{-# LANGUAGE CPP #-}
{-# LANGUAGE MultiWayIf #-}

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


import GHC.Prelude

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.InertSet
import GHC.Tc.Solver.Types( findFunEqsByTyCon )
import GHC.Tc.Types.Evidence
import GHC.Tc.Types.Constraint
import GHC.Tc.Types.Origin
import GHC.Tc.Utils.Unify
import GHC.Tc.Utils.TcType
import GHC.Tc.Instance.Family ( tcTopNormaliseNewTypeTF_maybe )
import GHC.Tc.Instance.FunDeps( FunDepEqn(..) )

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.Coercion.Axiom
import GHC.Core.Reduction
import GHC.Core.Unify( tcUnifyTyWithTFs )
import GHC.Core.FamInstEnv ( FamInstEnvs, FamInst(..), apartnessCheck
                           , lookupFamInstEnvByTyCon )
import GHC.Core

import GHC.Types.Var
import GHC.Types.Var.Env
import GHC.Types.Var.Set( anyVarSet )
import GHC.Types.Name.Reader
import GHC.Types.Basic

import GHC.Builtin.Types ( anyTypeOfKind )

import GHC.Utils.Outputable
import GHC.Utils.Panic
import GHC.Utils.Panic.Plain
import GHC.Utils.Misc
import GHC.Utils.Monad
import GHC.Utils.Constants( debugIsOn )

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 -> Type -> Type -> SolverStage Void
solveEquality CtEvidence
ev EqRel
eq_rel Type
ty1 Type
ty2
  = do { Pair Type
ty1' Type
ty2' <- CtEvidence -> EqRel -> Type -> Type -> SolverStage TypeEqn
zonkEqTypes CtEvidence
ev EqRel
eq_rel Type
ty1 Type
ty2
       ; let ev' :: CtEvidence
ev' | Bool
debugIsOn = HasDebugCallStack => CtEvidence -> Type -> CtEvidence
CtEvidence -> Type -> CtEvidence
setCtEvPredType CtEvidence
ev (Type -> CtEvidence) -> Type -> CtEvidence
forall a b. (a -> b) -> a -> b
$
                               Role -> Type -> Type -> Type
mkPrimEqPredRole (EqRel -> Role
eqRelRole EqRel
eq_rel) Type
ty1' Type
ty2'
                 | Bool
otherwise = CtEvidence
ev
                   -- ev': satisfy the precondition of can_eq_nc

       ; Either IrredCt EqCt
mb_canon <- CtEvidence
-> EqRel -> Type -> Type -> SolverStage (Either IrredCt EqCt)
canonicaliseEquality CtEvidence
ev' EqRel
eq_rel Type
ty1' Type
ty2'

       ; case Either IrredCt EqCt
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 ()
tryFunDeps  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)
       ; TcLevel
tc_lvl <- TcS TcLevel
getTcLevel
       ; (InertCans -> InertCans) -> TcS ()
updInertCans (TcLevel -> EqCt -> InertCans -> InertCans
addEqToCans TcLevel
tc_lvl EqCt
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 -> Type -> Type -> SolverStage TypeEqn
zonkEqTypes CtEvidence
ev EqRel
eq_rel Type
ty1 Type
ty2
  = TcS (StopOrContinue TypeEqn) -> SolverStage TypeEqn
forall a. TcS (StopOrContinue a) -> SolverStage a
Stage (TcS (StopOrContinue TypeEqn) -> SolverStage TypeEqn)
-> TcS (StopOrContinue TypeEqn) -> SolverStage TypeEqn
forall a b. (a -> b) -> a -> b
$ do { Either TypeEqn Type
res <- Type -> Type -> TcS (Either TypeEqn Type)
go Type
ty1 Type
ty2
               ; case Either TypeEqn Type
res of
                    Left TypeEqn
pair -> TypeEqn -> TcS (StopOrContinue TypeEqn)
forall a. a -> TcS (StopOrContinue a)
continueWith TypeEqn
pair
                    Right Type
ty  -> CtEvidence -> EqRel -> Type -> TcS (StopOrContinue TypeEqn)
forall a. CtEvidence -> EqRel -> Type -> TcS (StopOrContinue a)
canEqReflexive CtEvidence
ev EqRel
eq_rel Type
ty }
  where
    go :: TcType -> TcType -> TcS (Either (Pair TcType) TcType)
    go :: Type -> Type -> TcS (Either TypeEqn Type)
go (TyVarTy TyVar
tv1) (TyVarTy TyVar
tv2) = TyVar -> TyVar -> TcS (Either TypeEqn Type)
tyvar_tyvar TyVar
tv1 TyVar
tv2
    go (TyVarTy TyVar
tv1) Type
ty2           = SwapFlag -> TyVar -> Type -> TcS (Either TypeEqn Type)
tyvar SwapFlag
NotSwapped TyVar
tv1 Type
ty2
    go Type
ty1 (TyVarTy TyVar
tv2)           = SwapFlag -> TyVar -> Type -> TcS (Either TypeEqn Type)
tyvar SwapFlag
IsSwapped  TyVar
tv2 Type
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 Type
w1 Type
arg1 Type
res1) (FunTy FunTyFlag
af2 Type
w2 Type
arg2 Type
res2)
      | FunTyFlag
af1 FunTyFlag -> FunTyFlag -> Bool
forall a. Eq a => a -> a -> Bool
== FunTyFlag
af2
      , Type -> Type -> Bool
eqType Type
w1 Type
w2
      = do { Either TypeEqn Type
res_a <- Type -> Type -> TcS (Either TypeEqn Type)
go Type
arg1 Type
arg2
           ; Either TypeEqn Type
res_b <- Type -> Type -> TcS (Either TypeEqn Type)
go Type
res1 Type
res2
           ; Either TypeEqn Type -> TcS (Either TypeEqn Type)
forall a. a -> TcS a
forall (m :: * -> *) a. Monad m => a -> m a
return (Either TypeEqn Type -> TcS (Either TypeEqn Type))
-> Either TypeEqn Type -> TcS (Either TypeEqn Type)
forall a b. (a -> b) -> a -> b
$ (Type -> Type -> Type)
-> Either TypeEqn Type
-> Either TypeEqn Type
-> Either TypeEqn Type
forall a b c.
(a -> b -> c)
-> Either (Pair b) b -> Either (Pair a) a -> Either (Pair c) c
combine_rev (FunTyFlag -> Type -> Type -> Type -> Type
FunTy FunTyFlag
af1 Type
w1) Either TypeEqn Type
res_b Either TypeEqn Type
res_a }

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

    go Type
ty1 Type
ty2
      | Just (TyCon
tc1, [Type]
tys1) <- Type -> Maybe (TyCon, [Type])
splitTyConAppNoView_maybe Type
ty1
      , Just (TyCon
tc2, [Type]
tys2) <- Type -> Maybe (TyCon, [Type])
splitTyConAppNoView_maybe Type
ty2
      = if TyCon
tc1 TyCon -> TyCon -> Bool
forall a. Eq a => a -> a -> Bool
== TyCon
tc2 Bool -> Bool -> Bool
&& [Type]
tys1 [Type] -> [Type] -> Bool
forall a b. [a] -> [b] -> Bool
`equalLength` [Type]
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 -> [Type] -> [Type] -> TcS (Either TypeEqn Type)
tycon TyCon
tc1 [Type]
tys1 [Type]
tys2
        else Type -> Type -> TcS (Either TypeEqn Type)
forall {m :: * -> *} {a} {b}.
Monad m =>
a -> a -> m (Either (Pair a) b)
bale_out Type
ty1 Type
ty2

    go Type
ty1 Type
ty2
      | Just (Type
ty1a, Type
ty1b) <- Type -> Maybe (Type, Type)
tcSplitAppTyNoView_maybe Type
ty1
      , Just (Type
ty2a, Type
ty2b) <- Type -> Maybe (Type, Type)
tcSplitAppTyNoView_maybe Type
ty2
      = do { Either TypeEqn Type
res_a <- Type -> Type -> TcS (Either TypeEqn Type)
go Type
ty1a Type
ty2a
           ; Either TypeEqn Type
res_b <- Type -> Type -> TcS (Either TypeEqn Type)
go Type
ty1b Type
ty2b
           ; Either TypeEqn Type -> TcS (Either TypeEqn Type)
forall a. a -> TcS a
forall (m :: * -> *) a. Monad m => a -> m a
return (Either TypeEqn Type -> TcS (Either TypeEqn Type))
-> Either TypeEqn Type -> TcS (Either TypeEqn Type)
forall a b. (a -> b) -> a -> b
$ (Type -> Type -> Type)
-> Either TypeEqn Type
-> Either TypeEqn Type
-> Either TypeEqn Type
forall a b c.
(a -> b -> c)
-> Either (Pair b) b -> Either (Pair a) a -> Either (Pair c) c
combine_rev Type -> Type -> Type
mkAppTy Either TypeEqn Type
res_b Either TypeEqn Type
res_a }

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

    go Type
ty1 Type
ty2 = Type -> Type -> TcS (Either TypeEqn Type)
forall {m :: * -> *} {a} {b}.
Monad m =>
a -> a -> m (Either (Pair a) b)
bale_out Type
ty1 Type
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 -> Type -> TcS (Either TypeEqn Type)
tyvar SwapFlag
swapped TyVar
tv Type
ty
      = case TyVar -> TcTyVarDetails
tcTyVarDetails TyVar
tv of
          MetaTv { mtv_ref :: TcTyVarDetails -> IORef MetaDetails
mtv_ref = IORef MetaDetails
ref }
            -> do { MetaDetails
cts <- IORef MetaDetails -> TcS MetaDetails
forall a. TcRef a -> TcS a
readTcRef IORef MetaDetails
ref
                  ; case MetaDetails
cts of
                      MetaDetails
Flexi        -> TcS (Either TypeEqn Type)
give_up
                      Indirect Type
ty' -> do { TyVar -> Type -> TcS ()
forall {a} {a}. (Outputable a, Outputable a) => a -> a -> TcS ()
trace_indirect TyVar
tv Type
ty'
                                         ; SwapFlag
-> (Type -> Type -> TcS (Either TypeEqn Type))
-> Type
-> Type
-> TcS (Either TypeEqn Type)
forall a b. SwapFlag -> (a -> a -> b) -> a -> a -> b
unSwap SwapFlag
swapped Type -> Type -> TcS (Either TypeEqn Type)
go Type
ty' Type
ty } }
          TcTyVarDetails
_ -> TcS (Either TypeEqn Type)
give_up
      where
        give_up :: TcS (Either TypeEqn Type)
give_up = Either TypeEqn Type -> TcS (Either TypeEqn Type)
forall a. a -> TcS a
forall (m :: * -> *) a. Monad m => a -> m a
return (Either TypeEqn Type -> TcS (Either TypeEqn Type))
-> Either TypeEqn Type -> TcS (Either TypeEqn Type)
forall a b. (a -> b) -> a -> b
$ TypeEqn -> Either TypeEqn Type
forall a b. a -> Either a b
Left (TypeEqn -> Either TypeEqn Type) -> TypeEqn -> Either TypeEqn Type
forall a b. (a -> b) -> a -> b
$ SwapFlag -> (Type -> Type -> TypeEqn) -> Type -> Type -> TypeEqn
forall a b. SwapFlag -> (a -> a -> b) -> a -> a -> b
unSwap SwapFlag
swapped Type -> Type -> TypeEqn
forall a. a -> a -> Pair a
Pair (TyVar -> Type
mkTyVarTy TyVar
tv) Type
ty

