stuck, because w4 is insoluble. On the other hand, if we
see w2 first, setting alpha := Maybe, all is well, as we can decompose
Maybe b ~R Maybe a into b ~R a.

Another example:
    newtype Phant x = MkPhant Int
    [W] w1 :: Phant Int ~R alpha Bool
    [W] w2 :: alpha ~ Phant

If we see w1 first, decomposing would be disastrous, as we would then try
to solve Int ~ Bool. Instead, spotting w2 allows us to simplify w1 to become
    [W] w1' :: Phant Int ~R Phant Bool

which can then (assuming MkPhant is in scope) be simplified to Int ~R Int,
and all will be well. See also Note [Unwrap newtypes first].

Bottom line:
* Always decompose AppTy at nominal role: can_eq_app
* Never decompose AppTy at representational role (neither Given nor Wanted):
  the lack of an equation in can_eq_nc

Extra points
{1}  Decomposing a Given AppTy over a representational role is simply
     unsound. For example, if we have co1 :: Phant Int ~R a Bool (for
     the newtype Phant, above), then we surely don't want any relationship
     between Int and Bool, lest we also have co2 :: Phant ~ a around.

{2} The role on the AppCo coercion is a conservative choice, because we don't
    know the role signature of the function. For example, let's assume we could
    have a representational role on the second argument of AppCo. Then, consider

    data G a where    -- G will have a nominal role, as G is a GADT
      MkG :: G Int
    newtype Age = MkAge Int

    co1 :: G ~R a        -- by assumption
    co2 :: Age ~R Int    -- by newtype axiom
    co3 = AppCo co1 co2 :: G Age ~R a Int    -- by our broken AppCo

    and now co3 can be used to cast MkG to have type G Age, in violation of
    the way GADTs are supposed to work (which is to use nominal equality).
-}

canDecomposableTyConAppOK :: CtEvidence -> EqRel
                          -> TyCon
                          -> (Type,[TcType]) -> (Type,[TcType])
                          -> TcS (StopOrContinue a)
-- Precondition: tys1 and tys2 are the same finite length, hence "OK"
canDecomposableTyConAppOK :: forall a.
CtEvidence
-> EqRel
-> TyCon
-> (PredType, [PredType])
-> (PredType, [PredType])
-> TcS (StopOrContinue a)
canDecomposableTyConAppOK CtEvidence
ev EqRel
eq_rel TyCon
tc (PredType
ty1,[PredType]
tys1) (PredType
ty2,[PredType]
tys2)
  = Bool -> TcS (StopOrContinue a) -> TcS (StopOrContinue a)
forall a. HasCallStack => Bool -> a -> a
assert ([PredType]
tys1 [PredType] -> [PredType] -> Bool
forall a b. [a] -> [b] -> Bool
`equalLength` [PredType]
tys2) (TcS (StopOrContinue a) -> TcS (StopOrContinue a))
-> TcS (StopOrContinue a) -> TcS (StopOrContinue a)
forall a b. (a -> b) -> a -> b
$
    do { String -> SDoc -> TcS ()
traceTcS String
"canDecomposableTyConAppOK"
                  (CtEvidence -> SDoc
forall a. Outputable a => a -> SDoc
ppr CtEvidence
ev SDoc -> SDoc -> SDoc
forall doc. IsDoc doc => doc -> doc -> doc
$$ EqRel -> SDoc
forall a. Outputable a => a -> SDoc
ppr EqRel
eq_rel SDoc -> SDoc -> SDoc
forall doc. IsDoc doc => doc -> doc -> doc
$$ TyCon -> SDoc
forall a. Outputable a => a -> SDoc
ppr TyCon
tc SDoc -> SDoc -> SDoc
forall doc. IsDoc doc => doc -> doc -> doc
$$ [PredType] -> SDoc
forall a. Outputable a => a -> SDoc
ppr [PredType]
tys1 SDoc -> SDoc -> SDoc
forall doc. IsDoc doc => doc -> doc -> doc
$$ [PredType] -> SDoc
forall a. Outputable a => a -> SDoc
ppr [PredType]
tys2)
       ; case CtEvidence
ev of
           CtWanted (WantedCt { ctev_dest :: WantedCtEvidence -> TcEvDest
ctev_dest = TcEvDest
dest })
             -- new_locs and tc_roles are both infinite, so we are
             -- guaranteed that cos has the same length as tys1 and tys2
             -- See Note [Fast path when decomposing TyConApps]
             -> do { co_plus_holes <- CtEvidence
-> Role -> (UnifyEnv -> TcM TcCoercion) -> TcS CoercionPlusHoles
wrapUnifierAndEmit CtEvidence
ev Role
role ((UnifyEnv -> TcM TcCoercion) -> TcS CoercionPlusHoles)
-> (UnifyEnv -> TcM TcCoercion) -> TcS CoercionPlusHoles
forall a b. (a -> b) -> a -> b
$ \UnifyEnv
uenv ->
                        do { cos <- (CtLoc -> Role -> PredType -> PredType -> TcM TcCoercion)
-> [CtLoc]
-> [Role]
-> [PredType]
-> [PredType]
-> IOEnv (Env TcGblEnv TcLclEnv) [TcCoercion]
forall (m :: * -> *) a b c d e.
Monad m =>
(a -> b -> c -> d -> m e) -> [a] -> [b] -> [c] -> [d] -> m [e]
zipWith4M (UnifyEnv -> CtLoc -> Role -> PredType -> PredType -> TcM TcCoercion
u_arg UnifyEnv
uenv) [CtLoc]
new_locs [Role]
tc_roles [PredType]
tys1 [PredType]
tys2
                                    -- zipWith4M: see Note [Work-list ordering]
                           ; return (mkTyConAppCo role tc cos) }
                   ; setWantedEq dest co_plus_holes }

           CtGiven (GivenCt { ctev_evar :: GivenCtEvidence -> TyVar
ctev_evar = TyVar
evar })
             | let pred_ty :: PredType
pred_ty = EqRel -> PredType -> PredType -> PredType
mkEqPred EqRel
eq_rel PredType
ty1 PredType
ty2
                   ev_co :: TcCoercion
ev_co   = TyVar -> TcCoercion
mkCoVarCo (TyVar -> PredType -> TyVar
setVarType TyVar
evar PredType
pred_ty)
                   -- setVarType: satisfy Note [mkSelCo precondition] in Coercion.hs
                   -- Remember: ty1/ty2 may be more fully zonked than evar
                   --           See the call to canonicaliseEquality in solveEquality.
             -> CtLoc -> [(Role, TcCoercion)] -> TcS ()
emitNewGivens CtLoc
loc
                       [ (Role
r, HasDebugCallStack => CoSel -> TcCoercion -> TcCoercion
CoSel -> TcCoercion -> TcCoercion
mkSelCo (Int -> Role -> CoSel
SelTyCon Int
i Role
r) TcCoercion
ev_co)
                       | (Role
r, PredType
ty1, PredType
ty2, Int
i) <- [Role]
-> [PredType]
-> [PredType]
-> [Int]
-> [(Role, PredType, PredType, Int)]
forall a b c d. [a] -> [b] -> [c] -> [d] -> [(a, b, c, d)]
zip4 [Role]
tc_roles [PredType]
tys1 [PredType]
tys2 [Int
0..]
                       , Role
r Role -> Role -> Bool
forall a. Eq a => a -> a -> Bool
/= Role
Phantom
                       , Bool -> Bool
not (PredType -> Bool
isCoercionTy PredType
ty1) Bool -> Bool -> Bool
&& Bool -> Bool
not (PredType -> Bool
isCoercionTy PredType
ty2) ]

    ; CtEvidence -> String -> TcS (StopOrContinue a)
forall a. CtEvidence -> String -> TcS (StopOrContinue a)
stopWith CtEvidence
ev String
"Decomposed TyConApp" }

  where
    loc :: CtLoc
loc  = CtEvidence -> CtLoc
ctEvLoc CtEvidence
ev
    role :: Role
role = EqRel -> Role
eqRelRole EqRel
eq_rel

    u_arg :: UnifyEnv -> CtLoc -> Role -> PredType -> PredType -> TcM TcCoercion
u_arg UnifyEnv
uenv CtLoc
arg_loc Role
arg_role = UnifyEnv -> PredType -> PredType -> TcM TcCoercion
uType UnifyEnv
arg_env
       where
         arg_env :: UnifyEnv
arg_env = UnifyEnv
uenv UnifyEnv -> Role -> UnifyEnv
`setUEnvRole` Role
arg_role
                        UnifyEnv -> (CtLoc -> CtLoc) -> UnifyEnv
`updUEnvLoc`  CtLoc -> CtLoc -> CtLoc
forall a b. a -> b -> a
const CtLoc
arg_loc

    -- Infinite, to allow for over-saturated TyConApps
    tc_roles :: [Role]
tc_roles = Role -> TyCon -> [Role]
tyConRoleListX Role
role TyCon
tc

      -- Add nuances to the location during decomposition:
      --  * if the argument is a kind argument, remember this, so that error
      --    messages say "kind", not "type". This is determined based on whether
      --    the corresponding tyConBinder is named (that is, dependent)
      --  * if the argument is invisible, note this as well, again by
      --    looking at the corresponding binder
      -- For oversaturated tycons, we need the (repeat loc) tail, which doesn't
      -- do either of these changes. (Forgetting to do so led to #16188)
      --
      -- NB: infinite in length
    new_locs :: [CtLoc]
new_locs = [ TyConBinder -> CtLoc -> CtLoc
adjustCtLocTyConBinder TyConBinder
bndr CtLoc
loc
               | TyConBinder
bndr <- TyCon -> [TyConBinder]
tyConBinders TyCon
tc ]
               [CtLoc] -> [CtLoc] -> [CtLoc]
forall a. [a] -> [a] -> [a]
++ CtLoc -> [CtLoc]
forall a. a -> [a]
repeat CtLoc
loc

canDecomposableFunTy :: CtEvidence -> EqRel -> FunTyFlag
                     -> (Type,Type,Type,Type)   -- (fun_ty,multiplicity,arg,res)
                     -> (Type,Type,Type,Type)   -- (fun_ty,multiplicity,arg,res)
                     -> TcS (StopOrContinue a)
canDecomposableFunTy :: forall a.
CtEvidence
-> EqRel
-> FunTyFlag
-> (PredType, PredType, PredType, PredType)
-> (PredType, PredType, PredType, PredType)
-> TcS (StopOrContinue a)
canDecomposableFunTy CtEvidence
ev EqRel
eq_rel FunTyFlag
af f1 :: (PredType, PredType, PredType, PredType)
f1@(PredType
ty1,PredType
m1,PredType
a1,PredType
r1) f2 :: (PredType, PredType, PredType, PredType)
f2@(PredType
ty2,PredType
m2,PredType
a2,PredType
r2)
  = do { String -> SDoc -> TcS ()
traceTcS String
"canDecomposableFunTy"
                  (CtEvidence -> SDoc
forall a. Outputable a => a -> SDoc
ppr CtEvidence
ev SDoc -> SDoc -> SDoc
forall doc. IsDoc doc => doc -> doc -> doc
$$ EqRel -> SDoc
forall a. Outputable a => a -> SDoc
ppr EqRel
eq_rel SDoc -> SDoc -> SDoc
forall doc. IsDoc doc => doc -> doc -> doc
$$ (PredType, PredType, PredType, PredType) -> SDoc
forall a. Outputable a => a -> SDoc
ppr (PredType, PredType, PredType, PredType)
f1 SDoc -> SDoc -> SDoc
forall doc. IsDoc doc => doc -> doc -> doc
$$ (PredType, PredType, PredType, PredType) -> SDoc
forall a. Outputable a => a -> SDoc
ppr (PredType, PredType, PredType, PredType)
f2)
       ; case CtEvidence
ev of
           CtWanted (WantedCt { ctev_dest :: WantedCtEvidence -> TcEvDest
ctev_dest = TcEvDest
dest })
             -> do { co_plus_holes <- CtEvidence
-> Role -> (UnifyEnv -> TcM TcCoercion) -> TcS CoercionPlusHoles
wrapUnifierAndEmit CtEvidence
ev Role
Nominal ((UnifyEnv -> TcM TcCoercion) -> TcS CoercionPlusHoles)
-> (UnifyEnv -> TcM TcCoercion) -> TcS CoercionPlusHoles
forall a b. (a -> b) -> a -> b
$ \ UnifyEnv
uenv ->
                        do { let mult_env :: UnifyEnv
mult_env = UnifyEnv
uenv UnifyEnv -> (CtLoc -> CtLoc) -> UnifyEnv
`updUEnvLoc` InvisibleBit -> CtLoc -> CtLoc
toInvisibleLoc InvisibleBit
InvisibleMultiplicity
                                                 UnifyEnv -> Role -> UnifyEnv
`setUEnvRole` Role -> FunSel -> Role
funRole Role
role FunSel
SelMult
                           ; mult <- UnifyEnv -> PredType -> PredType -> TcM TcCoercion
uType UnifyEnv
mult_env PredType
m1 PredType
m2
                             -- TODO: it would be good to set an environment for
                             -- RuntimeRep mismatches, but it's difficult to do so because
                             -- we don't explicit call uType on the RuntimeReps here.
                           ; arg  <- uType (uenv `setUEnvRole` funRole role SelArg) a1 a2
                           ; res  <- uType (uenv `setUEnvRole` funRole role SelRes) r1 r2
                           ; return (mkNakedFunCo role af mult arg res) }
                   ; setWantedEq dest co_plus_holes }

           CtGiven (GivenCt { ctev_evar :: GivenCtEvidence -> TyVar
ctev_evar = TyVar
evar })
             | let pred_ty :: PredType
pred_ty = EqRel -> PredType -> PredType -> PredType
mkEqPred EqRel
eq_rel PredType
ty1 PredType
ty2
                   ev_co :: TcCoercion
ev_co   = TyVar -> TcCoercion
mkCoVarCo (TyVar -> PredType -> TyVar
setVarType TyVar
evar PredType
pred_ty)
                   -- setVarType: satisfy Note [mkSelCo precondition] in Coercion.hs
                   -- Remember: ty1/ty2 may be more fully zonked than evar
                   --           See the call to canonicaliseEquality in solveEquality.
             -> CtLoc -> [(Role, TcCoercion)] -> TcS ()
emitNewGivens CtLoc
loc
                       [ (Role -> FunSel -> Role
funRole Role
role FunSel
fs, HasDebugCallStack => CoSel -> TcCoercion -> TcCoercion
CoSel -> TcCoercion -> TcCoercion
mkSelCo (FunSel -> CoSel
SelFun FunSel
fs) TcCoercion
ev_co)
                       | FunSel
fs <- [FunSel
SelMult, FunSel
SelArg, FunSel
SelRes] ]

    ; CtEvidence -> String -> TcS (StopOrContinue a)
forall a. CtEvidence -> String -> TcS (StopOrContinue a)
stopWith CtEvidence
ev String
"Decomposed TyConApp" }

  where
    loc :: CtLoc
loc  = CtEvidence -> CtLoc
ctEvLoc CtEvidence
ev
    role :: Role
role = EqRel -> Role
eqRelRole EqRel
eq_rel

-- | Call canEqSoftFailure when canonicalizing an equality fails, but if the
-- equality is representational, there is some hope for the future.
canEqSoftFailure :: CtIrredReason -> CtEvidence -> TcType -> TcType
                 -> TcS (StopOrContinue (Either IrredCt a))
canEqSoftFailure :: forall a.
CtIrredReason
-> CtEvidence
-> PredType
-> PredType
-> TcS (StopOrContinue (Either IrredCt a))
canEqSoftFailure CtIrredReason
reason CtEvidence
ev PredType
ty1 PredType
ty2
  = do { (redn1, rewriters1) <- CtEvidence -> PredType -> TcS (Reduction, CoHoleSet)
rewrite CtEvidence
ev PredType
ty1
       ; (redn2, rewriters2) <- rewrite ev ty2
            -- We must rewrite the types before putting them in the
            -- inert set, so that we are sure to kick them out when
            -- new equalities become available
       ; traceTcS "canEqSoftFailure" $
         vcat [ ppr ev, ppr redn1, ppr redn2 ]
       ; new_ev <- rewriteEqEvidence ev NotSwapped redn1 redn2
                                     (rewriters1 S.<> rewriters2)
       ; finishCanWithIrred reason new_ev }

-- | Call when canonicalizing an equality fails with utterly no hope.
canEqHardFailure :: CtEvidence -> TcType -> TcType
                 -> TcS (StopOrContinue (Either IrredCt a))
-- See Note [Make sure that insolubles are fully rewritten]
canEqHardFailure :: forall a.
CtEvidence
-> PredType -> PredType -> TcS (StopOrContinue (Either IrredCt a))
canEqHardFailure CtEvidence
ev PredType
ty1 PredType
ty2
  = do { String -> SDoc -> TcS ()
traceTcS String
"canEqHardFailure" (PredType -> SDoc
forall a. Outputable a => a -> SDoc
ppr PredType
ty1 SDoc -> SDoc -> SDoc
forall doc. IsDoc doc => doc -> doc -> doc
$$ PredType -> SDoc
forall a. Outputable a => a -> SDoc
ppr PredType
ty2)
       ; (redn1, rewriters1) <- CtEvidence -> PredType -> TcS (Reduction, CoHoleSet)
rewriteForErrors CtEvidence
ev PredType
ty1
       ; (redn2, rewriters2) <- rewriteForErrors ev ty2
       ; new_ev <- rewriteEqEvidence ev NotSwapped redn1 redn2
                                     (rewriters1 S.<> rewriters2)
       ; finishCanWithIrred ShapeMismatchReason new_ev }

{-
Note [Canonicalising type applications]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Given (s1 t1) ~ ty2, how should we proceed?
The simple thing is to see if ty2 is of form (s2 t2), and
decompose.

However, over-eager decomposition gives bad error messages
for things like
   a b ~ Maybe c
   e f ~ p -> q
Suppose (in the first example) we already know a~Array.  Then if we
decompose the application eagerly, yielding
   a ~ Maybe
   b ~ c
we get an error        "Can't match Array ~ Maybe",
but we'd prefer to get "Can't match Array b ~ Maybe c".

So instead can_eq_wanted_app rewrites the LHS and RHS, in the hope of
replacing (a b) by (Array b), before using try_decompose_app to
decompose it.

Note [Make sure that insolubles are fully rewritten]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
When an equality fails, we still want to rewrite the equality
all the way down, so that it accurately reflects
 (a) the mutable reference substitution in force at start of solving
 (b) any ty-binds in force at this point in solving
See Note [Rewrite insolubles] in GHC.Tc.Solver.InertSet.
And if we don't do this there is a bad danger that
GHC.Tc.Solver.applyTyVarDefaulting will find a variable
that has in fact been substituted.

Note [Do not decompose Given polytype equalities]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider [G] (forall a. t1 ~ forall a. t2).  Can we decompose this?
No -- what would the evidence look like?  So instead we simply discard
this given evidence.

Note [No top-level newtypes on RHS of representational equalities]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Suppose we're in this situation:

 work item:  [W] c1 : a ~R b
     inert:  [G] c2 : b ~R Id a

where
  newtype Id a = Id a

We want to make sure canEqCanLHS sees [W] a ~R a, after b is rewritten
and the Id newtype is unwrapped. This is assured by requiring only rewritten
types in canEqCanLHS *and* having the newtype-unwrapping check above
the tyvar check in can_eq_nc.

Note that this only applies to saturated applications of newtype TyCons, as
we can't rewrite an unsaturated application. See for example T22310, where
we ended up with:

  newtype Compose f g a = ...

  [W] t[tau] ~# Compose Foo Bar

Note [Put touchable variables on the left]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Ticket #10009, a very nasty example:

    f :: (UnF (F b) ~ b) => F b -> ()

    g :: forall a. (UnF (F a) ~ a) => a -> ()
    g _ = f (undefined :: F a)

For g we get [G]  g1 : UnF (F a) ~ a
             [W] w1 : UnF (F beta) ~ beta
             [W] w2 : F a ~ F beta

g1 is canonical (CEqCan). It is oriented as above because a is not touchable.
See canEqTyVarFunEq.

w1 is similarly canonical, though the occurs-check in canEqTyVarFunEq is key
here.

w2 is canonical. But which way should it be oriented? As written, we'll be
stuck. When w2 is added to the inert set, nothing gets kicked out: g1 is
a Given (and Wanteds don't rewrite Givens), and w2 doesn't mention the LHS
of w2. We'll thus lose.

But if w2 is swapped around, to

    [W] w3 : F beta ~ F a

then we'll kick w1 out of the inert set (it mentions the LHS of w3). We then
rewrite w1 to

    [W] w4 : UnF (F a) ~ beta

and then, using g1, to

    [W] w5 : a ~ beta

at which point we can unify and go on to glory. (This rewriting actually
happens all at once, in the call to rewrite during canonicalisation.)

But what about the new LHS makes it better? It mentions a variable (beta)
that can appear in a Wanted -- a touchable metavariable never appears
in a Given. On the other hand, the original LHS mentioned only variables
that appear in Givens. We thus choose to put variables that can appear
in Wanteds on the left.

Ticket #12526 is another good example of this in action.

-}

---------------------
canEqCanLHS :: CtEvidence            -- ev :: lhs ~ rhs
            -> EqRel -> SwapFlag
            -> CanEqLHS              -- lhs (or, if swapped, rhs)
            -> TcType                -- lhs: pretty lhs, already rewritten
            -> TcType -> TcType      -- rhs: already rewritten
            -> TcS (StopOrContinue (Either IrredCt EqCt))
canEqCanLHS :: CtEvidence
-> EqRel
-> SwapFlag
-> CanEqLHS
-> PredType
-> PredType
-> PredType
-> TcS (StopOrContinue (Either IrredCt EqCt))
canEqCanLHS CtEvidence
ev EqRel
eq_rel SwapFlag
swapped CanEqLHS
lhs1 PredType
ps_xi1 PredType
xi2 PredType
ps_xi2
  | PredType
k1 HasDebugCallStack => PredType -> PredType -> Bool
PredType -> PredType -> Bool
`tcEqType` PredType
k2
  = CtEvidence
-> EqRel
-> SwapFlag
-> CanEqLHS
-> PredType
-> PredType
-> PredType
-> TcS (StopOrContinue (Either IrredCt EqCt))
canEqCanLHSHomo CtEvidence
ev EqRel
eq_rel SwapFlag
swapped CanEqLHS
lhs1 PredType
ps_xi1 PredType
xi2 PredType
ps_xi2

  | Bool
otherwise
  = CtEvidence
-> EqRel
-> SwapFlag
-> CanEqLHS
-> PredType
-> PredType
-> PredType
-> PredType
-> PredType
-> TcS (StopOrContinue (Either IrredCt EqCt))
canEqCanLHSHetero CtEvidence
ev EqRel
eq_rel SwapFlag
swapped CanEqLHS
lhs1 PredType
ps_xi1 PredType
k1 PredType
xi2 PredType
ps_xi2 PredType
k2

  where
    k1 :: PredType
k1 = CanEqLHS -> PredType
canEqLHSKind CanEqLHS
lhs1
    k2 :: PredType
k2 = HasDebugCallStack => PredType -> PredType
PredType -> PredType
typeKind PredType
xi2


{- Historical Note [Kind Equality Orientation]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
This Note describes a solution to a problem that no longer exists;
it used to apply to `canEqCanLHSHetero`.

While in theory [W] x ~ y and [W] y ~ x ought to give us the same behaviour, in
the past it did not.  See Historical Note [Fundeps with instances, and equality
orientation] in GHC.Tc.Solver.FunDeps, where this is discussed at length.

As a rule of thumb: we keep the newest unification variables on the
left of the equality.  See also Note [Improvement orientation].

In particular, `canEqCanLHSHetero` produces the following constraint equalities

[X] (xi1 :: ki1) ~ (xi2 :: ki2)
  -->  [X] kco :: ki1 ~ ki2
       [X] co : xi1 :: ki1 ~ (xi2 |> sym kco) :: ki1

Note that the types in the LHS of the new constraints are the ones that were on the LHS of
the original constraint.

--- Historical note ---
We prevously used to flip the kco to avoid using a sym in the cast

[X] (xi1 :: ki1) ~ (xi2 :: ki2)
  -->  [X] kco :: ki2 ~ ki1
       [X] co : xi1 :: ki1 ~ (xi2 |> kco) :: ki1

But this sent solver in an infinite loop (see #19415).
-- End of historical note --
-}

canEqCanLHSHetero :: CtEvidence         -- :: (xi1 :: ki1) ~ (xi2 :: ki2)
                                        --    (or reversed if SwapFlag=IsSwapped)
                  -> EqRel -> SwapFlag
                  -> CanEqLHS -> TcType -- xi1
                  -> TcKind             -- ki1
                  -> TcType -> TcType   -- xi2
                  -> TcKind             -- ki2
                  -> TcS (StopOrContinue (Either IrredCt EqCt))
canEqCanLHSHetero :: CtEvidence
-> EqRel
-> SwapFlag
-> CanEqLHS
-> PredType
-> PredType
-> PredType
-> PredType
-> PredType
-> TcS (StopOrContinue (Either IrredCt EqCt))
canEqCanLHSHetero CtEvidence
ev EqRel
eq_rel SwapFlag
swapped CanEqLHS
lhs1 PredType
ps_xi1 PredType
ki1 PredType
xi2 PredType
ps_xi2 PredType
ki2
-- See Note [Equalities with heterogeneous kinds]
--    NotSwapped:
--        ev      :: (lhs1:ki1) ~r# (xi2:ki2)
--        kind_co :: ki2 ~# ki1
--        new_ev  :: lhs1 ~r# (xi2 |> kind_co)
--    Swapped
--        ev      :: (xi2:ki2) ~r# (lhs1:ki1)
--        kind_co :: ki2 ~# ki1
--        new_ev  :: (xi2 |> kind_co) ~r# lhs1

  = case CtEvidence
ev of
      CtGiven (GivenCt { ctev_evar :: GivenCtEvidence -> TyVar
ctev_evar = TyVar
evar, ctev_loc :: GivenCtEvidence -> CtLoc
ctev_loc = CtLoc
loc })
        -> do { let kind_co :: TcCoercion
kind_co  = SwapFlag -> TcCoercion -> TcCoercion
maybeSymCo (SwapFlag -> SwapFlag
flipSwap SwapFlag
swapped) (TcCoercion -> TcCoercion) -> TcCoercion -> TcCoercion
forall a b. (a -> b) -> a -> b
$
                               TcCoercion -> TcCoercion
mkKindCo (TyVar -> TcCoercion
mkCoVarCo TyVar
evar)
                    pred_ty :: PredType
pred_ty  = PredType -> PredType -> PredType
mkNomEqPred PredType
ki2 PredType
ki1
                    kind_loc :: CtLoc
kind_loc = PredType -> PredType -> CtLoc -> CtLoc
mkKindEqLoc PredType
xi1 PredType
xi2 CtLoc
loc
              ; kind_ev <- CtLoc -> (PredType, EvTerm) -> TcS GivenCtEvidence
newGivenEv CtLoc
kind_loc (PredType
pred_ty, TcCoercion -> EvTerm
evCoercion TcCoercion
kind_co)
              ; emitWorkNC [CtGiven kind_ev]
              ; finish emptyCoHoleSet (givenCtEvCoercion kind_ev) }

      CtWanted {}
         -> do { (unifs, (kind_co, eqs)) <- TcS (TcCoercion, Bag Ct) -> TcS (TcTyVarSet, (TcCoercion, Bag Ct))
forall a. TcS a -> TcS (TcTyVarSet, a)
reportFineGrainUnifications (TcS (TcCoercion, Bag Ct)
 -> TcS (TcTyVarSet, (TcCoercion, Bag Ct)))
-> TcS (TcCoercion, Bag Ct)
-> TcS (TcTyVarSet, (TcCoercion, Bag Ct))
forall a b. (a -> b) -> a -> b
$
                                            CtEvidence
-> Role -> (UnifyEnv -> TcM TcCoercion) -> TcS (TcCoercion, Bag Ct)
forall a.
CtEvidence -> Role -> (UnifyEnv -> TcM a) -> TcS (a, Bag Ct)
wrapUnifier CtEvidence
ev Role
Nominal ((UnifyEnv -> TcM TcCoercion) -> TcS (TcCoercion, Bag Ct))
-> (UnifyEnv -> TcM TcCoercion) -> TcS (TcCoercion, Bag Ct)
forall a b. (a -> b) -> a -> b
$ \UnifyEnv
uenv ->
                                            let uenv' :: UnifyEnv
uenv' = UnifyEnv -> (CtLoc -> CtLoc) -> UnifyEnv
updUEnvLoc UnifyEnv
uenv (PredType -> PredType -> CtLoc -> CtLoc
mkKindEqLoc PredType
xi1 PredType
xi2)
                                            in UnifyEnv -> PredType -> PredType -> TcM TcCoercion
uType UnifyEnv
uenv' PredType
ki2 PredType
ki1
                      -- kind_co :: ki2 ~N ki1
                      -- mkKindEqLoc: any new constraints, arising from the kind
                      -- unification, say they thay come from unifying xi1~xi2

               -- Kick out any inert constraints mentioning the unified variables
               ; kickOutAfterUnification unifs

               ; if not (isEmptyVarSet unifs)
                 then -- Unifications happened, so start again to do the zonking
                      -- Otherwise we might put something in the inert set that isn't inert
                      -- Since we are starting again we can ignore `eqs`, because they will
                      -- happen again next time round
                      startAgainWith (mkNonCanonical ev)
                 else

               -- Emit the deferred constraints
            do { emitChildEqs ev eqs

               ; assertPpr (not (isEmptyCts eqs))
                           (vcat [ppr ev, ppr ki1, ppr ki2, ppr unifs, ppr eqs]) $
                   -- assert: the constraints won't be empty because the two kinds differ,
                   -- and there are no unifications, so we must have emitted one or
                   -- more constraints
                 finish (rewriterSetFromCts eqs) kind_co }}
                         -- rewriterSetFromCts: record in the /type/ unification xi1~xi2 that
                         -- it has been rewritten by any (unsolved) constraints in `cts`; that
                         -- stops xi1~xi2 from unifying until `cts` are solved. See (EIK2).
  where
    xi1 :: PredType
xi1  = CanEqLHS -> PredType
canEqLHSType CanEqLHS
lhs1
    role :: Role
role = EqRel -> Role
eqRelRole EqRel
eq_rel

    finish :: CoHoleSet
-> TcCoercion -> TcS (StopOrContinue (Either IrredCt EqCt))
finish CoHoleSet
rewriters TcCoercion
kind_co
      = do { String -> SDoc -> TcS ()
traceTcS String
"Hetero equality gives rise to kind equality"
                 (SwapFlag -> SDoc
forall a. Outputable a => a -> SDoc
ppr SwapFlag
swapped SDoc -> SDoc -> SDoc
forall doc. IsDoc doc => doc -> doc -> doc
$$
                  TcCoercion -> SDoc
forall a. Outputable a => a -> SDoc
ppr TcCoercion
kind_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
sep [ PredType -> SDoc
forall a. Outputable a => a -> SDoc
ppr PredType
ki1, String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"~#", PredType -> SDoc
forall a. Outputable a => a -> SDoc
ppr PredType
ki2 ])
           ; new_ev <- CtEvidence
-> SwapFlag
-> Reduction
-> Reduction
-> CoHoleSet
-> TcS CtEvidence
rewriteEqEvidence CtEvidence
ev SwapFlag
swapped Reduction
lhs_redn Reduction
rhs_redn CoHoleSet
rewriters
                -- rewriteEqEvidence adds any as-yet-unsolved equalities
                -- from the kind equality (namely `rewriters`) to the
                -- rewriter set for `new_ev`.  See (EIK2).