    tyvar_tyvar :: TyVar -> TyVar -> TcS (Either TypeEqn Type)
tyvar_tyvar TyVar
tv1 TyVar
tv2
      | TyVar
tv1 TyVar -> TyVar -> Bool
forall a. Eq a => a -> a -> Bool
== TyVar
tv2 = Either TypeEqn Type -> TcS (Either TypeEqn Type)
forall a. a -> TcS a
forall (m :: * -> *) a. Monad m => a -> m a
return (Type -> Either TypeEqn Type
forall a b. b -> Either a b
Right (TyVar -> Type
mkTyVarTy TyVar
tv1))
      | Bool
otherwise  = do { (Type
ty1', Bool
progress1) <- TyVar -> TcS (Type, Bool)
quick_zonk TyVar
tv1
                        ; (Type
ty2', Bool
progress2) <- TyVar -> TcS (Type, Bool)
quick_zonk TyVar
tv2
                        ; if Bool
progress1 Bool -> Bool -> Bool
|| Bool
progress2
                          then Type -> Type -> TcS (Either TypeEqn Type)
go Type
ty1' Type
ty2'
                          else Either TypeEqn Type -> TcS (Either TypeEqn Type)
forall a. a -> TcS a
forall (m :: * -> *) a. Monad m => a -> m a
return (Either TypeEqn Type -> TcS (Either TypeEqn Type))
-> Either TypeEqn Type -> TcS (Either TypeEqn Type)
forall a b. (a -> b) -> a -> b
$ TypeEqn -> Either TypeEqn Type
forall a b. a -> Either a b
Left (Type -> Type -> TypeEqn
forall a. a -> a -> Pair a
Pair (TyVar -> Type
TyVarTy TyVar
tv1) (TyVar -> Type
TyVarTy TyVar
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 (Type, Bool)
quick_zonk TyVar
tv = case TyVar -> TcTyVarDetails
tcTyVarDetails TyVar
tv of
      MetaTv { mtv_ref :: TcTyVarDetails -> IORef MetaDetails
mtv_ref = IORef MetaDetails
ref }
        -> do { MetaDetails
cts <- IORef MetaDetails -> TcS MetaDetails
forall a. TcRef a -> TcS a
readTcRef IORef MetaDetails
ref
              ; case MetaDetails
cts of
                  MetaDetails
Flexi        -> (Type, Bool) -> TcS (Type, Bool)
forall a. a -> TcS a
forall (m :: * -> *) a. Monad m => a -> m a
return (TyVar -> Type
TyVarTy TyVar
tv, Bool
False)
                  Indirect Type
ty' -> do { TyVar -> Type -> TcS ()
forall {a} {a}. (Outputable a, Outputable a) => a -> a -> TcS ()
trace_indirect TyVar
tv Type
ty'
                                     ; (Type, Bool) -> TcS (Type, Bool)
forall a. a -> TcS a
forall (m :: * -> *) a. Monad m => a -> m a
return (Type
ty', Bool
True) } }
      TcTyVarDetails
_ -> (Type, Bool) -> TcS (Type, Bool)
forall a. a -> TcS a
forall (m :: * -> *) a. Monad m => a -> m a
return (TyVar -> Type
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 -> [Type] -> [Type] -> TcS (Either TypeEqn Type)
tycon TyCon
tc [Type]
tys1 [Type]
tys2
      = do { [Either TypeEqn Type]
results <- (Type -> Type -> TcS (Either TypeEqn Type))
-> [Type] -> [Type] -> TcS [Either TypeEqn Type]
forall (m :: * -> *) a b c.
Applicative m =>
(a -> b -> m c) -> [a] -> [b] -> m [c]
zipWithM Type -> Type -> TcS (Either TypeEqn Type)
go [Type]
tys1 [Type]
tys2
           ; Either TypeEqn Type -> TcS (Either TypeEqn Type)
forall a. a -> TcS a
forall (m :: * -> *) a. Monad m => a -> m a
return (Either TypeEqn Type -> TcS (Either TypeEqn Type))
-> Either TypeEqn Type -> TcS (Either TypeEqn Type)
forall a b. (a -> b) -> a -> b
$ case [Either TypeEqn Type] -> Either (Pair [Type]) [Type]
combine_results [Either TypeEqn Type]
results of
               Left Pair [Type]
tys  -> TypeEqn -> Either TypeEqn Type
forall a b. a -> Either a b
Left (TyCon -> [Type] -> Type
mkTyConApp TyCon
tc ([Type] -> Type) -> Pair [Type] -> TypeEqn
forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
<$> Pair [Type]
tys)
               Right [Type]
tys -> Type -> Either TypeEqn Type
forall a b. b -> Either a b
Right (TyCon -> [Type] -> Type
mkTyConApp TyCon
tc [Type]
tys) }

    combine_results :: [Either (Pair TcType) TcType]
                    -> Either (Pair [TcType]) [TcType]
    combine_results :: [Either TypeEqn Type] -> Either (Pair [Type]) [Type]
combine_results = (Pair [Type] -> Pair [Type])
-> ([Type] -> [Type])
-> Either (Pair [Type]) [Type]
-> Either (Pair [Type]) [Type]
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 (([Type] -> [Type]) -> Pair [Type] -> Pair [Type]
forall a b. (a -> b) -> Pair a -> Pair b
forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
fmap [Type] -> [Type]
forall a. [a] -> [a]
reverse) [Type] -> [Type]
forall a. [a] -> [a]
reverse (Either (Pair [Type]) [Type] -> Either (Pair [Type]) [Type])
-> ([Either TypeEqn Type] -> Either (Pair [Type]) [Type])
-> [Either TypeEqn Type]
-> Either (Pair [Type]) [Type]
forall b c a. (b -> c) -> (a -> b) -> a -> c
.
                      (Either (Pair [Type]) [Type]
 -> Either TypeEqn Type -> Either (Pair [Type]) [Type])
-> Either (Pair [Type]) [Type]
-> [Either TypeEqn Type]
-> Either (Pair [Type]) [Type]
forall b a. (b -> a -> b) -> b -> [a] -> b
forall (t :: * -> *) b a.
Foldable t =>
(b -> a -> b) -> b -> t a -> b
foldl' ((Type -> [Type] -> [Type])
-> Either (Pair [Type]) [Type]
-> Either TypeEqn Type
-> Either (Pair [Type]) [Type]
forall a b c.
(a -> b -> c)
-> Either (Pair b) b -> Either (Pair a) a -> Either (Pair c) c
combine_rev (:)) ([Type] -> Either (Pair [Type]) [Type]
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)


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

canonicaliseEquality
   :: CtEvidence -> EqRel
   -> Type -> Type     -- LHS and RHS
   -> SolverStage (Either IrredCt EqCt)
-- Precondition: in DEBUG mode, the `ctev_pred` of `ev` is (ps_ty1 ~# ps_ty2),
--               without zonking
-- This precondition is needed (only in DEBUG) to satisfy the assertions
--   in mkSelCo, called in canDecomposableTyConAppOK and canDecomposableFunTy

canonicaliseEquality :: CtEvidence
-> EqRel -> Type -> Type -> SolverStage (Either IrredCt EqCt)
canonicaliseEquality CtEvidence
ev EqRel
eq_rel Type
ty1 Type
ty2
  = 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, Type -> SDoc
forall a. Outputable a => a -> SDoc
ppr Type
ty1, Type -> SDoc
forall a. Outputable a => a -> SDoc
ppr Type
ty2 ]
               ; GlobalRdrEnv
rdr_env   <- TcS GlobalRdrEnv
getGlobalRdrEnvTcS
               ; (FamInstEnv, FamInstEnv)
fam_insts <- TcS (FamInstEnv, FamInstEnv)
getFamInstEnvs
               ; Bool
-> GlobalRdrEnv
-> (FamInstEnv, FamInstEnv)
-> CtEvidence
-> EqRel
-> Type
-> Type
-> Type
-> Type
-> TcS (StopOrContinue (Either IrredCt EqCt))
can_eq_nc Bool
False GlobalRdrEnv
rdr_env (FamInstEnv, FamInstEnv)
fam_insts CtEvidence
ev EqRel
eq_rel Type
ty1 Type
ty1 Type
ty2 Type
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 [Comparing nullary type synonyms] in GHC.Core.Type.
can_eq_nc :: Bool
-> GlobalRdrEnv
-> (FamInstEnv, FamInstEnv)
-> CtEvidence
-> EqRel
-> Type
-> Type
-> Type
-> Type
-> TcS (StopOrContinue (Either IrredCt EqCt))
can_eq_nc Bool
_flat GlobalRdrEnv
_rdr_env (FamInstEnv, FamInstEnv)
_envs CtEvidence
ev EqRel
eq_rel ty1 :: Type
ty1@(TyConApp TyCon
tc1 []) Type
_ps_ty1 (TyConApp TyCon
tc2 []) Type
_ps_ty2
  | TyCon
tc1 TyCon -> TyCon -> Bool
forall a. Eq a => a -> a -> Bool
== TyCon
tc2
  = CtEvidence
-> EqRel -> Type -> TcS (StopOrContinue (Either IrredCt EqCt))
forall a. CtEvidence -> EqRel -> Type -> TcS (StopOrContinue a)
canEqReflexive CtEvidence
ev EqRel
eq_rel Type
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 Type
ty1 Type
ps_ty1 Type
ty2 Type
ps_ty2
  | Just Type
ty1' <- Type -> Maybe Type
coreView Type
ty1 = Bool
-> GlobalRdrEnv
-> (FamInstEnv, FamInstEnv)
-> CtEvidence
-> EqRel
-> Type
-> Type
-> Type
-> Type
-> TcS (StopOrContinue (Either IrredCt EqCt))
can_eq_nc Bool
rewritten GlobalRdrEnv
rdr_env (FamInstEnv, FamInstEnv)
envs CtEvidence
ev EqRel
eq_rel Type
ty1' Type
ps_ty1 Type
ty2  Type
ps_ty2
  | Just Type
ty2' <- Type -> Maybe Type
coreView Type
ty2 = Bool
-> GlobalRdrEnv
-> (FamInstEnv, FamInstEnv)
-> CtEvidence
-> EqRel
-> Type
-> Type
-> Type
-> Type
-> TcS (StopOrContinue (Either IrredCt EqCt))
can_eq_nc Bool
rewritten GlobalRdrEnv
rdr_env (FamInstEnv, FamInstEnv)
envs CtEvidence
ev EqRel
eq_rel Type
ty1  Type
ps_ty1 Type
ty2' Type
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 Type
ty1 Type
_ Type
ty2 Type
_
  | Type
ty1 HasDebugCallStack => Type -> Type -> Bool
Type -> Type -> Bool
`tcEqType` Type
ty2
  = CtEvidence
-> EqRel -> Type -> TcS (StopOrContinue (Either IrredCt EqCt))
forall a. CtEvidence -> EqRel -> Type -> TcS (StopOrContinue a)
canEqReflexive CtEvidence
ev EqRel
ReprEq Type
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 Type
ty1 Type
ps_ty1 Type
ty2 Type
ps_ty2
  | EqRel
ReprEq <- EqRel
eq_rel
  , Just ((Bag GlobalRdrElt, TcCoercion), Type)
stuff1 <- (FamInstEnv, FamInstEnv)
-> GlobalRdrEnv
-> Type
-> Maybe ((Bag GlobalRdrElt, TcCoercion), Type)
tcTopNormaliseNewTypeTF_maybe (FamInstEnv, FamInstEnv)
envs GlobalRdrEnv
rdr_env Type
ty1
  = GlobalRdrEnv
-> (FamInstEnv, FamInstEnv)
-> CtEvidence
-> SwapFlag
-> Type
-> ((Bag GlobalRdrElt, TcCoercion), Type)
-> Type
-> Type
-> TcS (StopOrContinue (Either IrredCt EqCt))
can_eq_newtype_nc GlobalRdrEnv
rdr_env (FamInstEnv, FamInstEnv)
envs CtEvidence
ev SwapFlag
NotSwapped Type
ty1 ((Bag GlobalRdrElt, TcCoercion), Type)
stuff1 Type
ty2 Type
ps_ty2