           -- Now `new_ev` is homogenous: carry on!
           ; canEqCanLHSHomo new_ev eq_rel NotSwapped lhs1 ps_xi1 new_xi2 new_xi2 }

      where
        -- kind_co :: ki2 ~N ki1
        lhs_redn :: Reduction
lhs_redn    = Role -> PredType -> Reduction
mkReflRedn Role
role PredType
ps_xi1
        rhs_redn :: Reduction
rhs_redn    = Role -> PredType -> TcCoercion -> Reduction
mkGReflRightRedn Role
role PredType
xi2 TcCoercion
kind_co
        new_xi2 :: PredType
new_xi2     = PredType -> TcCoercion -> PredType
mkCastTy PredType
ps_xi2 TcCoercion
kind_co

canEqCanLHSHomo :: CtEvidence          -- lhs ~ rhs
                                       -- or, if swapped: rhs ~ lhs
                -> EqRel -> SwapFlag
                -> CanEqLHS -> TcType  -- lhs, pretty lhs
                -> TcType   -> TcType  -- rhs, pretty rhs
                -> TcS (StopOrContinue (Either IrredCt EqCt))
-- Guaranteed that typeKind lhs == typeKind rhs
canEqCanLHSHomo :: CtEvidence
-> EqRel
-> SwapFlag
-> CanEqLHS
-> PredType
-> PredType
-> PredType
-> TcS (StopOrContinue (Either IrredCt EqCt))
canEqCanLHSHomo CtEvidence
ev EqRel
eq_rel SwapFlag
swapped CanEqLHS
lhs1 PredType
ps_xi1 PredType
xi2 PredType
ps_xi2
  | (PredType
xi2', MCoercion
mco) <- PredType -> (PredType, MCoercion)
split_cast_ty PredType
xi2
  , Just CanEqLHS
lhs2 <- PredType -> Maybe CanEqLHS
canEqLHS_maybe PredType
xi2'
  = CtEvidence
-> EqRel
-> SwapFlag
-> CanEqLHS
-> PredType
-> CanEqLHS
-> PredType
-> MCoercion
-> TcS (StopOrContinue (Either IrredCt EqCt))
canEqCanLHS2 CtEvidence
ev EqRel
eq_rel SwapFlag
swapped CanEqLHS
lhs1 PredType
ps_xi1 CanEqLHS
lhs2 (PredType
ps_xi2 PredType -> MCoercion -> PredType
`mkCastTyMCo` MCoercion -> MCoercion
mkSymMCo MCoercion
mco) MCoercion
mco

  | Bool
otherwise
  = CtEvidence
-> EqRel
-> SwapFlag
-> CanEqLHS
-> PredType
-> TcS (StopOrContinue (Either IrredCt EqCt))
canEqCanLHSFinish CtEvidence
ev EqRel
eq_rel SwapFlag
swapped CanEqLHS
lhs1 PredType
ps_xi2

  where
    split_cast_ty :: PredType -> (PredType, MCoercion)
split_cast_ty (CastTy PredType
ty TcCoercion
co) = (PredType
ty, TcCoercion -> MCoercion
MCo TcCoercion
co)
    split_cast_ty PredType
other          = (PredType
other, MCoercion
MRefl)

-- This function deals with the case that both LHS and RHS are potential
-- CanEqLHSs.
canEqCanLHS2 :: CtEvidence              -- lhs ~ (rhs |> mco)
                                        -- or, if swapped: (rhs |> mco) ~ lhs
             -> EqRel -> SwapFlag
             -> CanEqLHS                -- lhs (or, if swapped, rhs)
             -> TcType                  -- pretty lhs
             -> CanEqLHS                -- rhs
             -> TcType                  -- pretty rhs
             -> MCoercion               -- :: kind(rhs) ~N kind(lhs)
             -> TcS (StopOrContinue (Either IrredCt EqCt))
canEqCanLHS2 :: CtEvidence
-> EqRel
-> SwapFlag
-> CanEqLHS
-> PredType
-> CanEqLHS
-> PredType
-> MCoercion
-> TcS (StopOrContinue (Either IrredCt EqCt))
canEqCanLHS2 CtEvidence
ev EqRel
eq_rel SwapFlag
swapped CanEqLHS
lhs1 PredType
ps_xi1 CanEqLHS
lhs2 PredType
ps_xi2 MCoercion
mco
  | CanEqLHS
lhs1 CanEqLHS -> CanEqLHS -> Bool
`eqCanEqLHS` CanEqLHS
lhs2
    -- It must be the case that mco is reflexive
  = CtEvidence
-> EqRel -> PredType -> TcS (StopOrContinue (Either IrredCt EqCt))
forall a. CtEvidence -> EqRel -> PredType -> TcS (StopOrContinue a)
canEqReflexive CtEvidence
ev EqRel
eq_rel PredType
lhs1_ty

  | TyVarLHS TyVar
tv1 <- CanEqLHS
lhs1
  , TyVarLHS TyVar
tv2 <- CanEqLHS
lhs2
  = -- See Note [TyVar/TyVar orientation] in GHC.Tc.Utils.Unify
    do { String -> SDoc -> TcS ()
traceTcS String
"canEqLHS2 swapOver" (TyVar -> SDoc
forall a. Outputable a => a -> SDoc
ppr TyVar
tv1 SDoc -> SDoc -> SDoc
forall doc. IsDoc doc => doc -> doc -> doc
$$ TyVar -> SDoc
forall a. Outputable a => a -> SDoc
ppr TyVar
tv2 SDoc -> SDoc -> SDoc
forall doc. IsDoc doc => doc -> doc -> doc
$$ SwapFlag -> SDoc
forall a. Outputable a => a -> SDoc
ppr SwapFlag
swapped)
       ; if Bool -> TyVar -> TyVar -> Bool
swapOverTyVars (CtEvidence -> Bool
isGiven CtEvidence
ev) TyVar
tv1 TyVar
tv2
         then TcS (StopOrContinue (Either IrredCt EqCt))
finish_with_swapping
         else TcS (StopOrContinue (Either IrredCt EqCt))
finish_without_swapping }

  | TyVarLHS {} <- CanEqLHS
lhs1
  , TyFamLHS {} <- CanEqLHS
lhs2
  = if Bool
put_tyvar_on_lhs
    then TcS (StopOrContinue (Either IrredCt EqCt))
finish_without_swapping
    else TcS (StopOrContinue (Either IrredCt EqCt))
finish_with_swapping

  | TyFamLHS {} <- CanEqLHS
lhs1
  , TyVarLHS {} <- CanEqLHS
lhs2
  = if Bool
put_tyvar_on_lhs
    then TcS (StopOrContinue (Either IrredCt EqCt))
finish_with_swapping
    else TcS (StopOrContinue (Either IrredCt EqCt))
finish_without_swapping

  | TyFamLHS TyCon
_fun_tc1 [PredType]
fun_args1 <- CanEqLHS
lhs1
  , TyFamLHS TyCon
_fun_tc2 [PredType]
fun_args2 <- CanEqLHS
lhs2
  -- Don't decompose, but maybe re-orient
  = do { String -> SDoc -> TcS ()
traceTcS String
"canEqCanLHS2 two type families" (CanEqLHS -> SDoc
forall a. Outputable a => a -> SDoc
ppr CanEqLHS
lhs1 SDoc -> SDoc -> SDoc
forall doc. IsDoc doc => doc -> doc -> doc
$$ CanEqLHS -> SDoc
forall a. Outputable a => a -> SDoc
ppr CanEqLHS
lhs2)
       ; tclvl <- TcS TcLevel
getTcLevel
       ; let tvs1 = [PredType] -> TcTyVarSet
tyCoVarsOfTypes [PredType]
fun_args1
             tvs2 = [PredType] -> TcTyVarSet
tyCoVarsOfTypes [PredType]
fun_args2

             -- See Note [Orienting TyFamLHS/TyFamLHS]
             swap_for_size = [PredType] -> Int
typesSize [PredType]
fun_args2 Int -> Int -> Bool
forall a. Ord a => a -> a -> Bool
> [PredType] -> Int
typesSize [PredType]
fun_args1

             -- See Note [Orienting TyFamLHS/TyFamLHS]
             meta_tv_lhs = (TyVar -> Bool) -> TcTyVarSet -> Bool
anyVarSet (TcLevel -> TyVar -> Bool
isTouchableMetaTyVar TcLevel
tclvl) TcTyVarSet
tvs1
             meta_tv_rhs = (TyVar -> Bool) -> TcTyVarSet -> Bool
anyVarSet (TcLevel -> TyVar -> Bool
isTouchableMetaTyVar TcLevel
tclvl) TcTyVarSet
tvs2
             swap_for_rewriting = Bool
meta_tv_rhs Bool -> Bool -> Bool
&& Bool -> Bool
not Bool
meta_tv_lhs
                                  -- See Note [Put touchable variables on the left]
                                  -- This second check is just to avoid unfruitful swapping

         -- It's important that we don't flip-flop (#T24134)
         -- So swap_for_rewriting "wins", and we only try swap_for_size
         -- if swap_for_rewriting doesn't care either way
       ; if swap_for_rewriting || (meta_tv_lhs == meta_tv_rhs && swap_for_size)
         then finish_with_swapping
         else finish_without_swapping }
  where
    sym_mco :: MCoercion
sym_mco = MCoercion -> MCoercion
mkSymMCo MCoercion
mco
    role :: Role
role    = EqRel -> Role
eqRelRole EqRel
eq_rel
    lhs1_ty :: PredType
lhs1_ty  = CanEqLHS -> PredType
canEqLHSType CanEqLHS
lhs1
    lhs2_ty :: PredType
lhs2_ty  = CanEqLHS -> PredType
canEqLHSType CanEqLHS
lhs2

    finish_without_swapping :: TcS (StopOrContinue (Either IrredCt EqCt))
finish_without_swapping
      = CtEvidence
-> EqRel
-> SwapFlag
-> CanEqLHS
-> PredType
-> TcS (StopOrContinue (Either IrredCt EqCt))
canEqCanLHSFinish CtEvidence
ev EqRel
eq_rel SwapFlag
swapped CanEqLHS
lhs1 (PredType
ps_xi2 PredType -> MCoercion -> PredType
`mkCastTyMCo` MCoercion
mco)

    -- Swapping. We have   ev : lhs1 ~ lhs2 |> co
    -- We swap to      new_ev : lhs2 ~ lhs1 |> sym co
    --                     ev = grefl1 ; sym new_ev ; grefl2
    --      where grefl1 : lhs1 ~ lhs1 |> sym co
    --            grefl2 : lhs2 ~ lhs2 |> co
    finish_with_swapping :: TcS (StopOrContinue (Either IrredCt EqCt))
finish_with_swapping
      = do { let lhs1_redn :: Reduction
lhs1_redn = Role -> PredType -> MCoercion -> Reduction
mkGReflRightMRedn Role
role PredType
lhs1_ty MCoercion
sym_mco
                 lhs2_redn :: Reduction
lhs2_redn = Role -> PredType -> MCoercion -> Reduction
mkGReflLeftMRedn  Role
role PredType
lhs2_ty MCoercion
mco
           ; new_ev <-CtEvidence
-> SwapFlag
-> Reduction
-> Reduction
-> CoHoleSet
-> TcS CtEvidence
rewriteEqEvidence CtEvidence
ev SwapFlag
swapped Reduction
lhs1_redn Reduction
lhs2_redn CoHoleSet
emptyCoHoleSet
           ; canEqCanLHSFinish new_ev eq_rel IsSwapped lhs2 (ps_xi1 `mkCastTyMCo` sym_mco) }

    put_tyvar_on_lhs :: Bool
put_tyvar_on_lhs = CtEvidence -> Bool
isWanted CtEvidence
ev Bool -> Bool -> Bool
&& EqRel
eq_rel EqRel -> EqRel -> Bool
forall a. Eq a => a -> a -> Bool
== EqRel
NomEq
    -- See Note [Orienting TyVarLHS/TyFamLHS]
    -- Same conditions as for canEqCanLHSFinish_try_unification
    -- which we are setting ourselves up for here

{- Note [Orienting TyVarLHS/TyFamLHS]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
What if one side is a TyVarLHS and the other is a TyFamLHS, (a ~ F tys)?
Which to put on the left?  Answer:

* If there is no chance of unifying, put the type family on the left,
  (F tys ~ a), because it's generally better to rewrite away function
  calls.  See `put_tyvar_on_lhs` in canEqCanLHS2; and
  Note [Orienting TyVarLHS/TyFamLHS]

* But if there /is/ a chance of unifying, put the tyvar on the left,
  (a ~ F tys), as this may be our only shot to unify. Again see
  `put_tyvar_on_lhs`.

* But if we /fail/ to unify then flip back to (F tys ~ a) because it's
  generally better to rewrite away function calls. See the call to
  `swapAndFinish` in `canEqCanLHSFinish_try_unification`

  It's important to flip back. Consider
    [W] F alpha ~ alpha
    [W] F alpha ~ beta
    [W] G alpha beta ~ Int   ( where we have type instance G a a = a )
  If we end up with a stuck alpha ~ F alpha, we won't be able to solve this.
  Test case: indexed-types/should_compile/CEqCanOccursCheck

Note [Orienting TyFamLHS/TyFamLHS]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
If we have a TyFamLHS on both sides, we choose how to orient it.

* swap_for_size.  If we have
      S a ~ F (G (H (Maybe a)))
  then we swap so that we tend to rewrite the bigger type (F (G (H (Maybe a))))
  into the smaller one (S a).  This same test tends to avoid occurs-check
  errors.  E.g.
      S g ~ F (G (S g))
  Here (S g) occurs on the RHS, so this is not canonical.  But if we swap it
  around, it /is/ canonical
      F (G (S g)) ~ S g

* swap_for_rewriting: put touchable meta-tyvars on the left:
  see Note [Put touchable variables on the left]
-}

-- The RHS here is either not CanEqLHS, or it's one that we
-- want to rewrite the LHS to (as per e.g. swapOverTyVars)
canEqCanLHSFinish, canEqCanLHSFinish_try_unification,
                   canEqCanLHSFinish_no_unification
    :: CtEvidence           -- (lhs ~ rhs) or if swapped (rhs ~ lhs)
    -> EqRel -> SwapFlag
    -> CanEqLHS             -- lhs
    -> TcType               -- rhs
    -> TcS (StopOrContinue (Either IrredCt EqCt))
    -- RHS is fully rewritten, but with type synonyms
    --   preserved as much as possible
    -- Guaranteed preconditions that
    --    (TyEq:K)  handled in canEqCanLHSHomo
    --    (TyEq:N)  checked in can_eq_nc
    --    (TyEq:TV) handled in canEqCanLHS2

---------------------------
canEqCanLHSFinish :: CtEvidence
-> EqRel
-> SwapFlag
-> CanEqLHS
-> PredType
-> TcS (StopOrContinue (Either IrredCt EqCt))
canEqCanLHSFinish CtEvidence
ev EqRel
eq_rel SwapFlag
swapped CanEqLHS
lhs PredType
rhs
  = do { String -> SDoc -> TcS ()
traceTcS String
"canEqCanLHSFinish" (SDoc -> TcS ()) -> SDoc -> TcS ()
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
"ev:" SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> CtEvidence -> SDoc
forall a. Outputable a => a -> SDoc
ppr CtEvidence
ev
              , String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"swapped:" SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> SwapFlag -> SDoc
forall a. Outputable a => a -> SDoc
ppr SwapFlag
swapped
              , String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"lhs:" SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> CanEqLHS -> SDoc
forall a. Outputable a => a -> SDoc
ppr CanEqLHS
lhs
              , String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"rhs:" SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> PredType -> SDoc
forall a. Outputable a => a -> SDoc
ppr PredType
rhs ]

         -- Assertion: (TyEq:K) is already satisfied
       ; Bool -> TcS ()
forall (m :: * -> *). (HasCallStack, Applicative m) => Bool -> m ()
massert (CanEqLHS -> PredType
canEqLHSKind CanEqLHS
lhs HasCallStack => PredType -> PredType -> Bool
PredType -> PredType -> Bool
`eqType` HasDebugCallStack => PredType -> PredType
PredType -> PredType
typeKind PredType
rhs)

         -- Assertion: (TyEq:N) is already satisfied (if applicable)
       ; TcS Bool -> SDoc -> TcS ()
forall (m :: * -> *).
(HasCallStack, Monad m) =>
m Bool -> SDoc -> m ()
assertPprM TcS Bool
ty_eq_N_OK (SDoc -> TcS ()) -> SDoc -> TcS ()
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
"CanEqCanLHSFinish: (TyEq:N) not satisfied"
                , String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"rhs:" SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> PredType -> SDoc
forall a. Outputable a => a -> SDoc
ppr PredType
rhs ]

       ; CtEvidence
-> EqRel
-> SwapFlag
-> CanEqLHS
-> PredType
-> TcS (StopOrContinue (Either IrredCt EqCt))
canEqCanLHSFinish_try_unification CtEvidence
ev EqRel
eq_rel SwapFlag
swapped CanEqLHS
lhs PredType
rhs }

  where
    -- This is about (TyEq:N): check that we don't have a saturated application
    -- of a newtype TyCon at the top level of the RHS, if the constructor
    -- of the newtype is in scope.
    ty_eq_N_OK :: TcS Bool
    ty_eq_N_OK :: TcS Bool
ty_eq_N_OK
      | EqRel
ReprEq <- EqRel
eq_rel
      , Just (TyCon
tc, [PredType]
tc_args) <- HasDebugCallStack => PredType -> Maybe (TyCon, [PredType])
PredType -> Maybe (TyCon, [PredType])
splitTyConApp_maybe PredType
rhs
      , Just DataCon
con <- TyCon -> Maybe DataCon
newTyConDataCon_maybe TyCon
tc
      -- #22310: only a problem if the newtype TyCon is saturated.
      , [PredType]
tc_args [PredType] -> Int -> Bool
forall a. [a] -> Int -> Bool
`lengthAtLeast` TyCon -> Int
tyConArity TyCon
tc
      -- #21010: only a problem if the newtype constructor is in scope.
      = do { rdr_env <- TcS GlobalRdrEnv
getGlobalRdrEnvTcS
           ; let con_in_scope = Maybe GlobalRdrElt -> Bool
forall a. Maybe a -> Bool
isJust (Maybe GlobalRdrElt -> Bool) -> Maybe GlobalRdrElt -> Bool
forall a b. (a -> b) -> a -> b
$ GlobalRdrEnv -> Name -> Maybe GlobalRdrElt
forall info.
Outputable info =>
GlobalRdrEnvX info -> Name -> Maybe (GlobalRdrEltX info)
lookupGRE_Name GlobalRdrEnv
rdr_env (DataCon -> Name
dataConName DataCon
con)
           ; return $ not con_in_scope }
      | Bool
otherwise
      = Bool -> TcS Bool
forall a. a -> TcS a
forall (m :: * -> *) a. Monad m => a -> m a
return Bool
True

-----------------------
canEqCanLHSFinish_try_unification :: CtEvidence
-> EqRel
-> SwapFlag
-> CanEqLHS
-> PredType
-> TcS (StopOrContinue (Either IrredCt EqCt))
canEqCanLHSFinish_try_unification CtEvidence
ev EqRel
eq_rel SwapFlag
swapped CanEqLHS
lhs PredType
rhs
  -- Try unification; for Wanted, Nominal equalities with a meta-tyvar on the LHS
  | CtWanted WantedCtEvidence
wev <- CtEvidence
ev         -- See Note [Do not unify Givens]
  , EqRel
NomEq <- EqRel
eq_rel            -- See Note [Do not unify representational equalities]
  , WantedCtEvidence -> Bool
wantedCtHasNoRewriters WantedCtEvidence
wev -- See Note [Unify only if the rewriter set is empty]
  , TyVarLHS TyVar
lhs_tv <- CanEqLHS
lhs
  = do  { given_eq_lvl <- TcS TcLevel
getInnermostGivenEqLevel
        ; case simpleUnifyCheck UC_Solver given_eq_lvl lhs_tv rhs of
            SimpleUnifyResult
SUC_CanUnify ->
              TyVar -> Reduction -> TcS (StopOrContinue (Either IrredCt EqCt))
unify TyVar
lhs_tv (Role -> PredType -> Reduction
mkReflRedn Role
Nominal PredType
rhs)
            SimpleUnifyResult
SUC_CannotUnify
              | Just CanEqLHS
can_rhs <- PredType -> Maybe CanEqLHS
canTyFamEqLHS_maybe PredType
rhs
              -> TyVar -> CanEqLHS -> TcS (StopOrContinue (Either IrredCt EqCt))
swap_and_finish TyVar
lhs_tv CanEqLHS
can_rhs -- See Note [Orienting TyVarLHS/TyFamLHS]
              | Bool
otherwise
              -> TcS (StopOrContinue (Either IrredCt EqCt))
finish_no_unify
            SimpleUnifyResult
SUC_NotSure ->
              -- We have a touchable unification variable on the left,
              -- and the top-shape check succeeded. These are both guaranteed
              -- by the fact that simpleUnifyCheck did not return SUC_CannotUnify.
              do  { let flags :: TyEqFlags (IOEnv (Env TcGblEnv TcLclEnv)) Ct
flags = CtEvidence -> TyVar -> TyEqFlags (IOEnv (Env TcGblEnv TcLclEnv)) Ct
unifyingLHSMetaTyVar_TEFTask CtEvidence
ev TyVar
lhs_tv
                  ; check_result <- TcM (PuResult Ct Reduction) -> TcS (PuResult Ct Reduction)
forall a. TcM a -> TcS a
wrapTcS (TyEqFlags (IOEnv (Env TcGblEnv TcLclEnv)) Ct
-> PredType -> TcM (PuResult Ct Reduction)
forall (m :: * -> *) a.
Monad m =>
TyEqFlags m a -> PredType -> m (PuResult a Reduction)
checkTyEqRhs TyEqFlags (IOEnv (Env TcGblEnv TcLclEnv)) Ct
flags PredType
rhs)
                  ; case check_result of
                      PuOK Bag Ct
cts Reduction
rhs_redn ->
                        do { Bag Ct -> TcS ()
emitWork Bag Ct
cts
                           ; TyVar -> Reduction -> TcS (StopOrContinue (Either IrredCt EqCt))
unify TyVar
lhs_tv Reduction
rhs_redn }
                      PuFail CheckTyEqResult
reason
                        | Just CanEqLHS
can_rhs <- PredType -> Maybe CanEqLHS
canTyFamEqLHS_maybe PredType
rhs
                        -> TyVar -> CanEqLHS -> TcS (StopOrContinue (Either IrredCt EqCt))
swap_and_finish TyVar
lhs_tv CanEqLHS
can_rhs -- See Note [Orienting TyVarLHS/TyFamLHS]
                        | CheckTyEqResult
reason CheckTyEqResult -> CheckTyEqResult -> Bool
`cterHasOnlyProblems` CheckTyEqResult
do_not_prevent_rewriting
                        ->
                          -- ContinueWith, to allow using this constraint for
                          -- rewriting (e.g. alpha[2] ~ beta[3]).
                          do { new_ev <- CtEvidence
-> EqRel -> SwapFlag -> CanEqLHS -> PredType -> TcS CtEvidence
rewriteEqEvidenceSwapOnly CtEvidence
ev EqRel
eq_rel SwapFlag
swapped CanEqLHS
lhs PredType
rhs
                             ; continueWith $ Right $
                                 EqCt { eq_ev  = new_ev, eq_eq_rel = eq_rel
                                      , eq_lhs = lhs , eq_rhs = rhs }
                             }
                        | Bool
otherwise
                        -> CheckTyEqResult -> TcS (StopOrContinue (Either IrredCt EqCt))
try_irred CheckTyEqResult
reason
                  }
         }
  -- Otherwise unification is off the table
  | Bool
otherwise
  = TcS (StopOrContinue (Either IrredCt EqCt))
finish_no_unify

  where
    -- We can't unify, but this equality can go in the inert set
    -- and be used to rewrite other constraints.
    finish_no_unify :: TcS (StopOrContinue (Either IrredCt EqCt))
finish_no_unify =
      CtEvidence
-> EqRel
-> SwapFlag
-> CanEqLHS
-> PredType
-> TcS (StopOrContinue (Either IrredCt EqCt))
canEqCanLHSFinish_no_unification CtEvidence
ev EqRel
eq_rel SwapFlag
swapped CanEqLHS
lhs PredType
rhs

    -- We can't unify, and this equality should not be used to rewrite
    -- other constraints (e.g. because it has an occurs check).
    -- So add it to the inert Irreds.
    try_irred :: CheckTyEqResult -> TcS (StopOrContinue (Either IrredCt EqCt))
try_irred CheckTyEqResult
reason =
      CheckTyEqResult
-> CtEvidence
-> EqRel
-> SwapFlag
-> CanEqLHS
-> PredType
-> TcS (StopOrContinue (Either IrredCt EqCt))
forall unused.
CheckTyEqResult
-> CtEvidence
-> EqRel
-> SwapFlag
-> CanEqLHS
-> PredType
-> TcS (StopOrContinue (Either IrredCt unused))
tryIrredInstead CheckTyEqResult
reason CtEvidence
ev EqRel
eq_rel SwapFlag
swapped CanEqLHS
lhs PredType
rhs

    -- We can't unify as-is, and want to flip the equality around.
    -- Example: alpha ~ F tys, flip it around to become the canonical
    -- equality f tys ~ alpha.
    swap_and_finish :: TyVar -> CanEqLHS -> TcS (StopOrContinue (Either IrredCt EqCt))
swap_and_finish TyVar
tv CanEqLHS
can_rhs =
      CtEvidence
-> EqRel
-> SwapFlag
-> PredType
-> CanEqLHS
-> TcS (StopOrContinue (Either IrredCt EqCt))
forall unused.
CtEvidence
-> EqRel
-> SwapFlag
-> PredType
-> CanEqLHS
-> TcS (StopOrContinue (Either unused EqCt))
swapAndFinish CtEvidence
ev EqRel
eq_rel SwapFlag
swapped (TyVar -> PredType
mkTyVarTy TyVar
tv) CanEqLHS
can_rhs

    -- Phew! Finally!  We can unify; go ahead and do so.
    unify :: TyVar -> Reduction -> TcS (StopOrContinue (Either IrredCt EqCt))
unify TyVar
tv Reduction
rhs_redn =
      do { -- In the common case where rhs_redn is Refl, we don't need to rewrite
           -- the evidence, even if swapped=IsSwapped.   Suppose the original was
           --     [W] co : Int ~ alpha
           -- We unify alpha := Int, and set co := <Int>.  No need to
           -- swap to   co = sym co'
           --           co' = <Int>
           new_ev <- if TcCoercion -> Bool
isReflCo (Reduction -> TcCoercion
reductionCoercion Reduction
rhs_redn)
                     then CtEvidence -> TcS CtEvidence
forall a. a -> TcS a
forall (m :: * -> *) a. Monad m => a -> m a
return CtEvidence
ev
                     else CtEvidence
-> SwapFlag
-> Reduction
-> Reduction
-> CoHoleSet
-> TcS CtEvidence
rewriteEqEvidence CtEvidence
ev SwapFlag
swapped
                              (Role -> PredType -> Reduction
mkReflRedn Role
Nominal (TyVar -> PredType
mkTyVarTy TyVar
tv)) Reduction
rhs_redn
                              CoHoleSet
emptyCoHoleSet
                              -- emptyCoHoleSet: rhs_redn has no CoercionHoles

         ; let tv_ty     = TyVar -> PredType
mkTyVarTy TyVar
tv
               final_rhs = Reduction -> PredType
reductionReducedType Reduction
rhs_redn

         ; traceTcS "Sneaky unification:" $
           vcat [text "Unifies:" <+> ppr tv <+> text ":=" <+> ppr final_rhs,
                 text "Coercion:" <+> pprEq tv_ty final_rhs,
                 text "Left Kind is:" <+> ppr (typeKind tv_ty),
                 text "Right Kind is:" <+> ppr (typeKind final_rhs) ]

         -- Update the unification variable itself
         ; unifyTyVar tv final_rhs

         -- Provide Refl evidence for the constraint
         -- Ignore 'swapped' because it's Refl!
         ; setEqIfWanted new_ev (mkReflCPH NomEq final_rhs)

         -- Kick out any constraints that can now be rewritten
         ; kickOutAfterUnification (unitVarSet tv)