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

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

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

-- Literals
can_eq_nc Bool
_rewritten GlobalRdrEnv
_rdr_env (FamInstEnv, FamInstEnv)
_envs CtEvidence
ev EqRel
eq_rel ty1 :: Type
ty1@(LitTy TyLit
l1) Type
_ (LitTy TyLit
l2) Type
_
 | TyLit
l1 TyLit -> TyLit -> Bool
forall a. Eq a => a -> a -> Bool
== TyLit
l2
  = do { CtEvidence -> Bool -> EvTerm -> TcS ()
setEvBindIfWanted CtEvidence
ev Bool
True (TcCoercion -> EvTerm
evCoercion (TcCoercion -> EvTerm) -> TcCoercion -> EvTerm
forall a b. (a -> b) -> a -> b
$ Role -> Type -> TcCoercion
mkReflCo (EqRel -> Role
eqRelRole EqRel
eq_rel) Type
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
           (FunTy { ft_mult :: Type -> Type
ft_mult = Type
am1, ft_af :: Type -> FunTyFlag
ft_af = FunTyFlag
af1, ft_arg :: Type -> Type
ft_arg = Type
ty1a, ft_res :: Type -> Type
ft_res = Type
ty1b }) Type
_ps_ty1
           (FunTy { ft_mult :: Type -> Type
ft_mult = Type
am2, ft_af :: Type -> FunTyFlag
ft_af = FunTyFlag
af2, ft_arg :: Type -> Type
ft_arg = Type
ty2a, ft_res :: Type -> Type
ft_res = Type
ty2b }) Type
_ps_ty2
  | FunTyFlag
af1 FunTyFlag -> FunTyFlag -> Bool
forall a. Eq a => a -> a -> Bool
== FunTyFlag
af2  -- See Note [Decomposing FunTy]
  = CtEvidence
-> EqRel
-> FunTyFlag
-> (Type, Type, Type)
-> (Type, Type, Type)
-> TcS (StopOrContinue (Either IrredCt EqCt))
forall a.
CtEvidence
-> EqRel
-> FunTyFlag
-> (Type, Type, Type)
-> (Type, Type, Type)
-> TcS (StopOrContinue a)
canDecomposableFunTy CtEvidence
ev EqRel
eq_rel FunTyFlag
af1 (Type
am1,Type
ty1a,Type
ty1b) (Type
am2,Type
ty2a,Type
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 Type
ty1 Type
_ Type
ty2 Type
_
  | Just (TyCon
tc1, [Type]
tys1) <- HasDebugCallStack => Type -> Maybe (TyCon, [Type])
Type -> Maybe (TyCon, [Type])
tcSplitTyConApp_maybe Type
ty1
  , Just (TyCon
tc2, [Type]
tys2) <- HasDebugCallStack => Type -> Maybe (TyCon, [Type])
Type -> Maybe (TyCon, [Type])
tcSplitTyConApp_maybe Type
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
-> TyCon
-> [Type]
-> TyCon
-> [Type]
-> TcS (StopOrContinue (Either IrredCt EqCt))
canTyConApp CtEvidence
ev EqRel
eq_rel Bool
both_generative TyCon
tc1 [Type]
tys1 TyCon
tc2 [Type]
tys2

can_eq_nc Bool
_rewritten GlobalRdrEnv
_rdr_env (FamInstEnv, FamInstEnv)
_envs CtEvidence
ev EqRel
eq_rel
           s1 :: Type
s1@ForAllTy{} Type
_
           s2 :: Type
s2@ForAllTy{} Type
_
  = CtEvidence
-> EqRel
-> Type
-> Type
-> TcS (StopOrContinue (Either IrredCt EqCt))
can_eq_nc_forall CtEvidence
ev EqRel
eq_rel Type
s1 Type
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 Type
ty1 Type
_ Type
ty2 Type
_
  | Just (Type
t1, Type
s1) <- Type -> Maybe (Type, Type)
tcSplitAppTy_maybe Type
ty1
  , Just (Type
t2, Type
s2) <- Type -> Maybe (Type, Type)
tcSplitAppTy_maybe Type
ty2
  = CtEvidence
-> Type
-> Type
-> Type
-> Type
-> TcS (StopOrContinue (Either IrredCt EqCt))
can_eq_app CtEvidence
ev Type
t1 Type
s1 Type
t2 Type
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 Type
_ Type
ps_ty1 Type
_ Type
ps_ty2
  = -- Rewrite the two types and try again
    do { (redn1 :: Reduction
redn1@(Reduction TcCoercion
_ Type
xi1), RewriterSet
rewriters1) <- CtEvidence -> Type -> TcS (Reduction, RewriterSet)
rewrite CtEvidence
ev Type
ps_ty1
       ; (redn2 :: Reduction
redn2@(Reduction TcCoercion
_ Type
xi2), RewriterSet
rewriters2) <- CtEvidence -> Type -> TcS (Reduction, RewriterSet)
rewrite CtEvidence
ev Type
ps_ty2
       ; CtEvidence
new_ev <- RewriterSet
-> CtEvidence
-> SwapFlag
-> Reduction
-> Reduction
-> TcS CtEvidence
rewriteEqEvidence (RewriterSet
rewriters1 RewriterSet -> RewriterSet -> RewriterSet
forall a. Semigroup a => a -> a -> a
S.<> RewriterSet
rewriters2) CtEvidence
ev SwapFlag
NotSwapped Reduction
redn1 Reduction
redn2
       ; String -> SDoc -> TcS ()
traceTcS String
"can_eq_nc: go round again" (CtEvidence -> SDoc
forall a. Outputable a => a -> SDoc
ppr CtEvidence
new_ev SDoc -> SDoc -> SDoc
forall doc. IsDoc doc => doc -> doc -> doc
$$ Type -> SDoc
forall a. Outputable a => a -> SDoc
ppr Type
xi1 SDoc -> SDoc -> SDoc
forall doc. IsDoc doc => doc -> doc -> doc
$$ Type -> SDoc
forall a. Outputable a => a -> SDoc
ppr Type
xi2)
       ; Bool
-> GlobalRdrEnv
-> (FamInstEnv, FamInstEnv)
-> CtEvidence
-> EqRel
-> Type
-> Type
-> Type
-> Type
-> TcS (StopOrContinue (Either IrredCt EqCt))
can_eq_nc Bool
True GlobalRdrEnv
rdr_env (FamInstEnv, FamInstEnv)
envs CtEvidence
new_ev EqRel
eq_rel Type
xi1 Type
xi1 Type
xi2 Type
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 Type
ty1 Type
ps_ty1 Type
ty2 Type
ps_ty2
  | Just CanEqLHS
can_eq_lhs1 <- Type -> Maybe CanEqLHS
canEqLHS_maybe Type
ty1
  = do { String -> SDoc -> TcS ()
traceTcS String
"can_eq1" (Type -> SDoc
forall a. Outputable a => a -> SDoc
ppr Type
ty1 SDoc -> SDoc -> SDoc
forall doc. IsDoc doc => doc -> doc -> doc
$$ Type -> SDoc
forall a. Outputable a => a -> SDoc
ppr Type
ty2)
       ; CtEvidence
-> EqRel
-> SwapFlag
-> CanEqLHS
-> Type
-> Type
-> Type
-> TcS (StopOrContinue (Either IrredCt EqCt))
canEqCanLHS CtEvidence
ev EqRel
eq_rel SwapFlag
NotSwapped CanEqLHS
can_eq_lhs1 Type
ps_ty1 Type
ty2 Type
ps_ty2 }

  | Just CanEqLHS
can_eq_lhs2 <- Type -> Maybe CanEqLHS
canEqLHS_maybe Type
ty2
  = do { String -> SDoc -> TcS ()
traceTcS String
"can_eq2" (Type -> SDoc
forall a. Outputable a => a -> SDoc
ppr Type
ty1 SDoc -> SDoc -> SDoc
forall doc. IsDoc doc => doc -> doc -> doc
$$ Type -> SDoc
forall a. Outputable a => a -> SDoc
ppr Type
ty2)
       ; CtEvidence
-> EqRel
-> SwapFlag
-> CanEqLHS
-> Type
-> Type
-> Type
-> TcS (StopOrContinue (Either IrredCt EqCt))
canEqCanLHS CtEvidence
ev EqRel
eq_rel SwapFlag
IsSwapped CanEqLHS
can_eq_lhs2 Type
ps_ty2 Type
ty1 Type
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 Type
_ Type
ps_ty1 Type
_ Type
ps_ty2
  = do { String -> SDoc -> TcS ()
traceTcS String
"can_eq_nc catch-all case" (Type -> SDoc
forall a. Outputable a => a -> SDoc
ppr Type
ps_ty1 SDoc -> SDoc -> SDoc
forall doc. IsDoc doc => doc -> doc -> doc
$$ Type -> SDoc
forall a. Outputable a => a -> SDoc
ppr Type
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))
-- (forall as. phi1) ~ (forall bs. phi2)
-- Check for length match of as, bs
-- Then build an implication constraint: forall as. phi1 ~ phi2[as/bs]
-- But remember also to unify the kinds of as and bs
--  (this is the 'go' loop), and actually substitute phi2[as |> cos / bs]
-- Remember also that we might have forall z (a:z). blah
--  so we must proceed one binder at a time (#13879)