         ; return (Stop new_ev (text "Solved by unification")) }

---------------------------
-- Unification is off the table
canEqCanLHSFinish_no_unification :: CtEvidence
-> EqRel
-> SwapFlag
-> CanEqLHS
-> PredType
-> TcS (StopOrContinue (Either IrredCt EqCt))
canEqCanLHSFinish_no_unification CtEvidence
ev EqRel
eq_rel SwapFlag
swapped CanEqLHS
lhs PredType
rhs
  = do { -- Do checkTypeEq to guarantee (TyEq:OC), (TyEq:F)
         -- Must do the occurs check even on tyvar/tyvar equalities,
         -- in case have  x ~ (y :: ..x...); this is #12593.
       ; check_result <- CtEvidence
-> EqRel -> CanEqLHS -> PredType -> TcS (PuResult Ct Reduction)
checkTypeEq CtEvidence
ev EqRel
eq_rel CanEqLHS
lhs PredType
rhs

       ; let lhs_ty = CanEqLHS -> PredType
canEqLHSType CanEqLHS
lhs
       ; case check_result of
            PuFail CheckTyEqResult
reason

              -- If we had F a ~ G (F a), which gives an occurs check,
              -- then swap it to G (F a) ~ F a, which does not
              -- However `swap_for_size` above will orient it with (G (F a)) on
              -- the left anwyway.  `swap_for_rewriting` "wins", but that doesn't
              -- matter: in the occurs check case swap_for_rewriting will be moot.
              -- TL;DR: the next four lines of code are redundant
              -- I'm leaving them here in case they become relevant again
--              | TyFamLHS {} <- lhs
--              , Just can_rhs <- canTyFamEqLHS_maybe rhs
--              , reason `cterHasOnlyProblem` cteSolubleOccurs
--              -> swapAndFinish ev eq_rel swapped lhs_ty can_rhs
--              | otherwise

              | CheckTyEqResult
reason CheckTyEqResult -> CheckTyEqResult -> Bool
`cterHasOnlyProblems` CheckTyEqResult
do_not_prevent_rewriting
              -> do { new_ev <- CtEvidence
-> EqRel -> SwapFlag -> CanEqLHS -> PredType -> TcS CtEvidence
rewriteEqEvidenceSwapOnly CtEvidence
ev EqRel
eq_rel SwapFlag
swapped CanEqLHS
lhs PredType
rhs
                    ; continueWith $ Right $
                        EqCt { eq_ev  = new_ev, eq_eq_rel = eq_rel
                             , eq_lhs = lhs , eq_rhs = rhs }
                    }

              | Bool
otherwise
              -> CheckTyEqResult
-> CtEvidence
-> EqRel
-> SwapFlag
-> CanEqLHS
-> PredType
-> TcS (StopOrContinue (Either IrredCt EqCt))
forall unused.
CheckTyEqResult
-> CtEvidence
-> EqRel
-> SwapFlag
-> CanEqLHS
-> PredType
-> TcS (StopOrContinue (Either IrredCt unused))
tryIrredInstead CheckTyEqResult
reason CtEvidence
ev EqRel
eq_rel SwapFlag
swapped CanEqLHS
lhs PredType
rhs

            PuOK Bag Ct
new_eqs Reduction
rhs_redn
              -> do { Bag Ct -> TcS ()
emitWork Bag Ct
new_eqs
                    ; let new_holes :: CoHoleSet
new_holes = Bag Ct -> CoHoleSet
rewriterSetFromCts Bag Ct
new_eqs
                          lhs_redn :: Reduction
lhs_redn  = Role -> PredType -> Reduction
mkReflRedn (EqRel -> Role
eqRelRole EqRel
eq_rel) PredType
lhs_ty
                    ; new_ev <- CtEvidence
-> SwapFlag
-> Reduction
-> Reduction
-> CoHoleSet
-> TcS CtEvidence
rewriteEqEvidence CtEvidence
ev SwapFlag
swapped Reduction
lhs_redn Reduction
rhs_redn CoHoleSet
new_holes

                    -- Important: even if the coercion is Refl,
                    --   * new_ev has reductionReducedType on the RHS
                    --   * eq_rhs is set to reductionReducedType
                    -- See Note [Forgetful synonyms in checkTyConApp] in GHC.Tc.Utils.Unify
                    ; continueWith $ Right $
                      EqCt { eq_ev  = new_ev, eq_eq_rel = eq_rel
                           , eq_lhs = lhs
                           , eq_rhs = reductionReducedType rhs_redn } } }

-- | Some problems prevent /unification/ but not /rewriting/:
--
-- Skolem-escape: if we have [W] alpha[2] ~ Maybe b[3]
--    we can't unify (skolem-escape); but it /is/ canonical,
--    and hence we /can/ use it for rewriting
--
-- Concrete-ness:  alpha[conc] ~ b[sk]
--    We can use it to rewrite; we still have to solve the original
do_not_prevent_rewriting :: CheckTyEqResult
do_not_prevent_rewriting :: CheckTyEqResult
do_not_prevent_rewriting = CheckTyEqProblem -> CheckTyEqResult
cteProblem CheckTyEqProblem
cteSkolemEscape CheckTyEqResult -> CheckTyEqResult -> CheckTyEqResult
forall a. Semigroup a => a -> a -> a
S.<>
                           CheckTyEqProblem -> CheckTyEqResult
cteProblem CheckTyEqProblem
cteConcrete

----------------------
swapAndFinish :: CtEvidence -> EqRel -> SwapFlag
              -> TcType -> CanEqLHS      -- ty ~ F tys
              -> TcS (StopOrContinue (Either unused EqCt))
-- We have an equality alpha ~ F tys, that we can't unify e.g because 'tys'
-- mentions alpha, it would not be a canonical constraint as-is.
-- We want to flip it to (F tys ~ a), whereupon it is canonical
swapAndFinish :: forall unused.
CtEvidence
-> EqRel
-> SwapFlag
-> PredType
-> CanEqLHS
-> TcS (StopOrContinue (Either unused EqCt))
swapAndFinish CtEvidence
ev EqRel
eq_rel SwapFlag
swapped PredType
lhs_ty CanEqLHS
can_rhs
  = do { new_ev <- CtEvidence
-> EqRel -> SwapFlag -> CanEqLHS -> PredType -> TcS CtEvidence
rewriteEqEvidenceSwapOnly CtEvidence
ev EqRel
eq_rel (SwapFlag -> SwapFlag
flipSwap SwapFlag
swapped) CanEqLHS
can_rhs PredType
lhs_ty
       ; continueWith $ Right $
         EqCt { eq_ev  = new_ev, eq_eq_rel = eq_rel
              , eq_lhs = can_rhs, eq_rhs = lhs_ty } }

----------------------
tryIrredInstead :: CheckTyEqResult -> CtEvidence
                -> EqRel -> SwapFlag -> CanEqLHS -> TcType
                -> TcS (StopOrContinue (Either IrredCt unused))
-- We have a non-canonical equality
-- We still swap it if 'swapped' says so, so that it is oriented
-- in the direction that the error-reporting machinery
-- expects it; e.g.  (m ~ t m) rather than (t m ~ m)
-- This is not very important, and only affects error reporting.
tryIrredInstead :: forall unused.
CheckTyEqResult
-> CtEvidence
-> EqRel
-> SwapFlag
-> CanEqLHS
-> PredType
-> TcS (StopOrContinue (Either IrredCt unused))
tryIrredInstead CheckTyEqResult
reason CtEvidence
ev EqRel
eq_rel SwapFlag
swapped CanEqLHS
lhs PredType
rhs
  = do { String -> SDoc -> TcS ()
traceTcS String
"cantMakeCanonical" (CheckTyEqResult -> SDoc
forall a. Outputable a => a -> SDoc
ppr CheckTyEqResult
reason SDoc -> SDoc -> SDoc
forall doc. IsDoc doc => doc -> doc -> doc
$$ CanEqLHS -> SDoc
forall a. Outputable a => a -> SDoc
ppr CanEqLHS
lhs SDoc -> SDoc -> SDoc
forall doc. IsDoc doc => doc -> doc -> doc
$$ PredType -> SDoc
forall a. Outputable a => a -> SDoc
ppr PredType
rhs)
       ; new_ev <- CtEvidence
-> EqRel -> SwapFlag -> CanEqLHS -> PredType -> TcS CtEvidence
rewriteEqEvidenceSwapOnly CtEvidence
ev EqRel
eq_rel SwapFlag
swapped CanEqLHS
lhs PredType
rhs
       ; finishCanWithIrred (NonCanonicalReason reason) new_ev }

finishCanWithIrred :: CtIrredReason -> CtEvidence
                   -> TcS (StopOrContinue (Either IrredCt a))
finishCanWithIrred :: forall a.
CtIrredReason
-> CtEvidence -> TcS (StopOrContinue (Either IrredCt a))
finishCanWithIrred CtIrredReason
reason CtEvidence
ev
  = do { -- Abort fast if we have any insoluble Wanted constraints,
         -- and the TcS abort-if-insoluble flag is on.
         Bool -> TcS () -> TcS ()
forall (f :: * -> *). Applicative f => Bool -> f () -> f ()
when (CtIrredReason -> Bool
isInsolubleReason CtIrredReason
reason) TcS ()
tryEarlyAbortTcS

       ; Either IrredCt a -> TcS (StopOrContinue (Either IrredCt a))
forall a. a -> TcS (StopOrContinue a)
continueWith (Either IrredCt a -> TcS (StopOrContinue (Either IrredCt a)))
-> Either IrredCt a -> TcS (StopOrContinue (Either IrredCt a))
forall a b. (a -> b) -> a -> b
$ IrredCt -> Either IrredCt a
IrredCt -> Either IrredCt a
forall a b. a -> Either a b
Left (IrredCt -> Either IrredCt a) -> IrredCt -> Either IrredCt a
forall a b. (a -> b) -> a -> b
$ IrredCt { ir_ev :: CtEvidence
ir_ev = CtEvidence
ev, ir_reason :: CtIrredReason
ir_reason = CtIrredReason
reason } }

-----------------------
-- | Solve a reflexive equality constraint
canEqReflexive :: CtEvidence    -- ty ~ ty
               -> EqRel
               -> TcType        -- ty
               -> TcS (StopOrContinue a)   -- always Stop
canEqReflexive :: forall a. CtEvidence -> EqRel -> PredType -> TcS (StopOrContinue a)
canEqReflexive CtEvidence
ev EqRel
eq_rel PredType
ty
  = do { CtEvidence -> CoercionPlusHoles -> TcS ()
setEqIfWanted CtEvidence
ev (EqRel -> PredType -> CoercionPlusHoles
mkReflCPH EqRel
eq_rel PredType
ty)
       ; CtEvidence -> String -> TcS (StopOrContinue a)
forall a. CtEvidence -> String -> TcS (StopOrContinue a)
stopWith CtEvidence
ev String
"Solved by reflexivity" }

{- Note [Equalities with heterogeneous kinds]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
What do we do when we have an equality

  (tv :: k1) ~ (rhs :: k2)

where k1 and k2 differ? Easy: we create a coercion that relates k1 and
k2 and use this to cast. To wit, from

  [X] co1 :: (tv :: k1) ~ (rhs :: k2)

(where [X] is [G] or [W]), we go to

  co1 = co2 ; sym (GRefl kco)
  [X] co2 :: (tv :: k1) ~ ((rhs |> sym kco) :: k1)
  [X] kco :: k1 ~ k2

Wrinkles:

(EIK1) When X=Wanted, the new type-level wanted for `co` is effectively rewritten by
     the kind-level one. We thus include the kind-level wanted in the CoHoleSet
     for the type-level one. See Note [Wanteds rewrite Wanteds: rewriter-sets] in
     GHC.Tc.Types.Constraint.  This is done in canEqCanLHSHetero.

(EIK2) Suppose we have [W] (a::Type) ~ (b::Type->Type). The above rewrite will produce
        [W] w (rewriters: {kw}) : a ~ (b |> kw)
        [W] kw                  : Type ~ (Type->Type)

     We track `w` as having `kw` in its rewriter set.  That will stop us unifying `w`
     (see Note [Unify only if the rewriter set is empty] in GHC.Tc.Solver.Equality).

    But `w` is still /canonical/, and used for rewriting other constraints.
    See Note [Wanteds rewrite Wanteds: rewriter-sets] in GHC.Tc.Types.Constraint.
    That's important in general. Consider:
        [W] kw : k  ~ Type
        [W] w  : a ~ F k t
     We can rewrite `w` with `kw` like this:
        [W] w' (rewriters: {kw}) : a ~ F Type (t |> kw)
     The cast on the second argument of `F` is necessary to keep the appliation well-kinded.
     There is nothing special here; no reason not treat w' as canonical, and use it for
     rewriting. Indeed test JuanLopez only typechecks if we do.

  So here is our implementation:
     * When doing the kind unification, any equality constraints we can't solve
       immediately get an origin that tells that the constraint arises from
       the kind of the parent type-equality.  See the calls to `mkKindEqLoc`
       in `canEqCanLHSHetero`.

     * We /also/ add these unsolved kind equalities to the `CoHoleSet` of the
       parent constraint; see the call to `rewriteEqEvidence` in `finish` in
       `canEqCanLHSHetero`.

     * When filling a coercion hole we kick out any equality constraints whose
       rewriter set mentions this hole.  See `kickOutAfterFillingCoercionHole`

(EIK3) Suppose we have [W] co1 : (a :: k1) ~ (rhs :: k2). We duly follow the
     algorithm detailed here, producing [W] kco :: k1 ~ k2, and adding
     [W] co2 : (a :: k1) ~ ((rhs |> sym kco) :: k1) to the inert set.
     Some time later, we solve `kco`, and fill in kco's coercion hole.
     This kicks out the inert equality `co2`

     But now, during canonicalization, we see the cast and remove it, in
     `canEqCast`. By the time we get into `canEqCanLHS`, the equality is
     heterogeneous again, and the process repeats!

     To avoid this, we don't strip casts off a type if the other type in the
     equality is a CanEqLHS.  See the `CastTy` case of `can_eq_nc`.
     (The scenario above can happen with a type family, too.
      testcase: typecheck/should_compile/T13822).

     And this is an improvement regardless: because tyvars can, generally,
     unify with casted types, there's no reason to go through the work of
     stripping off the cast when the cast appears opposite a tyvar.

Historical note:

We used to do this via emitting a Derived kind equality and then parking
the heterogeneous equality as irreducible. But this new approach is much
more direct. And it doesn't produce duplicate Deriveds (as the old one did).

Note [Type synonyms and canonicalization]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
We treat type synonym applications as xi types, that is, they do not
count as type function applications.  However, we do need to be a bit
careful with type synonyms: like type functions they may not be
generative or injective.  However, unlike type functions, they are
parametric, so there is no problem in expanding them whenever we see
them, since we do not need to know anything about their arguments in
order to expand them; this is what justifies not having to treat them
as specially as type function applications.  The thing that causes
some subtleties is that we prefer to leave type synonym applications
*unexpanded* whenever possible, in order to generate better error
messages.