can_eq_nc_forall :: CtEvidence
-> EqRel
-> Type
-> Type
-> TcS (StopOrContinue (Either IrredCt EqCt))
can_eq_nc_forall CtEvidence
ev EqRel
eq_rel Type
s1 Type
s2
 | CtWanted { ctev_loc :: CtEvidence -> CtLoc
ctev_loc = CtLoc
loc, ctev_dest :: CtEvidence -> TcEvDest
ctev_dest = TcEvDest
orig_dest, ctev_rewriters :: CtEvidence -> RewriterSet
ctev_rewriters = RewriterSet
rewriters } <- CtEvidence
ev
 = do { let free_tvs :: TyCoVarSet
free_tvs       = [Type] -> TyCoVarSet
tyCoVarsOfTypes [Type
s1,Type
s2]
            ([TyVarBinder]
bndrs1, Type
phi1) = Type -> ([TyVarBinder], Type)
tcSplitForAllTyVarBinders Type
s1
            ([TyVarBinder]
bndrs2, Type
phi2) = Type -> ([TyVarBinder], Type)
tcSplitForAllTyVarBinders Type
s2
            flags1 :: [ForAllTyFlag]
flags1 = [TyVarBinder] -> [ForAllTyFlag]
forall tv argf. [VarBndr tv argf] -> [argf]
binderFlags [TyVarBinder]
bndrs1
            flags2 :: [ForAllTyFlag]
flags2 = [TyVarBinder] -> [ForAllTyFlag]
forall tv argf. [VarBndr tv argf] -> [argf]
binderFlags [TyVarBinder]
bndrs2
      ; if 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" (SDoc -> TcS ()) -> SDoc -> TcS ()
forall a b. (a -> b) -> a -> b
$
                     [SDoc] -> SDoc
forall doc. IsDoc doc => [doc] -> doc
vcat [ Type -> SDoc
forall a. Outputable a => a -> SDoc
ppr Type
s1, Type -> SDoc
forall a. Outputable a => a -> SDoc
ppr Type
s2, [TyVarBinder] -> SDoc
forall a. Outputable a => a -> SDoc
ppr [TyVarBinder]
bndrs1, [TyVarBinder] -> SDoc
forall a. Outputable a => a -> SDoc
ppr [TyVarBinder]
bndrs2
                          , [ForAllTyFlag] -> SDoc
forall a. Outputable a => a -> SDoc
ppr [ForAllTyFlag]
flags1, [ForAllTyFlag] -> SDoc
forall a. Outputable a => a -> SDoc
ppr [ForAllTyFlag]
flags2 ]
                ; CtEvidence
-> Type -> Type -> TcS (StopOrContinue (Either IrredCt EqCt))
forall a.
CtEvidence
-> Type -> Type -> TcS (StopOrContinue (Either IrredCt a))
canEqHardFailure CtEvidence
ev Type
s1 Type
s2 }
        else
   do { String -> SDoc -> TcS ()
traceTcS String
"Creating implication for polytype equality" (SDoc -> TcS ()) -> SDoc -> TcS ()
forall a b. (a -> b) -> a -> b
$ CtEvidence -> SDoc
forall a. Outputable a => a -> SDoc
ppr CtEvidence
ev
      ; let empty_subst1 :: Subst
empty_subst1 = InScopeSet -> Subst
mkEmptySubst (InScopeSet -> Subst) -> InScopeSet -> Subst
forall a b. (a -> b) -> a -> b
$ TyCoVarSet -> InScopeSet
mkInScopeSet TyCoVarSet
free_tvs
      ; SkolemInfo
skol_info <- SkolemInfoAnon -> TcS SkolemInfo
forall (m :: * -> *). MonadIO m => SkolemInfoAnon -> m SkolemInfo
mkSkolemInfo (Type -> SkolemInfoAnon
UnifyForAllSkol Type
phi1)
      ; (Subst
subst1, [TyVar]
skol_tvs) <- SkolemInfo -> Subst -> [TyVar] -> TcS (Subst, [TyVar])
tcInstSkolTyVarsX SkolemInfo
skol_info Subst
empty_subst1 ([TyVar] -> TcS (Subst, [TyVar]))
-> [TyVar] -> TcS (Subst, [TyVar])
forall a b. (a -> b) -> a -> b
$
                              [TyVarBinder] -> [TyVar]
forall tv argf. [VarBndr tv argf] -> [tv]
binderVars [TyVarBinder]
bndrs1

      ; let phi1' :: Type
phi1' = HasDebugCallStack => Subst -> Type -> Type
Subst -> Type -> Type
substTy Subst
subst1 Type
phi1

            -- Unify the kinds, extend the substitution
            go :: [TcTyVar] -> Subst -> [TyVarBinder]
               -> TcS (TcCoercion, Cts)
            go :: [TyVar] -> Subst -> [TyVarBinder] -> TcS (TcCoercion, Cts)
go (TyVar
skol_tv:[TyVar]
skol_tvs) Subst
subst (TyVarBinder
bndr2:[TyVarBinder]
bndrs2)
              = do { let tv2 :: TyVar
tv2 = TyVarBinder -> TyVar
forall tv argf. VarBndr tv argf -> tv
binderVar TyVarBinder
bndr2
                   ; (TcCoercion
kind_co, Cts
wanteds1) <- CtLoc
-> RewriterSet -> Role -> Type -> Type -> TcS (TcCoercion, Cts)
unify CtLoc
loc RewriterSet
rewriters Role
Nominal (TyVar -> Type
tyVarKind TyVar
skol_tv)
                                                  (HasDebugCallStack => Subst -> Type -> Type
Subst -> Type -> Type
substTy Subst
subst (TyVar -> Type
tyVarKind TyVar
tv2))
                   ; let subst' :: Subst
subst' = Subst -> TyVar -> Type -> Subst
extendTvSubstAndInScope Subst
subst TyVar
tv2
                                       (Type -> TcCoercion -> Type
mkCastTy (TyVar -> Type
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"
                   ; (TcCoercion
co, Cts
wanteds2) <- [TyVar] -> Subst -> [TyVarBinder] -> TcS (TcCoercion, Cts)
go [TyVar]
skol_tvs Subst
subst' [TyVarBinder]
bndrs2
                   ; (TcCoercion, Cts) -> TcS (TcCoercion, Cts)
forall a. a -> TcS a
forall (m :: * -> *) a. Monad m => a -> m a
return ( TyVar -> TcCoercion -> TcCoercion -> TcCoercion
mkForAllCo TyVar
skol_tv TcCoercion
kind_co TcCoercion
co
                            , Cts
wanteds1 Cts -> Cts -> Cts
forall a. Bag a -> Bag a -> Bag a
`unionBags` Cts
wanteds2 ) }

            -- Done: unify phi1 ~ phi2
            go [] Subst
subst [TyVarBinder]
bndrs2
              = Bool -> TcS (TcCoercion, Cts) -> TcS (TcCoercion, Cts)
forall a. HasCallStack => Bool -> a -> a
assert ([TyVarBinder] -> Bool
forall a. [a] -> Bool
forall (t :: * -> *) a. Foldable t => t a -> Bool
null [TyVarBinder]
bndrs2) (TcS (TcCoercion, Cts) -> TcS (TcCoercion, Cts))
-> TcS (TcCoercion, Cts) -> TcS (TcCoercion, Cts)
forall a b. (a -> b) -> a -> b
$
                CtLoc
-> RewriterSet -> Role -> Type -> Type -> TcS (TcCoercion, Cts)
unify CtLoc
loc RewriterSet
rewriters (EqRel -> Role
eqRelRole EqRel
eq_rel) Type
phi1' (Subst -> Type -> Type
substTyUnchecked Subst
subst Type
phi2)

            go [TyVar]
_ Subst
_ [TyVarBinder]
_ = String -> TcS (TcCoercion, Cts)
forall a. HasCallStack => String -> a
panic String
"cna_eq_nc_forall"  -- case (s:ss) []

            empty_subst2 :: Subst
empty_subst2 = InScopeSet -> Subst
mkEmptySubst (Subst -> InScopeSet
getSubstInScope Subst
subst1)

      ; (TcLevel
lvl, (TcCoercion
all_co, Cts
wanteds)) <- SDoc -> TcS (TcCoercion, Cts) -> TcS (TcLevel, (TcCoercion, Cts))
forall a. SDoc -> TcS a -> TcS (TcLevel, a)
pushLevelNoWorkList (SkolemInfo -> SDoc
forall a. Outputable a => a -> SDoc
ppr SkolemInfo
skol_info) (TcS (TcCoercion, Cts) -> TcS (TcLevel, (TcCoercion, Cts)))
-> TcS (TcCoercion, Cts) -> TcS (TcLevel, (TcCoercion, Cts))
forall a b. (a -> b) -> a -> b
$
                                    [TyVar] -> Subst -> [TyVarBinder] -> TcS (TcCoercion, Cts)
go [TyVar]
skol_tvs Subst
empty_subst2 [TyVarBinder]
bndrs2
      ; TcLevel -> SkolemInfoAnon -> [TyVar] -> Cts -> TcS ()
emitTvImplicationTcS TcLevel
lvl (SkolemInfo -> SkolemInfoAnon
getSkolemInfo SkolemInfo
skol_info) [TyVar]
skol_tvs Cts
wanteds

      ; HasDebugCallStack => TcEvDest -> TcCoercion -> TcS ()
TcEvDest -> TcCoercion -> TcS ()
setWantedEq TcEvDest
orig_dest TcCoercion
all_co
      ; CtEvidence -> String -> TcS (StopOrContinue (Either IrredCt EqCt))
forall a. CtEvidence -> String -> TcS (StopOrContinue a)
stopWith CtEvidence
ev String
"Deferred polytype equality" } }

 | 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
$
        Type -> Type -> SDoc
pprEq Type
s1 Type
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
    unify :: CtLoc -> RewriterSet -> Role -> TcType -> TcType -> TcS (TcCoercion, Cts)
    -- This version returns the wanted constraint rather
    -- than putting it in the work list
    unify :: CtLoc
-> RewriterSet -> Role -> Type -> Type -> TcS (TcCoercion, Cts)
unify CtLoc
loc RewriterSet
rewriters Role
role Type
ty1 Type
ty2
      | Type
ty1 HasDebugCallStack => Type -> Type -> Bool
Type -> Type -> Bool
`tcEqType` Type
ty2
      = (TcCoercion, Cts) -> TcS (TcCoercion, Cts)
forall a. a -> TcS a
forall (m :: * -> *) a. Monad m => a -> m a
return (Role -> Type -> TcCoercion
mkReflCo Role
role Type
ty1, Cts
forall a. Bag a
emptyBag)
      | Bool
otherwise
      = do { (CtEvidence
wanted, TcCoercion
co) <- CtLoc
-> RewriterSet
-> Role
-> Type
-> Type
-> TcS (CtEvidence, TcCoercion)
newWantedEq CtLoc
loc RewriterSet
rewriters Role
role Type
ty1 Type
ty2
           ; (TcCoercion, Cts) -> TcS (TcCoercion, Cts)
forall a. a -> TcS a
forall (m :: * -> *) a. Monad m => a -> m a
return (TcCoercion
co, Ct -> Cts
forall a. a -> Bag a
unitBag (CtEvidence -> Ct
mkNonCanonical CtEvidence
wanted)) }


{- 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
-> Type
-> ((Bag GlobalRdrElt, TcCoercion), Type)
-> Type
-> Type
-> TcS (StopOrContinue (Either IrredCt EqCt))
can_eq_newtype_nc GlobalRdrEnv
rdr_env (FamInstEnv, FamInstEnv)
envs CtEvidence
ev SwapFlag
swapped Type
ty1 ((Bag GlobalRdrElt
gres, TcCoercion
co1), Type
ty1') Type
ty2 Type
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, Type -> SDoc
forall a. Outputable a => a -> SDoc
ppr Type
ty1', Type -> SDoc
forall a. Outputable a => a -> SDoc
ppr Type
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 -> Type -> TcS ()
checkReductionDepth CtLoc
loc Type
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 -> Type -> Reduction
mkReduction TcCoercion
co1 Type
ty1'

       ; CtEvidence
new_ev <- RewriterSet
-> CtEvidence
-> SwapFlag
-> Reduction
-> Reduction
-> TcS CtEvidence
rewriteEqEvidence RewriterSet
emptyRewriterSet CtEvidence
ev' SwapFlag
swapped
                     Reduction
redn1 (Role -> Type -> Reduction
mkReflRedn Role
Representational Type
ps_ty2)

       ; Bool
-> GlobalRdrEnv
-> (FamInstEnv, FamInstEnv)
-> CtEvidence
-> EqRel
-> Type
-> Type
-> Type
-> Type
-> TcS (StopOrContinue (Either IrredCt EqCt))
can_eq_nc Bool
False GlobalRdrEnv
rdr_env (FamInstEnv, FamInstEnv)
envs CtEvidence
new_ev EqRel
ReprEq Type
ty1' Type
ty1' Type
ty2 Type
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
-> Type
-> Type
-> Type
-> Type
-> TcS (StopOrContinue (Either IrredCt EqCt))
can_eq_app CtEvidence
ev Type
s1 Type
t1 Type
s2 Type
t2
  | CtWanted { ctev_dest :: CtEvidence -> 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
<+> Type -> SDoc
forall a. Outputable a => a -> SDoc
ppr Type
s1, String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"t1:" SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> Type -> SDoc
forall a. Outputable a => a -> SDoc
ppr Type
t1
                                     , String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"s2:" SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> Type -> SDoc
forall a. Outputable a => a -> SDoc
ppr Type
s2, String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"t2:" SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> Type -> SDoc
forall a. Outputable a => a -> SDoc
ppr Type
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 (Type -> Bool
isNextArgVisible Type
s1) ])
       ; (TcCoercion
co,Cts
_,[TyVar]
_) <- CtEvidence
-> Role
-> (UnifyEnv -> TcM TcCoercion)
-> TcS (TcCoercion, Cts, [TyVar])
forall a.
CtEvidence -> Role -> (UnifyEnv -> TcM a) -> TcS (a, Cts, [TyVar])
wrapUnifierTcS CtEvidence
ev Role
Nominal ((UnifyEnv -> TcM TcCoercion) -> TcS (TcCoercion, Cts, [TyVar]))
-> (UnifyEnv -> TcM TcCoercion) -> TcS (TcCoercion, Cts, [TyVar])
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 arg_env :: UnifyEnv
arg_env = UnifyEnv -> (CtLoc -> CtLoc) -> UnifyEnv
updUEnvLoc UnifyEnv
uenv (Bool -> Bool -> CtLoc -> CtLoc
adjustCtLoc (Type -> Bool
isNextArgVisible Type
s1) Bool
False)
               ; TcCoercion
co_t <- UnifyEnv -> Type -> Type -> TcM TcCoercion
uType UnifyEnv
arg_env Type
t1 Type
t2
               ; TcCoercion
co_s <- UnifyEnv -> Type -> Type -> TcM TcCoercion
uType UnifyEnv
uenv Type
s1 Type
s2
               ; TcCoercion -> TcM TcCoercion
forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
forall (m :: * -> *) a. Monad m => a -> m a
return (TcCoercion -> TcCoercion -> TcCoercion
mkAppCo TcCoercion
co_s TcCoercion
co_t) }
       ; HasDebugCallStack => TcEvDest -> TcCoercion -> TcS ()
TcEvDest -> TcCoercion -> TcS ()
setWantedEq TcEvDest
dest TcCoercion
co
       ; CtEvidence -> String -> TcS (StopOrContinue (Either IrredCt EqCt))
forall a. CtEvidence -> String -> TcS (StopOrContinue a)
stopWith CtEvidence
ev String
"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
  | Type
s1k Type -> Type -> Bool
`mismatches` Type
s2k
  = CtEvidence
-> Type -> Type -> TcS (StopOrContinue (Either IrredCt EqCt))
forall a.
CtEvidence
-> Type -> Type -> TcS (StopOrContinue (Either IrredCt a))
canEqHardFailure CtEvidence
ev (Type
s1 Type -> Type -> Type
`mkAppTy` Type
t1) (Type
s2 Type -> Type -> Type
`mkAppTy` Type
t2)

  | CtGiven { ctev_evar :: CtEvidence -> 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
       ; CtEvidence
evar_s <- CtLoc -> (Type, EvTerm) -> TcS CtEvidence
newGivenEvVar CtLoc
loc ( CtEvidence -> Type -> Type -> Type
mkTcEqPredLikeEv CtEvidence
ev Type
s1 Type
s2
                                     , TcCoercion -> EvTerm
evCoercion TcCoercion
co_s )
       ; CtEvidence
evar_t <- CtLoc -> (Type, EvTerm) -> TcS CtEvidence
newGivenEvVar CtLoc
loc ( CtEvidence -> Type -> Type -> Type
mkTcEqPredLikeEv CtEvidence
ev Type
t1 Type
t2
                                     , TcCoercion -> EvTerm
evCoercion TcCoercion
co_t )
       ; [CtEvidence] -> TcS ()
emitWorkNC [CtEvidence
evar_t]
       ; Ct -> TcS (StopOrContinue (Either IrredCt EqCt))
forall a. Ct -> TcS (StopOrContinue a)
startAgainWith (CtEvidence -> Ct
mkNonCanonical CtEvidence
evar_s) }

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

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

    Type
k1 mismatches :: Type -> Type -> Bool
`mismatches` Type
k2
      =  Type -> Bool
isForAllTy Type
k1 Bool -> Bool -> Bool
&& Bool -> Bool
not (Type -> Bool
isForAllTy Type
k2)
      Bool -> Bool -> Bool
|| Bool -> Bool
not (Type -> Bool
isForAllTy Type
k1) Bool -> Bool -> Bool
&& Type -> Bool
isForAllTy Type
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
-> Type
-> TcCoercion
-> Type
-> Type
-> TcS (StopOrContinue (Either IrredCt EqCt))
canEqCast Bool
rewritten GlobalRdrEnv
rdr_env (FamInstEnv, FamInstEnv)
envs CtEvidence
ev EqRel
eq_rel SwapFlag
swapped Type
ty1 TcCoercion
co1 Type
ty2 Type
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
                                           , Type -> SDoc
forall a. Outputable a => a -> SDoc
ppr Type
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
                                           , Type -> SDoc
forall a. Outputable a => a -> SDoc
ppr Type
ps_ty2 ])
       ; CtEvidence
new_ev <- RewriterSet
-> CtEvidence
-> SwapFlag
-> Reduction
-> Reduction
-> TcS CtEvidence
rewriteEqEvidence RewriterSet
emptyRewriterSet CtEvidence
ev SwapFlag
swapped
                      (Role -> Type -> TcCoercion -> Reduction
mkGReflLeftRedn Role
role Type
ty1 TcCoercion
co1)
                      (Role -> Type -> Reduction
mkReflRedn Role
role Type
ps_ty2)
       ; Bool
-> GlobalRdrEnv
-> (FamInstEnv, FamInstEnv)
-> CtEvidence
-> EqRel
-> Type
-> Type
-> Type
-> Type
-> TcS (StopOrContinue (Either IrredCt EqCt))
can_eq_nc Bool
rewritten GlobalRdrEnv
rdr_env (FamInstEnv, FamInstEnv)
envs CtEvidence
new_ev EqRel
eq_rel Type
ty1 Type
ty1 Type
ty2 Type
ps_ty2 }
  where
    role :: Role
role = EqRel -> Role
eqRelRole EqRel
eq_rel

------------------------
canTyConApp :: CtEvidence -> EqRel
            -> Bool  -- Both TyCons are generative
            -> TyCon -> [TcType]
            -> 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
-> TyCon
-> [Type]
-> TyCon
-> [Type]
-> TcS (StopOrContinue (Either IrredCt EqCt))
canTyConApp CtEvidence
ev EqRel
eq_rel Bool
both_generative TyCon
tc1 [Type]
tys1 TyCon
tc2 [Type]
tys2
  | TyCon
tc1 TyCon -> TyCon -> Bool
forall a. Eq a => a -> a -> Bool
== TyCon
tc2
  , [Type]
tys1 [Type] -> [Type] -> Bool
forall a b. [a] -> [b] -> Bool
`equalLength` [Type]
tys2
  = do { InertSet
inerts <- TcS InertSet
getInertSet
       ; if InertSet -> Bool
can_decompose InertSet
inerts
         then CtEvidence
-> EqRel
-> TyCon
-> [Type]
-> [Type]
-> TcS (StopOrContinue (Either IrredCt EqCt))
forall a.
CtEvidence
-> EqRel -> TyCon -> [Type] -> [Type] -> TcS (StopOrContinue a)
canDecomposableTyConAppOK CtEvidence
ev EqRel
eq_rel TyCon
tc1 [Type]
tys1 [Type]
tys2
         else CtEvidence
-> EqRel
-> Type
-> Type
-> TcS (StopOrContinue (Either IrredCt EqCt))
forall a.
CtEvidence
-> EqRel -> Type -> Type -> TcS (StopOrContinue (Either IrredCt a))
canEqSoftFailure CtEvidence
ev EqRel
eq_rel Type
ty1 Type
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 }

  | Bool
otherwise  -- Different TyCons
  = if Bool
both_generative -- See (TC2) and (TC3) in
                       -- Note [Canonicalising TyCon/TyCon equalities]
    then CtEvidence
-> Type -> Type -> TcS (StopOrContinue (Either IrredCt EqCt))
forall a.
CtEvidence
-> Type -> Type -> TcS (StopOrContinue (Either IrredCt a))
canEqHardFailure CtEvidence
ev Type
ty1 Type
ty2
    else CtEvidence
-> EqRel
-> Type
-> Type
-> TcS (StopOrContinue (Either IrredCt EqCt))
forall a.
CtEvidence
-> EqRel -> Type -> Type -> TcS (StopOrContinue (Either IrredCt a))
canEqSoftFailure CtEvidence
ev EqRel
eq_rel Type
ty1 Type
ty2
  where
    -- Reconstruct the types for error messages. This would do
    -- the wrong thing (from a pretty printing point of view)
    -- for functions, because we've lost the FunTyFlag; but
    -- in fact we never call canTyConApp on a saturated FunTyCon
    ty1 :: Type
ty1 = TyCon -> [Type] -> Type
mkTyConApp TyCon
tc1 [Type]
tys1
    ty2 :: Type
ty2 = TyCon -> [Type] -> Type
mkTyConApp TyCon
tc2 [Type]
tys2

     -- 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 wrapUnifierTcS 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: see Note [Decomposing type family applications]
  * 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 canEqLHSHetero.  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`.

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

* `extendWorkListEqs` and `selectWorkItem` together arrange that the
  list of constraints given to `extendWorkListEqs` 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 type family applications]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Supose we have
   [G/W]  (F ty1) ~r  (F ty2)
This is handled by the TyFamLHS/TyFamLHS case of canEqCanLHS2.

We never decompose to
   [G/W]  ty1 ~r' ty2

Instead

* For Givens we do nothing. Injective type families have no corresponding
  evidence of their injectivity, so we cannot decompose an
  injective-type-family Given.

* For Wanteds, for the Nominal role only, we emit new Wanteds rather like
  functional dependencies, for each injective argument position.

  E.g type family F a b   -- injective in first arg, but not second
      [W] (F s1 t1) ~N (F s2 t2)
  Emit new Wanteds
      [W] s1 ~N s2
  But retain the existing, unsolved constraint.

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): 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"?

  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.)