If we encounter an equality constraint with type synonym applications
on both sides, or a type synonym application on one side and some sort
of type application on the other, we simply must expand out the type
synonyms in order to continue decomposing the equality constraint into
primitive equality constraints.  For example, suppose we have

  type F a = [Int]

and we encounter the equality

  F a ~ [b]

In order to continue we must expand F a into [Int], giving us the
equality

  [Int] ~ [b]

which we can then decompose into the more primitive equality
constraint

  Int ~ b.

However, if we encounter an equality constraint with a type synonym
application on one side and a variable on the other side, we should
NOT (necessarily) expand the type synonym, since for the purpose of
good error messages we want to leave type synonyms unexpanded as much
as possible.  Hence the ps_xi1, ps_xi2 argument passed to canEqCanLHS.

Note [Type equality cycles]
~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider this situation (from indexed-types/should_compile/GivenLoop):

  instance C (Maybe b)
  *[G] a ~ Maybe (F a)
  [W] C a

or (typecheck/should_compile/T19682b):

  instance C (a -> b)
  *[W] alpha ~ (Arg alpha -> Res alpha)
  [W] C alpha

or (typecheck/should_compile/T21515):

  type family Code a
  *[G] Code a ~ '[ '[ Head (Head (Code a)) ] ]
  [W] Code a ~ '[ '[ alpha ] ]

In order to solve the final Wanted, we must use the starred constraint
for rewriting. But note that all starred constraints have occurs-check failures,
and so we can't straightforwardly add these to the inert set and
use them for rewriting. (NB: A rigid type constructor is at the
top of all RHSs, preventing reorienting in canEqTyVarFunEq in the tyvar
cases.)

The key idea is to replace the outermost type family applications in the RHS of
the starred constraints with a fresh variable, which we'll call a cycle-breaker
variable, or cbv. Then, relate the cbv back with the original type family
application via new equality constraints. Our situations thus become:

  instance C (Maybe b)
  [G] a ~ Maybe cbv
  [G] F a ~ cbv
  [W] C a

or

  instance C (a -> b)
  [W] alpha ~ (cbv1 -> cbv2)
  [W] Arg alpha ~ cbv1
  [W] Res alpha ~ cbv2
  [W] C alpha

or

  [G] Code a ~ '[ '[ cbv ] ]
  [G] Head (Head (Code a)) ~ cbv
  [W] Code a ~ '[ '[ alpha ] ]

This transformation (creating the new types and emitting new equality
constraints) is done by the `FamAppBreaker` field of `TEFA_Break`, which
in turn lives in the `tef_fam_app` field of `TyEqFlags`.  And that in
turn controls the behaviour of the workhorse: GHC.Tc.Utils.Unify.checkTyEqRhs.

The details depend on whether we're working with a Given or a Wanted.

Given
-----
We emit a new Given, [G] F a ~ cbv, equating the type family application
to our new cbv. This is actually done by `break_given` in
`GHC.Tc.Solver.Monad.checkTypeEq`.

Note its orientation: The type family ends up on the left; see
Note [Orienting TyFamLHS/TyFamLHS]. No special treatment for
CycleBreakerTvs is necessary. This scenario is now easily soluble, by using
the first Given to rewrite the Wanted, which can now be solved.

(The first Given actually also rewrites the second one, giving
[G] F (Maybe cbv) ~ cbv, but this causes no trouble.)

Of course, we don't want our fresh variables leaking into e.g. error
messages.  So we fill in the metavariables with their original type family
applications after we're done running the solver (in nestImplicTcS and
runTcSWithEvBinds).  This is done by `restoreTyVarCycles`, which uses the
`inert_cycle_breakers` field in InertSet, which contains the pairings
invented in `break_given`.

That is, we transform
  [G] g : lhs ~ ...(F lhs)...
to
  [G] (Refl lhs) : F lhs ~ cbv      -- CEqCan
  [G] g          : lhs ~ ...cbv...  -- CEqCan

Note that
* `cbv` is a fresh cycle breaker variable.
* `cbv` is a meta-tyvar, but it is completely untouchable.
* We track the cycle-breaker variables in inert_cycle_breakers in InertSet
* We eventually fill in the cycle-breakers, with `cbv := F lhs`.
  No one else fills in CycleBreakerTvs!
* The evidence for the new `F lhs ~ cbv` constraint is Refl, because we know
  this fill-in is ultimately going to happen.
* In `inert_cycle_breakers`, we remember the (cbv, F lhs) pair; that is, we
  remember the /original/ type.  The [G] F lhs ~ cbv constraint may be rewritten
  by other givens (eg if we have another [G] lhs ~ (b,c)), but at the end we
  still fill in with cbv := F lhs
* This fill-in is done when solving is complete, by restoreTyVarCycles
  in nestImplicTcS and runTcSWithEvBinds.

Wanted
------
First, we do not cycle-break unless the LHS is a unifiable type variable
See Note [Don't cycle-break Wanteds when not unifying] in GHC.Tc.Solver.Monad.

OK, so suppose the LHS is a unifiable type variable.  The fresh cycle-breaker
variables here must actually be normal, touchable metavariables. That is, they
are TauTvs. Nothing at all unusual. Repeating the example from above, we have

  *[W] alpha ~ (Arg alpha -> Res alpha)

and we turn this into

  *[W] alpha ~ (cbv1 -> cbv2)
  [W] Arg alpha ~ cbv1
  [W] Res alpha ~ cbv2

where cbv1 and cbv2 are fresh TauTvs.  This is actually done within checkTyEqRhs,
called within canEqCanLHSFinish_try_unification, which will use the BreakWanted
FamAppBreaker.

Why TauTvs? See [Why TauTvs] below.

Critically, we emit the two new constraints (the last two above)
directly instead of calling wrapUnifier. (Otherwise, we'd end up
unifying cbv1 and cbv2 immediately, achieving nothing.)  Next, we
unify alpha := cbv1 -> cbv2, having eliminated the occurs check. This
unification happens immediately following a successful call to
checkTyEqRhs, in canEqCanLHSFinish_try_unification.

Now, we're here (including further context from our original example,
from the top of the Note):

  instance C (a -> b)
  [W] Arg (cbv1 -> cbv2) ~ cbv1
  [W] Res (cbv1 -> cbv2) ~ cbv2
  [W] C (cbv1 -> cbv2)

The first two W constraints reduce to reflexivity and are discarded,
and the last is easily soluble.

[Why TauTvs]:
Let's look at another example (typecheck/should_compile/T19682) where we need
to unify the cbvs:

  class    (AllEqF xs ys, SameShapeAs xs ys) => AllEq xs ys
  instance (AllEqF xs ys, SameShapeAs xs ys) => AllEq xs ys

  type family SameShapeAs xs ys :: Constraint where
    SameShapeAs '[] ys      = (ys ~ '[])
    SameShapeAs (x : xs) ys = (ys ~ (Head ys : Tail ys))

  type family AllEqF xs ys :: Constraint where
    AllEqF '[]      '[]      = ()
    AllEqF (x : xs) (y : ys) = (x ~ y, AllEq xs ys)

  [W] alpha ~ (Head alpha : Tail alpha)
  [W] AllEqF '[Bool] alpha

Without the logic detailed in this Note, we're stuck here, as AllEqF cannot
reduce and alpha cannot unify. Let's instead apply our cycle-breaker approach,
just as described above. We thus invent cbv1 and cbv2 and unify
alpha := cbv1 -> cbv2, yielding (after zonking)

  [W] Head (cbv1 : cbv2) ~ cbv1
  [W] Tail (cbv1 : cbv2) ~ cbv2
  [W] AllEqF '[Bool] (cbv1 : cbv2)

The first two W constraints simplify to reflexivity and are discarded.
But the last reduces:

  [W] Bool ~ cbv1
  [W] AllEq '[] cbv2

The first of these is solved by unification: cbv1 := Bool. The second
is solved by the instance for AllEq to become

  [W] AllEqF '[] cbv2
  [W] SameShapeAs '[] cbv2

While the first of these is stuck, the second makes progress, to lead to

  [W] AllEqF '[] cbv2
  [W] cbv2 ~ '[]

This second constraint is solved by unification: cbv2 := '[]. We now
have

  [W] AllEqF '[] '[]

which reduces to

  [W] ()

which is trivially satisfiable. Hooray!

Note that we need to unify the cbvs here; if we did not, there would be
no way to solve those constraints. That's why the cycle-breakers are
ordinary TauTvs.

How all this is implemented
---------------------------
We implement all this via the `TEFA_Break` constructor of `TyEqFamApp`,
itself stored in the `tef_fam_app` field of `TyEqFlags`, which controls
the behaviour of `GHC.Tc.Utils.Unify.checkTyEqRhs`.  The `TEFA_Break`
stuff happens when `checkTyEqRhs` encounters a family application.

We try the cycle-breaking trick:
* For Wanteds, when there is a touchable unification variable on the left
* For Givens, regardless of the LHS

EXCEPT that, in both cases, as `GHC.Tc.Solver.Monad.mkTEFA_Break` shows, we
don't use this trick:

* When the constraint we are looking at was itself created by cycle-breaking;
  see Detail (7) below.

* For representational equalities, as there is no concrete use case where it is
  helpful (unlike for nominal equalities).

  Furthermore, because function applications can be CanEqLHSs, but newtype
  applications cannot, the disparities between the cases are enough that it
  would be effortful to expand the idea to representational equalities. A quick
  attempt, with
      data family N a b
      f :: (Coercible a (N a b), Coercible (N a b) b) => a -> b
      f = coerce
  failed with "Could not match 'b' with 'b'." Further work is held off
  until when we have a concrete incentive to explore this dark corner.

More details:

 (1) We don't look under foralls, at all, in `checkTyEqRhs`.  There might be
     a cyclic occurrence underneath, in a case like
          [G] lhs ~ forall b. ... lhs ....
     but it doesn't matter because we will classify the constraint as Irred,
     so it will not be used for rewriting.

     Earlier versions required an extra, post-breaking, check.  Skipping this
     check causes typecheck/should_fail/GivenForallLoop and polykinds/T18451 to
     loop.  But now it is all simpler, with no need for a second check.

 (2) Historical Note: our goal here is to avoid loops in rewriting. We can thus
     skip looking in coercions, as we don't rewrite in coercions in the
     algorithm in GHC.Solver.Rewrite.  This doesn't seem relevant any more.
     We cycle break to make the constraint canonical.

 (3) As we cycle-break as described in this Note, we can build ill-kinded
     types. For example, if we have Proxy (F a) b, where (b :: F a), then
     replacing this with Proxy cbv b is ill-kinded. However, we will later
     set cbv := F a, and so the zonked type will be well-kinded again.
     The temporary ill-kinded type hurts no one, and avoiding this would
     be quite painfully difficult.

     Specifically, this detail does not contravene the Purely Kinded Type Invariant
     (Note [The Purely Kinded Type Invariant (PKTI)] in GHC.Tc.Gen.HsType).
     The PKTI says that we can call typeKind on any type, without failure.
     It would be violated if we, say, replaced a kind (a -> b) with a kind c,
     because an arrow kind might be consulted in piResultTys. Here, we are
     replacing one opaque type like (F a b c) with another, cbv (opaque in
     that we never assume anything about its structure, like that it has a
     result type or a RuntimeRep argument).

 (4) The evidence for the produced Givens is all just reflexive, because we
     will eventually set the cycle-breaker variable to be the type family, and
     then, after the zonk, all will be well. See also the notes at the end of
     the Given section of this Note.

 (5) The implementation in `checkTyEqRhs` is efficient because it only replaces
     a type family application with a type variable, if that particular
     appplication is implicated in the occurs check.  For example:
         [W] alpha ~ Maybe (F alpha, G beta)
     We'll end up calling GHC.Tc.Utils.Unify.checkFamApp
       * On `F alpha`, which fail and calls the cycle-breaker in TEFA_Break
       * On `G beta`, which succeeds no problem.

     However, we make no attempt to detect cases like a ~ (F a, F a) and use the
     same tyvar to replace F a. The constraint solver will common them up later!
     (Cf. Note [Apartness and type families] in GHC.Core.Unify, which goes to
     this extra effort.) However, this is really a very small corner case.  The
     investment to craft a clever, performant solution seems unworthwhile.

 (6) We often get the predicate associated with a constraint from its evidence
     with ctPred. We thus must not only make sure the generated CEqCan's fields
     have the updated RHS type (that is, the one produced by replacing type
     family applications with fresh variables), but we must also update the
     evidence itself. This is done by the call to rewriteEqEvidence in
     canEqCanLHSFinish.

 (7) We don't wish to apply this magic on the equalities created
     by this very same process. Consider this, from
     typecheck/should_compile/ContextStack2:

       type instance TF (a, b) = (TF a, TF b)
       t :: (a ~ TF (a, Int)) => ...

       [G] a ~ TF (a, Int)

     The RHS reduces, so we get

       [G] a ~ (TF a, TF Int)

     We then break cycles, to get

       [G] g1 :: a ~ (cbv1, cbv2)
       [G] g2 :: TF a ~ cbv1
       [G] g3 :: TF Int ~ cbv2

     g1 gets added to the inert set, as written. But then g2 becomes
     the work item. g1 rewrites g2 to become

       [G] TF (cbv1, cbv2) ~ cbv1

     which then uses the type instance to become

       [G] (TF cbv1, TF cbv2) ~ cbv1

     which looks remarkably like the Given we started with. If left unchecked,
     this will end up breaking cycles again, looping ad infinitum (and
     resulting in a context-stack reduction error, not an outright loop). The
     solution is easy: don't break cycles on an equality generated by breaking
     cycles. Instead, we mark this final Given as a CIrredCan with a
     NonCanonicalReason with the soluble occurs-check bit set (only).

     We track these equalities by giving them a special CtOrigin,
     CycleBreakerOrigin. This works for both Givens and Wanteds, as we need the
     logic in the W case for e.g. typecheck/should_fail/T17139.  Because this
     logic needs to work for Wanteds, too, we cannot simply look for a
     CycleBreakerTv on the left: Wanteds don't use them.