  The currently-implemented compromise is this:

    we decompose [W] N s ~R# N t unless there is a [G] N s' ~ N t'

  that is, a Given Irred 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 desribed 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 -> [TcType] -> [TcType]
                          -> TcS (StopOrContinue a)
-- Precondition: tys1 and tys2 are the same finite length, hence "OK"
canDecomposableTyConAppOK :: forall a.
CtEvidence
-> EqRel -> TyCon -> [Type] -> [Type] -> TcS (StopOrContinue a)
canDecomposableTyConAppOK CtEvidence
ev EqRel
eq_rel TyCon
tc [Type]
tys1 [Type]
tys2
  = Bool -> TcS (StopOrContinue a) -> TcS (StopOrContinue a)
forall a. HasCallStack => Bool -> a -> a
assert ([Type]
tys1 [Type] -> [Type] -> Bool
forall a b. [a] -> [b] -> Bool
`equalLength` [Type]
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
$$ [Type] -> SDoc
forall a. Outputable a => a -> SDoc
ppr [Type]
tys1 SDoc -> SDoc -> SDoc
forall doc. IsDoc doc => doc -> doc -> doc
$$ [Type] -> SDoc
forall a. Outputable a => a -> SDoc
ppr [Type]
tys2)
       ; case CtEvidence
ev of
           CtWanted { ctev_dest :: CtEvidence -> 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 { (TcCoercion
co, Cts
_, [TyVar]
_) <- CtEvidence
-> Role
-> (UnifyEnv -> TcM TcCoercion)
-> TcS (TcCoercion, Cts, [TyVar])
forall a.
CtEvidence -> Role -> (UnifyEnv -> TcM a) -> TcS (a, Cts, [TyVar])
wrapUnifierTcS CtEvidence
ev Role
role ((UnifyEnv -> TcM TcCoercion) -> TcS (TcCoercion, Cts, [TyVar]))
-> (UnifyEnv -> TcM TcCoercion) -> TcS (TcCoercion, Cts, [TyVar])
forall a b. (a -> b) -> a -> b
$ \UnifyEnv
uenv ->
                        do { [TcCoercion]
cos <- (CtLoc -> Role -> Type -> Type -> TcM TcCoercion)
-> [CtLoc]
-> [Role]
-> [Type]
-> [Type]
-> 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 -> Type -> Type -> TcM TcCoercion
u_arg UnifyEnv
uenv) [CtLoc]
new_locs [Role]
tc_roles [Type]
tys1 [Type]
tys2
                                    -- zipWith4M: see Note [Work-list ordering]
                                    -- in GHC.Tc.Solved.Equality
                           ; TcCoercion -> TcM TcCoercion
forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
forall (m :: * -> *) a. Monad m => a -> m a
return (HasDebugCallStack => Role -> TyCon -> [TcCoercion] -> TcCoercion
Role -> TyCon -> [TcCoercion] -> TcCoercion
mkTyConAppCo Role
role TyCon
tc [TcCoercion]
cos) }
                   ; HasDebugCallStack => TcEvDest -> TcCoercion -> TcS ()
TcEvDest -> TcCoercion -> TcS ()
setWantedEq TcEvDest
dest TcCoercion
co }

           CtGiven { ctev_evar :: CtEvidence -> TyVar
ctev_evar = TyVar
evar }
             | let ev_co :: TcCoercion
ev_co = TyVar -> TcCoercion
mkCoVarCo TyVar
evar
             -> CtLoc -> [(Role, Type, Type, TcCoercion)] -> TcS ()
emitNewGivens CtLoc
loc
                       [ (Role
r, Type
ty1, Type
ty2, HasDebugCallStack => CoSel -> TcCoercion -> TcCoercion
CoSel -> TcCoercion -> TcCoercion
mkSelCo (Int -> Role -> CoSel
SelTyCon Int
i Role
r) TcCoercion
ev_co)
                       | (Role
r, Type
ty1, Type
ty2, Int
i) <- [Role] -> [Type] -> [Type] -> [Int] -> [(Role, Type, Type, Int)]
forall a b c d. [a] -> [b] -> [c] -> [d] -> [(a, b, c, d)]
zip4 [Role]
tc_roles [Type]
tys1 [Type]
tys2 [Int
0..]
                       , Role
r Role -> Role -> Bool
forall a. Eq a => a -> a -> Bool
/= Role
Phantom
                       , Bool -> Bool
not (Type -> Bool
isCoercionTy Type
ty1) Bool -> Bool -> Bool
&& Bool -> Bool
not (Type -> Bool
isCoercionTy Type
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 -> Type -> Type -> TcM TcCoercion
u_arg UnifyEnv
uenv CtLoc
arg_loc Role
arg_role = UnifyEnv -> Type -> Type -> 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)   -- (multiplicity,arg,res)
                     -> (Type,Type,Type)   -- (multiplicity,arg,res)
                     -> TcS (StopOrContinue a)
canDecomposableFunTy :: forall a.
CtEvidence
-> EqRel
-> FunTyFlag
-> (Type, Type, Type)
-> (Type, Type, Type)
-> TcS (StopOrContinue a)
canDecomposableFunTy CtEvidence
ev EqRel
eq_rel FunTyFlag
af f1 :: (Type, Type, Type)
f1@(Type
m1,Type
a1,Type
r1) f2 :: (Type, Type, Type)
f2@(Type
m2,Type
a2,Type
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
$$ EqRel -> SDoc
forall a. Outputable a => a -> SDoc
ppr EqRel
eq_rel SDoc -> SDoc -> SDoc
forall doc. IsDoc doc => doc -> doc -> doc
$$ (Type, Type, Type) -> SDoc
forall a. Outputable a => a -> SDoc
ppr (Type, Type, Type)
f1 SDoc -> SDoc -> SDoc
forall doc. IsDoc doc => doc -> doc -> doc
$$ (Type, Type, Type) -> SDoc
forall a. Outputable a => a -> SDoc
ppr (Type, Type, Type)
f2)
       ; case CtEvidence
ev of
           CtWanted { ctev_dest :: CtEvidence -> TcEvDest
ctev_dest = TcEvDest
dest }
             -> do { (TcCoercion
co, Cts
_, [TyVar]
_) <- CtEvidence
-> Role
-> (UnifyEnv -> TcM TcCoercion)
-> TcS (TcCoercion, Cts, [TyVar])
forall a.
CtEvidence -> Role -> (UnifyEnv -> TcM a) -> TcS (a, Cts, [TyVar])
wrapUnifierTcS CtEvidence
ev Role
Nominal ((UnifyEnv -> TcM TcCoercion) -> TcS (TcCoercion, Cts, [TyVar]))
-> (UnifyEnv -> TcM TcCoercion) -> TcS (TcCoercion, Cts, [TyVar])
forall a b. (a -> b) -> a -> b
$ \ UnifyEnv
uenv ->
                        do { let mult_env :: UnifyEnv
mult_env = UnifyEnv
uenv UnifyEnv -> (CtLoc -> CtLoc) -> UnifyEnv
`updUEnvLoc` CtLoc -> CtLoc
toInvisibleLoc
                                                 UnifyEnv -> Role -> UnifyEnv
`setUEnvRole` Role -> FunSel -> Role
funRole Role
role FunSel
SelMult
                           ; TcCoercion
mult <- UnifyEnv -> Type -> Type -> TcM TcCoercion
uType UnifyEnv
mult_env Type
m1 Type
m2
                           ; TcCoercion
arg  <- UnifyEnv -> Type -> Type -> TcM TcCoercion
uType (UnifyEnv
uenv UnifyEnv -> Role -> UnifyEnv
`setUEnvRole` Role -> FunSel -> Role
funRole Role
role FunSel
SelArg) Type
a1 Type
a2
                           ; TcCoercion
res  <- UnifyEnv -> Type -> Type -> TcM TcCoercion
uType (UnifyEnv
uenv UnifyEnv -> Role -> UnifyEnv
`setUEnvRole` Role -> FunSel -> Role
funRole Role
role FunSel
SelRes) Type
r1 Type
r2
                           ; TcCoercion -> TcM TcCoercion
forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
forall (m :: * -> *) a. Monad m => a -> m a
return (Role
-> FunTyFlag
-> TcCoercion
-> TcCoercion
-> TcCoercion
-> TcCoercion
mkNakedFunCo Role
role FunTyFlag
af TcCoercion
mult TcCoercion
arg TcCoercion
res) }
                   ; HasDebugCallStack => TcEvDest -> TcCoercion -> TcS ()
TcEvDest -> TcCoercion -> TcS ()
setWantedEq TcEvDest
dest TcCoercion
co }

           CtGiven { ctev_evar :: CtEvidence -> TyVar
ctev_evar = TyVar
evar }
             | let ev_co :: TcCoercion
ev_co = TyVar -> TcCoercion
mkCoVarCo TyVar
evar
             -> CtLoc -> [(Role, Type, Type, TcCoercion)] -> TcS ()
emitNewGivens CtLoc
loc
                       [ (Role -> FunSel -> Role
funRole Role
role FunSel
fs, Type
ty1, Type
ty2, HasDebugCallStack => CoSel -> TcCoercion -> TcCoercion
CoSel -> TcCoercion -> TcCoercion
mkSelCo (FunSel -> CoSel
SelFun FunSel
fs) TcCoercion
ev_co)
                       | (FunSel
fs, Type
ty1, Type
ty2) <- [ (FunSel
SelMult, Type
m1, Type
m2)
                                           , (FunSel
SelArg,  Type
a1, Type
a2)
                                           , (FunSel
SelRes,  Type
r1, Type
r2)] ]

    ; 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 :: CtEvidence -> EqRel -> TcType -> TcType
                 -> TcS (StopOrContinue (Either IrredCt a))
canEqSoftFailure :: forall a.
CtEvidence
-> EqRel -> Type -> Type -> TcS (StopOrContinue (Either IrredCt a))
canEqSoftFailure CtEvidence
ev EqRel
NomEq Type
ty1 Type
ty2
  = CtEvidence
-> Type -> Type -> TcS (StopOrContinue (Either IrredCt a))
forall a.
CtEvidence
-> Type -> Type -> TcS (StopOrContinue (Either IrredCt a))
canEqHardFailure CtEvidence
ev Type
ty1 Type
ty2
canEqSoftFailure CtEvidence
ev EqRel
ReprEq Type
ty1 Type
ty2
  = do { (Reduction
redn1, RewriterSet
rewriters1) <- CtEvidence -> Type -> TcS (Reduction, RewriterSet)
rewrite CtEvidence
ev Type
ty1
       ; (Reduction
redn2, RewriterSet
rewriters2) <- CtEvidence -> Type -> TcS (Reduction, RewriterSet)
rewrite CtEvidence
ev Type
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
       ; String -> SDoc -> TcS ()
traceTcS String
"canEqSoftFailure with ReprEq" (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, Reduction -> SDoc
forall a. Outputable a => a -> SDoc
ppr Reduction
redn1, Reduction -> SDoc
forall a. Outputable a => a -> SDoc
ppr Reduction
redn2 ]
       ; CtEvidence
new_ev <- RewriterSet
-> CtEvidence
-> SwapFlag
-> Reduction
-> Reduction
-> TcS CtEvidence
rewriteEqEvidence (RewriterSet
rewriters1 RewriterSet -> RewriterSet -> RewriterSet
forall a. Semigroup a => a -> a -> a
S.<> RewriterSet
rewriters2) CtEvidence
ev SwapFlag
NotSwapped Reduction
redn1 Reduction
redn2
       ; CtIrredReason
-> CtEvidence -> TcS (StopOrContinue (Either IrredCt a))
forall a.
CtIrredReason
-> CtEvidence -> TcS (StopOrContinue (Either IrredCt a))
finishCanWithIrred CtIrredReason
ReprEqReason CtEvidence
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
-> Type -> Type -> TcS (StopOrContinue (Either IrredCt a))
canEqHardFailure CtEvidence
ev Type
ty1 Type
ty2
  = do { String -> SDoc -> TcS ()
traceTcS String
"canEqHardFailure" (Type -> SDoc
forall a. Outputable a => a -> SDoc
ppr Type
ty1 SDoc -> SDoc -> SDoc
forall doc. IsDoc doc => doc -> doc -> doc
$$ Type -> SDoc
forall a. Outputable a => a -> SDoc
ppr Type
ty2)
       ; (Reduction
redn1, RewriterSet
rewriters1) <- CtEvidence -> Type -> TcS (Reduction, RewriterSet)
rewriteForErrors CtEvidence
ev Type
ty1
       ; (Reduction
redn2, RewriterSet
rewriters2) <- CtEvidence -> Type -> TcS (Reduction, RewriterSet)
rewriteForErrors CtEvidence
ev Type
ty2
       ; CtEvidence
new_ev <- RewriterSet
-> CtEvidence
-> SwapFlag
-> Reduction
-> Reduction
-> TcS CtEvidence
rewriteEqEvidence (RewriterSet
rewriters1 RewriterSet -> RewriterSet -> RewriterSet
forall a. Semigroup a => a -> a -> a
S.<> RewriterSet
rewriters2) CtEvidence
ev SwapFlag
NotSwapped Reduction
redn1 Reduction
redn2
       ; CtIrredReason
-> CtEvidence -> TcS (StopOrContinue (Either IrredCt a))
forall a.
CtIrredReason
-> CtEvidence -> TcS (StopOrContinue (Either IrredCt a))
finishCanWithIrred CtIrredReason
ShapeMismatchReason CtEvidence
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
-> Type
-> Type
-> Type
-> TcS (StopOrContinue (Either IrredCt EqCt))
canEqCanLHS CtEvidence
ev EqRel
eq_rel SwapFlag
swapped CanEqLHS
lhs1 Type
ps_xi1 Type
xi2 Type
ps_xi2
  | Type
k1 HasDebugCallStack => Type -> Type -> Bool
Type -> Type -> Bool
`tcEqType` Type
k2
  = CtEvidence
-> EqRel
-> SwapFlag
-> CanEqLHS
-> Type
-> Type
-> Type
-> TcS (StopOrContinue (Either IrredCt EqCt))
canEqCanLHSHomo CtEvidence
ev EqRel
eq_rel SwapFlag
swapped CanEqLHS
lhs1 Type
ps_xi1 Type
xi2 Type
ps_xi2

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

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


{-
Note [Kind Equality Orientation]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
While in theory [W] x ~ y and [W] y ~ x ought to give us the same behaviour, in
practice it does not.  See Note [Fundeps with instances, and equality
orientation] 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
-> Type
-> Type
-> Type
-> Type
-> Type
-> TcS (StopOrContinue (Either IrredCt EqCt))
canEqCanLHSHetero CtEvidence
ev EqRel
eq_rel SwapFlag
swapped CanEqLHS
lhs1 Type
ps_xi1 Type
ki1 Type
xi2 Type
ps_xi2 Type
ki2
-- See Note [Equalities with incompatible kinds]
-- See Note [Kind Equality Orientation]

-- NB: preserve left-to-right orientation!! See wrinkle (W2) in
-- Note [Fundeps with instances, and equality orientation] in GHC.Tc.Solver.Dict
--    NotSwapped:
--        ev      :: (lhs1:ki1) ~r# (xi2:ki2)
--        kind_co :: k11 ~# ki2               -- Same orientiation as ev
--        type_ev :: lhs1 ~r# (xi2 |> sym kind_co)
--    Swapped
--        ev      :: (xi2:ki2) ~r# (lhs1:ki1)
--        kind_co :: ki2 ~# ki1               -- Same orientiation as ev
--        type_ev :: (xi2 |> kind_co) ~r# lhs1

  = do { (TcCoercion
kind_co, RewriterSet
rewriters, Bool
unifs_happened) <- TcS (TcCoercion, RewriterSet, Bool)
mk_kind_eq   -- :: ki1 ~N ki2
       ; if Bool
unifs_happened
              -- Unifications happened, so start again to do the zonking
              -- Otherwise we might put something in the inert set that isn't inert
         then Ct -> TcS (StopOrContinue (Either IrredCt EqCt))
forall a. Ct -> TcS (StopOrContinue a)
startAgainWith (CtEvidence -> Ct
mkNonCanonical CtEvidence
ev)
         else
    do { let lhs_redn :: Reduction
lhs_redn = Role -> Type -> Reduction
mkReflRedn Role
role Type
ps_xi1
             rhs_redn :: Reduction
rhs_redn = Role -> Type -> TcCoercion -> Reduction
mkGReflRightRedn Role
role Type
xi2 TcCoercion
mb_sym_kind_co
             mb_sym_kind_co :: TcCoercion
mb_sym_kind_co = case SwapFlag
swapped of
                                SwapFlag
NotSwapped -> TcCoercion -> TcCoercion
mkSymCo TcCoercion
kind_co
                                SwapFlag
IsSwapped  -> TcCoercion
kind_co

       ; 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 [ Type -> SDoc
forall a. Outputable a => a -> SDoc
ppr Type
ki1, String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"~#", Type -> SDoc
forall a. Outputable a => a -> SDoc
ppr Type
ki2 ])
       ; CtEvidence
type_ev <- RewriterSet
-> CtEvidence
-> SwapFlag
-> Reduction
-> Reduction
-> TcS CtEvidence
rewriteEqEvidence RewriterSet
rewriters CtEvidence
ev SwapFlag
swapped Reduction
lhs_redn Reduction
rhs_redn

       ; let new_xi2 :: Type
new_xi2 = Type -> TcCoercion -> Type
mkCastTy Type
ps_xi2 TcCoercion
mb_sym_kind_co
       ; CtEvidence
-> EqRel
-> SwapFlag
-> CanEqLHS
-> Type
-> Type
-> Type
-> TcS (StopOrContinue (Either IrredCt EqCt))
canEqCanLHSHomo CtEvidence
type_ev EqRel
eq_rel SwapFlag
NotSwapped CanEqLHS
lhs1 Type
ps_xi1 Type
new_xi2 Type
new_xi2 }}

  where
    mk_kind_eq :: TcS (CoercionN, RewriterSet, Bool)
    -- Returned kind_co has kind (k1 ~ k2) if NotSwapped, (k2 ~ k1) if Swapped
    -- Returned Bool = True if unifications happened, so we should retry
    mk_kind_eq :: TcS (TcCoercion, RewriterSet, Bool)
mk_kind_eq = case CtEvidence
ev of
      CtGiven { ctev_evar :: CtEvidence -> TyVar
ctev_evar = TyVar
evar }
        -> do { let kind_co :: TcCoercion
kind_co  = TcCoercion -> TcCoercion
mkKindCo (TyVar -> TcCoercion
mkCoVarCo TyVar
evar)
                    pred_ty :: Type
pred_ty  = SwapFlag -> (Type -> Type -> Type) -> Type -> Type -> Type
forall a b. SwapFlag -> (a -> a -> b) -> a -> a -> b
unSwap SwapFlag
swapped (Type -> Type -> Type -> Type
mkNomPrimEqPred Type
liftedTypeKind) Type
ki1 Type
ki2
                    kind_loc :: CtLoc
kind_loc = Type -> Type -> CtLoc -> CtLoc
mkKindEqLoc Type
xi1 Type
xi2 (CtEvidence -> CtLoc
ctev_loc CtEvidence
ev)
              ; CtEvidence
kind_ev <- CtLoc -> (Type, EvTerm) -> TcS CtEvidence
newGivenEvVar CtLoc
kind_loc (Type
pred_ty, TcCoercion -> EvTerm
evCoercion TcCoercion
kind_co)
              ; [CtEvidence] -> TcS ()
emitWorkNC [CtEvidence
kind_ev]
              ; (TcCoercion, RewriterSet, Bool)
-> TcS (TcCoercion, RewriterSet, Bool)
forall a. a -> TcS a
forall (m :: * -> *) a. Monad m => a -> m a
return (HasDebugCallStack => CtEvidence -> TcCoercion
CtEvidence -> TcCoercion
ctEvCoercion CtEvidence
kind_ev, RewriterSet
emptyRewriterSet, Bool
False) }

      CtWanted {}
        -> do { (TcCoercion
kind_co, Cts
cts, [TyVar]
unifs) <- CtEvidence
-> Role
-> (UnifyEnv -> TcM TcCoercion)
-> TcS (TcCoercion, Cts, [TyVar])
forall a.
CtEvidence -> Role -> (UnifyEnv -> TcM a) -> TcS (a, Cts, [TyVar])
wrapUnifierTcS CtEvidence
ev Role
Nominal ((UnifyEnv -> TcM TcCoercion) -> TcS (TcCoercion, Cts, [TyVar]))
-> (UnifyEnv -> TcM TcCoercion) -> TcS (TcCoercion, Cts, [TyVar])
forall a b. (a -> b) -> a -> b
$ \UnifyEnv
uenv ->
                                         let uenv' :: UnifyEnv
uenv' = UnifyEnv -> (CtLoc -> CtLoc) -> UnifyEnv
updUEnvLoc UnifyEnv
uenv (Type -> Type -> CtLoc -> CtLoc
mkKindEqLoc Type
xi1 Type
xi2)
                                         in SwapFlag
-> (Type -> Type -> TcM TcCoercion)
-> Type
-> Type
-> TcM TcCoercion
forall a b. SwapFlag -> (a -> a -> b) -> a -> a -> b
unSwap SwapFlag
swapped (UnifyEnv -> Type -> Type -> TcM TcCoercion
uType UnifyEnv
uenv') Type
ki1 Type
ki2
              ; (TcCoercion, RewriterSet, Bool)
-> TcS (TcCoercion, RewriterSet, Bool)
forall a. a -> TcS a
forall (m :: * -> *) a. Monad m => a -> m a
return (TcCoercion
kind_co, Cts -> RewriterSet
rewriterSetFromCts Cts
cts, Bool -> Bool
not ([TyVar] -> Bool
forall a. [a] -> Bool
forall (t :: * -> *) a. Foldable t => t a -> Bool
null [TyVar]
unifs)) }

    xi1 :: Type
xi1  = CanEqLHS -> Type
canEqLHSType CanEqLHS
lhs1
    role :: Role
role = EqRel -> Role
eqRelRole EqRel
eq_rel

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
-> Type
-> Type
-> Type
-> TcS (StopOrContinue (Either IrredCt EqCt))
canEqCanLHSHomo CtEvidence
ev EqRel
eq_rel SwapFlag
swapped CanEqLHS
lhs1 Type
ps_xi1 Type
xi2 Type
ps_xi2
  | (Type
xi2', MCoercion
mco) <- Type -> (Type, MCoercion)
split_cast_ty Type
xi2
  , Just CanEqLHS
lhs2 <- Type -> Maybe CanEqLHS
canEqLHS_maybe Type
xi2'
  = CtEvidence
-> EqRel
-> SwapFlag
-> CanEqLHS
-> Type
-> CanEqLHS
-> Type
-> MCoercion
-> TcS (StopOrContinue (Either IrredCt EqCt))
canEqCanLHS2 CtEvidence
ev EqRel
eq_rel SwapFlag
swapped CanEqLHS
lhs1 Type
ps_xi1 CanEqLHS
lhs2 (Type
ps_xi2 Type -> MCoercion -> Type
`mkCastTyMCo` MCoercion -> MCoercion
mkSymMCo MCoercion
mco) MCoercion
mco