**********************************************************************
*                                                                    *
                   Rewriting evidence
*                                                                    *
**********************************************************************
-}

rewriteEqEvidence :: CtEvidence       -- Old evidence :: olhs ~ orhs (not swapped)
                                      --              or orhs ~ olhs (swapped)
                  -> SwapFlag
                  -> Reduction        -- lhs_co :: olhs ~ nlhs
                  -> Reduction        -- rhs_co :: orhs ~ nrhs
                  -> CoHoleSet        -- New rewriters free in lhs_redn and rhs_redn
                                      --   See GHC.Tc.Types.Constraint
                                      --   Note [Wanteds rewrite Wanteds: rewriter-sets]
                  -> TcS CtEvidence   -- Of type nlhs ~ nrhs
-- With reductions (Reduction lhs_co nlhs) (Reduction rhs_co nrhs),
-- rewriteEqEvidence yields, for a given equality (Given g olhs orhs):
-- If not swapped
--      g1 : nlhs ~ nrhs = sym lhs_co ; g ; rhs_co
-- If swapped
--      g1 : nlhs ~ nrhs = sym lhs_co ; Sym g ; rhs_co
--
-- For a wanted equality (Wanted w), we do the dual thing:
-- New  w1 : nlhs ~ nrhs
-- If not swapped
--      w : olhs ~ orhs = lhs_co ; w1 ; sym rhs_co
-- If swapped
--      w : orhs ~ olhs = rhs_co ; sym w1 ; sym lhs_co
--
-- It's all a form of rewriteEvidence, specialised for equalities
rewriteEqEvidence :: CtEvidence
-> SwapFlag
-> Reduction
-> Reduction
-> CoHoleSet
-> TcS CtEvidence
rewriteEqEvidence CtEvidence
old_ev SwapFlag
swapped (Reduction TcCoercion
lhs_co PredType
nlhs) (Reduction TcCoercion
rhs_co PredType
nrhs) CoHoleSet
new_holes
  | SwapFlag
NotSwapped <- SwapFlag
swapped
  , TcCoercion -> Bool
isReflCo TcCoercion
lhs_co      -- See Note [Rewriting with Refl]
  , TcCoercion -> Bool
isReflCo TcCoercion
rhs_co
  = CtEvidence -> TcS CtEvidence
forall a. a -> TcS a
forall (m :: * -> *) a. Monad m => a -> m a
return (HasDebugCallStack => CtEvidence -> PredType -> CtEvidence
CtEvidence -> PredType -> CtEvidence
setCtEvPredType CtEvidence
old_ev PredType
new_pred)

  | CtGiven (GivenCt { ctev_evar :: GivenCtEvidence -> TyVar
ctev_evar = TyVar
old_evar }) <- CtEvidence
old_ev
  = do { let new_tm :: EvTerm
new_tm = TcCoercion -> EvTerm
evCoercion ( TcCoercion -> TcCoercion
mkSymCo TcCoercion
lhs_co
                                  HasDebugCallStack => TcCoercion -> TcCoercion -> TcCoercion
TcCoercion -> TcCoercion -> TcCoercion
`mkTransCo` SwapFlag -> TcCoercion -> TcCoercion
maybeSymCo SwapFlag
swapped (TyVar -> TcCoercion
mkCoVarCo TyVar
old_evar)
                                  HasDebugCallStack => TcCoercion -> TcCoercion -> TcCoercion
TcCoercion -> TcCoercion -> TcCoercion
`mkTransCo` TcCoercion
rhs_co)
       ; GivenCtEvidence -> CtEvidence
GivenCtEvidence -> CtEvidence
CtGiven (GivenCtEvidence -> CtEvidence)
-> TcS GivenCtEvidence -> TcS CtEvidence
forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
<$> CtLoc -> (PredType, EvTerm) -> TcS GivenCtEvidence
newGivenEv CtLoc
loc (PredType
new_pred, EvTerm
new_tm) }

  | CtWanted (WantedCt { ctev_dest :: WantedCtEvidence -> TcEvDest
ctev_dest = TcEvDest
dest, ctev_rewriters :: WantedCtEvidence -> CoHoleSet
ctev_rewriters = CoHoleSet
rewriters }) <- CtEvidence
old_ev
  = do { let rewriters' :: CoHoleSet
rewriters' = CoHoleSet
rewriters CoHoleSet -> CoHoleSet -> CoHoleSet
forall a. Semigroup a => a -> a -> a
S.<> CoHoleSet
new_holes
       ; (new_ev, hole) <- CtLoc
-> CoHoleSet
-> Role
-> PredType
-> PredType
-> TcS (WantedCtEvidence, CoercionHole)
newWantedEq CtLoc
loc CoHoleSet
rewriters' (HasDebugCallStack => CtEvidence -> Role
CtEvidence -> Role
ctEvRewriteRole CtEvidence
old_ev) PredType
nlhs PredType
nrhs
       ; let co = SwapFlag -> TcCoercion -> TcCoercion
maybeSymCo SwapFlag
swapped (TcCoercion -> TcCoercion) -> TcCoercion -> TcCoercion
forall a b. (a -> b) -> a -> b
$
                  TcCoercion
lhs_co HasDebugCallStack => TcCoercion -> TcCoercion -> TcCoercion
TcCoercion -> TcCoercion -> TcCoercion
`mkTransCo` CoercionHole -> TcCoercion
mkHoleCo CoercionHole
hole HasDebugCallStack => TcCoercion -> TcCoercion -> TcCoercion
TcCoercion -> TcCoercion -> TcCoercion
`mkTransCo` TcCoercion -> TcCoercion
mkSymCo TcCoercion
rhs_co
       ; setWantedEq dest (CPH { cph_co    = co
                               , cph_holes = new_holes `mappend` unitCoHoleSet hole })
             -- cph_holes: new_holes has all the holes from lhs_co and rhs_co
       ; traceTcS "rewriteEqEvidence" (vcat [ ppr old_ev
                                            , ppr nlhs
                                            , ppr nrhs
                                            , ppr co
                                            , ppr new_holes ])
       ; return $ CtWanted new_ev }

  where
    new_pred :: PredType
new_pred = CtEvidence -> PredType -> PredType -> PredType
mkTcEqPredLikeEv CtEvidence
old_ev PredType
nlhs PredType
nrhs
    loc :: CtLoc
loc      = CtEvidence -> CtLoc
ctEvLoc CtEvidence
old_ev

rewriteEqEvidenceSwapOnly :: CtEvidence -> EqRel -> SwapFlag -> CanEqLHS -> TcType
                          -> TcS CtEvidence
rewriteEqEvidenceSwapOnly :: CtEvidence
-> EqRel -> SwapFlag -> CanEqLHS -> PredType -> TcS CtEvidence
rewriteEqEvidenceSwapOnly CtEvidence
ev EqRel
eq_rel SwapFlag
swapped CanEqLHS
lhs PredType
rhs
  = CtEvidence
-> SwapFlag
-> Reduction
-> Reduction
-> CoHoleSet
-> TcS CtEvidence
rewriteEqEvidence CtEvidence
ev SwapFlag
swapped
                      (Role -> PredType -> Reduction
mkReflRedn Role
role (CanEqLHS -> PredType
canEqLHSType CanEqLHS
lhs))
                      (Role -> PredType -> Reduction
mkReflRedn Role
role PredType
rhs)
                      CoHoleSet
emptyCoHoleSet
  where
    role :: Role
role = EqRel -> Role
eqRelRole EqRel
eq_rel

{-
**********************************************************************
*                                                                    *
                   tryInertEqs
*                                                                    *
**********************************************************************

Note [Combining equalities]
~~~~~~~~~~~~~~~~~~~~~~~~~~~
Suppose we have
   Inert:     g1 :: a ~ t
   Work item: g2 :: a ~ t

Then we can simply solve g2 from g1, thus g2 := g1.  Easy!
But it's not so simple:

(CE1) If t is a type variable, the equalties might be oriented differently:
      e.g. (g1 :: a~b) and (g2 :: b~a)
  So we look both ways round.  Hence the SwapFlag result to
  inertsCanDischarge.

(CE2) We can only do g2 := g1 if g1 can discharge g2; that depends on
  (a) the role and (b) the flavour.  E.g. a representational equality
  cannot discharge a nominal one; a Wanted cannot discharge a Given.
  The predicate is eqCanRewriteFR.

(CE3) Visibility. Suppose  S :: forall k. k -> Type, and consider unifying
      S @Type (a::Type)  ~   S @(Type->Type) (b::Type->Type)
  From the first argument we get (Type ~ Type->Type); from the second
  argument we get (a ~ b) which in turn gives (Type ~ Type->Type).
  See typecheck/should_fail/T16204c.

  That first argument is invisible in the source program (aside from
  visible type application), so we'd much prefer to get the error from
  the second. We track visibility in the uo_invisible field of a TypeEqOrigin.
  We use this to prioritise visible errors (see GHC.Tc.Errors.tryReporters,
  the partition on isVisibleOrigin).

  So when combining two otherwise-identical equalites, we want to
  keep the visible one, and discharge the invisible one.  Hence the
  call to strictly_more_visible.

(CE4) Suppose we have this set up (#25440):
   Inert:     [W] g1: F a ~ a Int    (arising from (F a ~ a Int)
   Work item: [W] g2: F alpha ~ F a  (arising from (F alpha ~ F a)
   We rewrite g2 with g1, to give
              [W] g2{rw:g1} : F alpha ~ a Int
   Now if F is injective we can get [W] alpha~a, and hence alpha:=a, and
   we kick out g1. Now we have two constraints
       [W] g1        : F a ~ a Int  (arising from (F a ~ a Int))
       [W] g2{rw:g1} : F a ~ a Int  (arising from (F alpha ~ F a))
   If we end up with g2 in the inert set (not g1) we'll get a very confusing
   error message that we can solve (F a ~ a Int)
       arising from F a ~ F a

   TL;DR: Better to hang on to `g1` (with no rewriters), in preference
   to `g2` (which has a rewriter).

   See (WRW11) in Note [Wanteds rewrite Wanteds: rewriter-sets]
   in GHC.Tc.Types.Constraint.
-}

tryInertEqs :: EqCt -> SolverStage ()
tryInertEqs :: EqCt -> SolverStage ()
tryInertEqs work_item :: EqCt
work_item@(EqCt { eq_ev :: EqCt -> CtEvidence
eq_ev = CtEvidence
ev, eq_eq_rel :: EqCt -> EqRel
eq_eq_rel = EqRel
eq_rel })
  = TcS (StopOrContinue ()) -> SolverStage ()
TcS (StopOrContinue ()) -> SolverStage ()
forall a. TcS (StopOrContinue a) -> SolverStage a
Stage (TcS (StopOrContinue ()) -> SolverStage ())
-> TcS (StopOrContinue ()) -> SolverStage ()
forall a b. (a -> b) -> a -> b
$
    do { inerts <- TcS InertCans
getInertCans
       ; if | Just (ev_i, swapped) <- inertsEqsCanDischarge inerts work_item
            -> case ev of
                 CtGiven {} -> CtEvidence -> String -> TcS (StopOrContinue ())
forall a. CtEvidence -> String -> TcS (StopOrContinue a)
stopWith CtEvidence
ev String
"Given solved from inert"
                 CtWanted (WantedCt { ctev_dest :: WantedCtEvidence -> TcEvDest
ctev_dest = TcEvDest
dest })
                    -> do { let co :: TcCoercion
co = SwapFlag -> TcCoercion -> TcCoercion
maybeSymCo SwapFlag
swapped (TcCoercion -> TcCoercion) -> TcCoercion -> TcCoercion
forall a b. (a -> b) -> a -> b
$
                                     Role -> Role -> TcCoercion -> TcCoercion
downgradeRole (EqRel -> Role
eqRelRole EqRel
eq_rel)
                                                   (HasDebugCallStack => CtEvidence -> Role
CtEvidence -> Role
ctEvRewriteRole CtEvidence
ev_i)
                                                   (HasDebugCallStack => CtEvidence -> TcCoercion
CtEvidence -> TcCoercion
ctEvCoercion CtEvidence
ev_i)
                          ; HasDebugCallStack => TcEvDest -> CoercionPlusHoles -> TcS ()
TcEvDest -> CoercionPlusHoles -> TcS ()
setWantedEq TcEvDest
dest (CPH { cph_co :: TcCoercion
cph_co    = TcCoercion
co
                                                  , cph_holes :: CoHoleSet
cph_holes = CtEvidence -> CoHoleSet
ctEvCoHoleSet CtEvidence
ev_i })
                          ; CtEvidence -> String -> TcS (StopOrContinue ())
forall a. CtEvidence -> String -> TcS (StopOrContinue a)
stopWith CtEvidence
ev String
"Wanted solved from inert" }

            | otherwise
            -> continueWith () }

inertsEqsCanDischarge :: InertCans -> EqCt
                      -> Maybe ( CtEvidence  -- The evidence for the inert
                               , SwapFlag )  -- Whether we need mkSymCo
inertsEqsCanDischarge :: InertCans -> EqCt -> Maybe (CtEvidence, SwapFlag)
inertsEqsCanDischarge InertCans
inerts (EqCt { eq_lhs :: EqCt -> CanEqLHS
eq_lhs = CanEqLHS
lhs_w, eq_rhs :: EqCt -> PredType
eq_rhs = PredType
rhs_w
                                , eq_ev :: EqCt -> CtEvidence
eq_ev = CtEvidence
ev_w, eq_eq_rel :: EqCt -> EqRel
eq_eq_rel = EqRel
eq_rel })
  | (CtEvidence
ev_i : [CtEvidence]
_) <- [ CtEvidence
ev_i | EqCt { eq_ev :: EqCt -> CtEvidence
eq_ev = CtEvidence
ev_i, eq_rhs :: EqCt -> PredType
eq_rhs = PredType
rhs_i
                                , eq_eq_rel :: EqCt -> EqRel
eq_eq_rel = EqRel
eq_rel }
                                  <- InertCans -> CanEqLHS -> [EqCt]
findEq InertCans
inerts CanEqLHS
lhs_w
                         , PredType
rhs_i HasDebugCallStack => PredType -> PredType -> Bool
PredType -> PredType -> Bool
`tcEqType` PredType
rhs_w
                         , CtEvidence -> EqRel -> Bool
inert_beats_wanted CtEvidence
ev_i EqRel
eq_rel ]
  =  -- Inert:     a ~ ty
     -- Work item: a ~ ty
    (CtEvidence, SwapFlag) -> Maybe (CtEvidence, SwapFlag)
forall a. a -> Maybe a
Just (CtEvidence
ev_i, SwapFlag
NotSwapped)

  | Just CanEqLHS
rhs_lhs <- PredType -> Maybe CanEqLHS
canEqLHS_maybe PredType
rhs_w
  , (CtEvidence
ev_i : [CtEvidence]
_) <- [ CtEvidence
ev_i | EqCt { eq_ev :: EqCt -> CtEvidence
eq_ev = CtEvidence
ev_i, eq_rhs :: EqCt -> PredType
eq_rhs = PredType
rhs_i
                                , eq_eq_rel :: EqCt -> EqRel
eq_eq_rel = EqRel
eq_rel }
                             <- InertCans -> CanEqLHS -> [EqCt]
findEq InertCans
inerts CanEqLHS
rhs_lhs
                         , PredType
rhs_i HasDebugCallStack => PredType -> PredType -> Bool
PredType -> PredType -> Bool
`tcEqType` CanEqLHS -> PredType
canEqLHSType CanEqLHS
lhs_w
                         , CtEvidence -> EqRel -> Bool
inert_beats_wanted CtEvidence
ev_i EqRel
eq_rel ]
  =  -- Inert:     a ~ b
     -- Work item: b ~ a
     (CtEvidence, SwapFlag) -> Maybe (CtEvidence, SwapFlag)
forall a. a -> Maybe a
Just (CtEvidence
ev_i, SwapFlag
IsSwapped)

  where
    loc_w :: CtLoc
loc_w  = CtEvidence -> CtLoc
ctEvLoc CtEvidence
ev_w
    flav_w :: CtFlavour
flav_w = CtEvidence -> CtFlavour
ctEvFlavour CtEvidence
ev_w
    fr_w :: CtFlavourRole
fr_w   = (CtFlavour
flav_w, EqRel
eq_rel)
    empty_rw_w :: Bool
empty_rw_w = CoHoleSet -> Bool
isEmptyCoHoleSet (CtEvidence -> CoHoleSet
ctEvRewriters CtEvidence
ev_w)

    inert_beats_wanted :: CtEvidence -> EqRel -> Bool
inert_beats_wanted CtEvidence
ev_i EqRel
eq_rel
      = -- eqCanRewriteFR:        see second bullet of Note [Combining equalities]
        CtFlavourRole
fr_i CtFlavourRole -> CtFlavourRole -> Bool
`eqCanRewriteFR` CtFlavourRole
fr_w
        Bool -> Bool -> Bool
&& Bool -> Bool
not (CtEvidence -> Bool
prefer_wanted CtEvidence
ev_i Bool -> Bool -> Bool
&& (CtFlavourRole
fr_w CtFlavourRole -> CtFlavourRole -> Bool
`eqCanRewriteFR` CtFlavourRole
fr_i))
      where
        fr_i :: CtFlavourRole
fr_i = (CtEvidence -> CtFlavour
ctEvFlavour CtEvidence
ev_i, EqRel
eq_rel)

    -- See (CE3) in Note [Combining equalities]
    strictly_more_visible :: CtLoc -> CtLoc -> Bool
strictly_more_visible CtLoc
loc1 CtLoc
loc2
       = Bool -> Bool
not (CtOrigin -> Bool
isVisibleOrigin (CtLoc -> CtOrigin
ctLocOrigin CtLoc
loc2)) Bool -> Bool -> Bool
&&
         CtOrigin -> Bool
isVisibleOrigin (CtLoc -> CtOrigin
ctLocOrigin CtLoc
loc1)

    prefer_wanted :: CtEvidence -> Bool
prefer_wanted CtEvidence
ev_i
      =  (CtLoc
loc_w CtLoc -> CtLoc -> Bool
`strictly_more_visible` CtEvidence -> CtLoc
ctEvLoc CtEvidence
ev_i)
             -- strictly_more_visible: see (CE3) in Note [Combining equalities]
      Bool -> Bool -> Bool
|| (Bool
empty_rw_w Bool -> Bool -> Bool
&& Bool -> Bool
not (CoHoleSet -> Bool
isEmptyCoHoleSet (CtEvidence -> CoHoleSet
ctEvRewriters CtEvidence
ev_i)))
             -- Prefer the one that has no rewriters
             -- See (CE4) in Note [Combining equalities]

inertsEqsCanDischarge InertCans
_ EqCt
_ = Maybe (CtEvidence, SwapFlag)
forall a. Maybe a
Nothing



{- Note [Do not unify Givens]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider this GADT match
   data T a where
      T1 :: T Int
      ...

   f x = case x of
           T1 -> True
           ...

So we get f :: T alpha[1] -> beta[1]
          x :: T alpha[1]
and from the T1 branch we get the implication
   forall[2] (alpha[1] ~ Int) => beta[1] ~ Bool

Now, clearly we don't want to unify alpha:=Int!  Yet at the moment we
process [G] alpha[1] ~ Int, we don't have any given-equalities in the
inert set, and hence there are no given equalities to make alpha untouchable.

NB: if it were alpha[2] ~ Int, this argument wouldn't hold.  But that
never happens: invariant (GivenInv) in Note [TcLevel invariants]
in GHC.Tc.Utils.TcType.

Simple solution: never unify in Givens!

Note [Do not unify representational equalities]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider   [W] alpha ~R# b
where alpha is touchable. Should we unify alpha := b?

Certainly not!  Unifying forces alpha and be to be the same; but they
only need to be representationally equal types.

For example, we might have another constraint [W] alpha ~# N b
where
  newtype N b = MkN b
and we want to get alpha := N b.

See also #15144, which was caused by unifying a representational
equality.

Note that it does however make sense to perform such unifications, as a last
resort, when doing top-level defaulting.
See Note [Defaulting representational equalities].

Note [Unify only if the rewriter set is empty]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider
    co (rewriters = {co1,co2}) :: alpha ~# blah
If we unify before solving `co1` and `co2` (which might well be insoluble)
we destroy the careful tracking of Note [Wanteds rewrite Wanteds: rewriter-sets]
in GHC.Tc.Types.Constraint.  So we decline to unify any equality with a
non-empty rewriter set: see (REWRITERS) in Note [Unification preconditions]
in GHC.Tc.Utils.

Wrinkles:

(URW1) We may, however, be willing to /default/ such an equality; see
   (DE6) in Note [Defaulting equalities] in GHC.Tc.Solver.Default.

(URW2) If we have `co` in the inert set, and we solve `co1` and `co2`,
   we should kick out `co` so that we can now unify it, which might
   unlock other stuff.  See `kickOutAfterFillingCoercionHole` in
   GHC.Tc.Solver.Monad.

Note [Solve by unification]
~~~~~~~~~~~~~~~~~~~~~~~~~~~
If we solve
   alpha[n] ~ ty
by unification, there are two cases to consider

* TouchableSameLevel: if the ambient level is 'n', then
  we can simply update alpha := ty, and do nothing else

* TouchableOuterLevel free_metas n: if the ambient level is greater than
  'n' (the level of alpha), in addition to setting alpha := ty we must
  do two other things:

  1. Promote all the free meta-vars of 'ty' to level n.  After all,
     alpha[n] is at level n, and so if we set, say,
          alpha[n] := Maybe beta[m],
     we must ensure that when unifying beta we do skolem-escape checks
     etc relevant to level n.  Simple way to do that: promote beta to
     level n.

  2. Set the Unification Level Flag to record that a level-n unification has
     taken place. See Note [WhatUnifications] in GHC.Tc.Utils.Unify

NB: TouchableSameLevel is just an optimisation for TouchableOuterLevel. Promotion
would be a no-op, and setting the unification flag unnecessarily would just
make the solver iterate more often.  (We don't need to iterate when unifying
at the ambient level because of the kick-out mechanism.)
-}


{-********************************************************************
*                                                                    *
          Final wrap-up for equalities
*                                                                    *
********************************************************************-}

{- Note [Looking up primitive equalities in quantified constraints]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
For equalities (a ~# b) look up (a ~ b), and then do a superclass
selection. This avoids having to support quantified constraints whose
kind is not Constraint, such as (forall a. F a ~# b)

See
 * Note [Evidence for quantified constraints] in GHC.Core.Predicate
 * Note [Equality superclasses in quantified constraints]
   in GHC.Tc.Solver.Dict
-}

--------------------
tryQCsIrredEqCt :: IrredCt -> SolverStage ()
tryQCsIrredEqCt :: IrredCt -> SolverStage ()
tryQCsIrredEqCt (IrredCt { ir_ev :: IrredCt -> CtEvidence
ir_ev = CtEvidence
ev })
  | EqPred EqRel
eq_rel PredType
t1 PredType
t2 <- HasDebugCallStack => PredType -> Pred
PredType -> Pred
classifyPredType (CtEvidence -> PredType
ctEvPred CtEvidence
ev)
  = CtEvidence -> EqRel -> PredType -> PredType -> SolverStage ()
lookup_eq_in_qcis CtEvidence
ev EqRel
eq_rel PredType
t1 PredType
t2

  | Bool
otherwise  -- All the calls come from in this module, where we deal only with
               -- equalities, so ctEvPred ev) must be an equality. Indeed, we could
               -- pass eq_rel, t1, t2 as arguments, to avoid this can't-happen case,
               -- but it's not a hot path, and this is simple and robust
  = String -> SDoc -> SolverStage ()
forall a. HasCallStack => String -> SDoc -> a
pprPanic String
"solveIrredEquality" (CtEvidence -> SDoc
forall a. Outputable a => a -> SDoc
ppr CtEvidence
ev)

--------------------
tryQCsEqCt :: EqCt -> SolverStage ()
tryQCsEqCt :: EqCt -> SolverStage ()
tryQCsEqCt (EqCt { eq_ev :: EqCt -> CtEvidence
eq_ev = CtEvidence
ev, eq_lhs :: EqCt -> CanEqLHS
eq_lhs = CanEqLHS
lhs, eq_rhs :: EqCt -> PredType
eq_rhs = PredType
rhs, eq_eq_rel :: EqCt -> EqRel
eq_eq_rel = EqRel
eq_rel })
  = CtEvidence -> EqRel -> PredType -> PredType -> SolverStage ()
lookup_eq_in_qcis CtEvidence
ev EqRel
eq_rel (CanEqLHS -> PredType
canEqLHSType CanEqLHS
lhs) PredType
rhs

--------------------
lookup_eq_in_qcis :: CtEvidence -> EqRel -> TcType -> TcType -> SolverStage ()
-- The "final QCI check" checks to see if we have
--    [W] t1 ~# t2
-- and a Given quantified contraint like (forall a b. blah => a ~ b)
-- Why?  See Note [Looking up primitive equalities in quantified constraints]
-- See also GHC.Tc.Solver.Dict Note [Equality superclasses in quantified constraints]
lookup_eq_in_qcis :: CtEvidence -> EqRel -> PredType -> PredType -> SolverStage ()
lookup_eq_in_qcis (CtGiven {}) EqRel
_ PredType
_ PredType
_
  = () -> SolverStage ()
forall a. a -> SolverStage a
nopStage ()  -- Never look up Givens in quantified constraints

lookup_eq_in_qcis ev :: CtEvidence
ev@(CtWanted (WantedCt { ctev_dest :: WantedCtEvidence -> TcEvDest
ctev_dest = TcEvDest
dest, ctev_loc :: WantedCtEvidence -> CtLoc
ctev_loc = CtLoc
loc })) EqRel
eq_rel PredType
lhs PredType
rhs
  = do { ev_binds_var <- TcS EvBindsVar -> SolverStage EvBindsVar
forall a. TcS a -> SolverStage a
simpleStage TcS EvBindsVar
getTcEvBindsVar
       ; ics          <- simpleStage getInertCans
       ; if null (inert_qcis ics)          -- Shortcut common case
            || isCoEvBindsVar ev_binds_var -- See Note [Instances in no-evidence implications]
         then nopStage ()
         else -- Try looking for both (lhs~rhs) anr (rhs~lhs); see #23333
              do { try NotSwapped; try IsSwapped } }
  where
    hole :: CoercionHole
hole = case TcEvDest
dest of
             HoleDest CoercionHole
hole -> CoercionHole
hole   -- Equality constraints have HoleDest
             TcEvDest
_ -> String -> SDoc -> CoercionHole
forall a. HasCallStack => String -> SDoc -> a
pprPanic String
"lookup_eq_in_qcis" (TcEvDest -> SDoc
forall a. Outputable a => a -> SDoc
ppr TcEvDest
dest)

    try :: SwapFlag -> SolverStage ()
    -- E.g. We are trying to solve (say)
    --             [W] g : [Int] ~# b)
    --      from   [G] forall x. blah => b ~ [x]   -- A quantified constraint
    -- We can solve it like this
    --     d::b~[Int] := $df @Int blah        -- Apply the quantified constraint
    --     g'::b~#[Int] := sc_sel d           -- Binding, extract the coercion from d
    --     g(co-hole) := sym g'               -- Fill the original coercion hole
    -- Here g' is a fresh coercion variable.
    try :: SwapFlag -> SolverStage ()
try SwapFlag
swap
       | Just (Class
cls, [PredType]
tys) <- SwapFlag
-> (PredType -> PredType -> Maybe (Class, [PredType]))
-> PredType
-> PredType
-> Maybe (Class, [PredType])
forall a b. SwapFlag -> (a -> a -> b) -> a -> a -> b
unSwap SwapFlag
swap (EqRel -> PredType -> PredType -> Maybe (Class, [PredType])
boxEqPred EqRel
eq_rel) PredType
lhs PredType
rhs
       = TcS (StopOrContinue ()) -> SolverStage ()
TcS (StopOrContinue ()) -> SolverStage ()
forall a. TcS (StopOrContinue a) -> SolverStage a
Stage (TcS (StopOrContinue ()) -> SolverStage ())
-> TcS (StopOrContinue ()) -> SolverStage ()
forall a b. (a -> b) -> a -> b
$
         do { let cls_pred :: PredType
cls_pred = Class -> [PredType] -> PredType
mkClassPred Class
cls [PredType]
tys
            ; res <- PredType -> CtLoc -> TcS ClsInstResult
matchLocalInst PredType
cls_pred CtLoc
loc
            ; traceTcS "lookup_eq_in_qcis:1" (ppr cls_pred)
            ; case res of
                OneInst {}
                  -> do { dict_ev <- CtLoc -> CoHoleSet -> PredType -> TcS WantedCtEvidence
newWantedEvVarNC CtLoc
loc CoHoleSet
emptyCoHoleSet PredType
cls_pred
                        ; chooseInstance dict_ev res
                        ; co_var <- newEvVar (unSwap swap (mkEqPred eq_rel) lhs rhs)
                        ; setEvBind (mkWantedEvBind co_var EvCanonical (mk_sc_sel cls tys dict_ev))
                        ; fillCoercionHole hole (CPH { cph_co    = maybeSymCo swap (mkCoVarCo co_var)
                                                     , cph_holes = emptyCoHoleSet })
                        ; stopWith ev "lookup_eq_in_qcis" }
                ClsInstResult
_ -> () -> TcS (StopOrContinue ())
forall a. a -> TcS (StopOrContinue a)
continueWith () }

       | Bool
otherwise
       = () -> SolverStage ()
forall a. a -> SolverStage a
nopStage ()

    mk_sc_sel :: Class -> [PredType] -> WantedCtEvidence -> EvTerm
mk_sc_sel Class
cls [PredType]
tys WantedCtEvidence
dict_ev
      | [TyVar
sc_id] <- Class -> [TyVar]
classSCSelIds Class
cls  -- Just one superclass for this
      = EvExpr -> EvTerm
EvExpr (TyVar -> EvExpr
forall b. TyVar -> Expr b
Var TyVar
sc_id EvExpr -> [PredType] -> EvExpr
forall b. Expr b -> [PredType] -> Expr b
`mkTyApps` [PredType]
tys EvExpr -> EvExpr -> EvExpr
forall b. Expr b -> Expr b -> Expr b
`App` TyVar -> EvExpr
forall b. TyVar -> Expr b
Var (WantedCtEvidence -> TyVar
wantedCtEvEvId WantedCtEvidence
dict_ev))
      | Bool
otherwise
      = String -> SDoc -> EvTerm
forall a. HasCallStack => String -> SDoc -> a
pprPanic String
"looup_eq_in_qcis" (WantedCtEvidence -> SDoc
forall a. Outputable a => a -> SDoc
ppr WantedCtEvidence
dict_ev)


{- Note [Instances in no-evidence implications]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In #15290 we had
  [G] forall p q. Coercible p q => Coercible (m p) (m q))   -- Quantified
  [W] forall <no-ev> a. m (Int, IntStateT m a)
                          ~R#
                        m (Int, StateT Int m a)

The Given is an ordinary quantified constraint; the Wanted is an implication
equality that arises from
  [W] (forall a. t1) ~R# (forall a. t2)

But because the (t1 ~R# t2) is solved "inside a type" (under that forall a)
we can't generate any term evidence.  So we can't actually use that
lovely quantified constraint.  Alas!

This test arranges to ignore the instance-based solution under these
(rare) circumstances.   It's sad, but I  really don't see what else we can do.
-}