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

  where
    split_cast_ty :: Type -> (Type, MCoercion)
split_cast_ty (CastTy Type
ty TcCoercion
co) = (Type
ty, TcCoercion -> MCoercion
MCo TcCoercion
co)
    split_cast_ty Type
other          = (Type
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
-> Type
-> CanEqLHS
-> Type
-> MCoercion
-> TcS (StopOrContinue (Either IrredCt EqCt))
canEqCanLHS2 CtEvidence
ev EqRel
eq_rel SwapFlag
swapped CanEqLHS
lhs1 Type
ps_xi1 CanEqLHS
lhs2 Type
ps_xi2 MCoercion
mco
  | CanEqLHS
lhs1 CanEqLHS -> CanEqLHS -> Bool
`eqCanEqLHS` CanEqLHS
lhs2
    -- It must be the case that mco is reflexive
  = CtEvidence
-> EqRel -> Type -> TcS (StopOrContinue (Either IrredCt EqCt))
forall a. CtEvidence -> EqRel -> Type -> TcS (StopOrContinue a)
canEqReflexive CtEvidence
ev EqRel
eq_rel Type
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 [Type]
fun_args1 <- CanEqLHS
lhs1
  , TyFamLHS TyCon
fun_tc2 [Type]
fun_args2 <- CanEqLHS
lhs2
  -- See Note [Decomposing type family applications]
  = 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)

       ; Bool
unifications_done <- CtEvidence
-> EqRel
-> TyCon
-> [Type]
-> TyCon
-> [Type]
-> MCoercion
-> TcS Bool
tryFamFamInjectivity CtEvidence
ev EqRel
eq_rel
                                   TyCon
fun_tc1 [Type]
fun_args1 TyCon
fun_tc2 [Type]
fun_args2 MCoercion
mco
       ; if Bool
unifications_done
         then -- Go round again, since the unifications affect lhs/rhs
              Ct -> TcS (StopOrContinue (Either IrredCt EqCt))
forall a. Ct -> TcS (StopOrContinue a)
startAgainWith (CtEvidence -> Ct
mkNonCanonical CtEvidence
ev)
         else
    do { TcLevel
tclvl <- TcS TcLevel
getTcLevel
       ; let tvs1 :: TyCoVarSet
tvs1 = [Type] -> TyCoVarSet
tyCoVarsOfTypes [Type]
fun_args1
             tvs2 :: TyCoVarSet
tvs2 = [Type] -> TyCoVarSet
tyCoVarsOfTypes [Type]
fun_args2

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

             -- See Note [Orienting TyFamLHS/TyFamLHS]
             meta_tv_lhs :: Bool
meta_tv_lhs = (TyVar -> Bool) -> TyCoVarSet -> Bool
anyVarSet (TcLevel -> TyVar -> Bool
isTouchableMetaTyVar TcLevel
tclvl) TyCoVarSet
tvs1
             meta_tv_rhs :: Bool
meta_tv_rhs = (TyVar -> Bool) -> TyCoVarSet -> Bool
anyVarSet (TcLevel -> TyVar -> Bool
isTouchableMetaTyVar TcLevel
tclvl) TyCoVarSet
tvs2
             swap_for_rewriting :: Bool
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 Bool
swap_for_rewriting Bool -> Bool -> Bool
|| (Bool
meta_tv_lhs Bool -> Bool -> Bool
forall a. Eq a => a -> a -> Bool
== Bool
meta_tv_rhs Bool -> Bool -> Bool
&& Bool
swap_for_size)
         then TcS (StopOrContinue (Either IrredCt EqCt))
finish_with_swapping
         else TcS (StopOrContinue (Either IrredCt EqCt))
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 :: Type
lhs1_ty  = CanEqLHS -> Type
canEqLHSType CanEqLHS
lhs1
    lhs2_ty :: Type
lhs2_ty  = CanEqLHS -> Type
canEqLHSType CanEqLHS
lhs2

    finish_without_swapping :: TcS (StopOrContinue (Either IrredCt EqCt))
finish_without_swapping
      = CtEvidence
-> EqRel
-> SwapFlag
-> CanEqLHS
-> Type
-> TcS (StopOrContinue (Either IrredCt EqCt))
canEqCanLHSFinish CtEvidence
ev EqRel
eq_rel SwapFlag
swapped CanEqLHS
lhs1 (Type
ps_xi2 Type -> MCoercion -> Type
`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 -> Type -> MCoercion -> Reduction
mkGReflRightMRedn Role
role Type
lhs1_ty MCoercion
sym_mco
                 lhs2_redn :: Reduction
lhs2_redn = Role -> Type -> MCoercion -> Reduction
mkGReflLeftMRedn  Role
role Type
lhs2_ty MCoercion
mco
           ; CtEvidence
new_ev <-RewriterSet
-> CtEvidence
-> SwapFlag
-> Reduction
-> Reduction
-> TcS CtEvidence
rewriteEqEvidence RewriterSet
emptyRewriterSet CtEvidence
ev SwapFlag
swapped Reduction
lhs1_redn Reduction
lhs2_redn
           ; CtEvidence
-> EqRel
-> SwapFlag
-> CanEqLHS
-> Type
-> TcS (StopOrContinue (Either IrredCt EqCt))
canEqCanLHSFinish CtEvidence
new_ev EqRel
eq_rel SwapFlag
IsSwapped CanEqLHS
lhs2 (Type
ps_xi1 Type -> MCoercion -> Type
`mkCastTyMCo` MCoercion
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
-> Type
-> TcS (StopOrContinue (Either IrredCt EqCt))
canEqCanLHSFinish CtEvidence
ev EqRel
eq_rel SwapFlag
swapped CanEqLHS
lhs Type
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
<+> Type -> SDoc
forall a. Outputable a => a -> SDoc
ppr Type
rhs ]

         -- Assertion: (TyEq:K) is already satisfied
       ; Bool -> TcS ()
forall (m :: * -> *). (HasCallStack, Applicative m) => Bool -> m ()
massert (CanEqLHS -> Type
canEqLHSKind CanEqLHS
lhs Type -> Type -> Bool
`eqType` HasDebugCallStack => Type -> Type
Type -> Type
typeKind Type
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
<+> Type -> SDoc
forall a. Outputable a => a -> SDoc
ppr Type
rhs ]

       ; CtEvidence
-> EqRel
-> SwapFlag
-> CanEqLHS
-> Type
-> TcS (StopOrContinue (Either IrredCt EqCt))
canEqCanLHSFinish_try_unification CtEvidence
ev EqRel
eq_rel SwapFlag
swapped CanEqLHS
lhs Type
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, [Type]
tc_args) <- HasDebugCallStack => Type -> Maybe (TyCon, [Type])
Type -> Maybe (TyCon, [Type])
splitTyConApp_maybe Type
rhs
      , Just DataCon
con <- TyCon -> Maybe DataCon
newTyConDataCon_maybe TyCon
tc
      -- #22310: only a problem if the newtype TyCon is saturated.
      , [Type]
tc_args [Type] -> 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 { GlobalRdrEnv
rdr_env <- TcS GlobalRdrEnv
getGlobalRdrEnvTcS
           ; let con_in_scope :: Bool
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)
           ; Bool -> TcS Bool
forall a. a -> TcS a
forall (m :: * -> *) a. Monad m => a -> m a
return (Bool -> TcS Bool) -> Bool -> TcS Bool
forall a b. (a -> b) -> a -> b
$ Bool -> Bool
not Bool
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
-> Type
-> TcS (StopOrContinue (Either IrredCt EqCt))
canEqCanLHSFinish_try_unification CtEvidence
ev EqRel
eq_rel SwapFlag
swapped CanEqLHS
lhs Type
rhs
  -- Try unification; for Wanted, Nominal equalities with a meta-tyvar on the LHS
  | CtEvidence -> Bool
isWanted CtEvidence
ev      -- See Note [Do not unify Givens]
  , EqRel
NomEq <- EqRel
eq_rel  -- See Note [Do not unify representational equalities]
  , TyVarLHS TyVar
tv <- CanEqLHS
lhs
  = do { TcLevel
given_eq_lvl <- TcS TcLevel
getInnermostGivenEqLevel
       ; if Bool -> Bool
not (TcLevel -> TyVar -> Type -> Bool
touchabilityAndShapeTest TcLevel
given_eq_lvl TyVar
tv Type
rhs)
         then if | Just CanEqLHS
can_rhs <- Type -> Maybe CanEqLHS
canTyFamEqLHS_maybe Type
rhs
                 -> CtEvidence
-> EqRel
-> SwapFlag
-> Type
-> CanEqLHS
-> TcS (StopOrContinue (Either IrredCt EqCt))
forall unused.
CtEvidence
-> EqRel
-> SwapFlag
-> Type
-> CanEqLHS
-> TcS (StopOrContinue (Either unused EqCt))
swapAndFinish CtEvidence
ev EqRel
eq_rel SwapFlag
swapped (TyVar -> Type
mkTyVarTy TyVar
tv) CanEqLHS
can_rhs
                    -- See Note [Orienting TyVarLHS/TyFamLHS]

                 | Bool
otherwise
                 -> CtEvidence
-> EqRel
-> SwapFlag
-> CanEqLHS
-> Type
-> TcS (StopOrContinue (Either IrredCt EqCt))
canEqCanLHSFinish_no_unification CtEvidence
ev EqRel
eq_rel SwapFlag
swapped CanEqLHS
lhs Type
rhs
         else

    -- We have a touchable unification variable on the left
    do { PuResult () Reduction
check_result <- CtEvidence -> TyVar -> Type -> TcS (PuResult () Reduction)
checkTouchableTyVarEq CtEvidence
ev TyVar
tv Type
rhs
       ; case PuResult () Reduction
check_result of {
            PuFail CheckTyEqResult
reason
              | Just CanEqLHS
can_rhs <- Type -> Maybe CanEqLHS
canTyFamEqLHS_maybe Type
rhs
              -> CtEvidence
-> EqRel
-> SwapFlag
-> Type
-> CanEqLHS
-> TcS (StopOrContinue (Either IrredCt EqCt))
forall unused.
CtEvidence
-> EqRel
-> SwapFlag
-> Type
-> CanEqLHS
-> TcS (StopOrContinue (Either unused EqCt))
swapAndFinish CtEvidence
ev EqRel
eq_rel SwapFlag
swapped (TyVar -> Type
mkTyVarTy TyVar
tv) CanEqLHS
can_rhs
                -- Swap back: see Note [Orienting TyVarLHS/TyFamLHS]

              | CheckTyEqResult
reason CheckTyEqResult -> CheckTyEqResult -> Bool
`cterHasOnlyProblems` CheckTyEqResult
do_not_prevent_rewriting
              -> CtEvidence
-> EqRel
-> SwapFlag
-> CanEqLHS
-> Type
-> TcS (StopOrContinue (Either IrredCt EqCt))
canEqCanLHSFinish_no_unification CtEvidence
ev EqRel
eq_rel SwapFlag
swapped CanEqLHS
lhs Type
rhs

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

            PuOK Bag ()
_ Reduction
rhs_redn ->

    -- Success: we can solve by unification
    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>
         CtEvidence
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 RewriterSet
-> CtEvidence
-> SwapFlag
-> Reduction
-> Reduction
-> TcS CtEvidence
rewriteEqEvidence RewriterSet
emptyRewriterSet CtEvidence
ev SwapFlag
swapped
                            (Role -> Type -> Reduction
mkReflRedn Role
Nominal (TyVar -> Type
mkTyVarTy TyVar
tv)) Reduction
rhs_redn

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

       ; String -> SDoc -> TcS ()
traceTcS String
"Sneaky unification:" (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
"Unifies:" SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> TyVar -> SDoc
forall a. Outputable a => a -> SDoc
ppr TyVar
tv 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
<+> Type -> SDoc
forall a. Outputable a => a -> SDoc
ppr Type
final_rhs,
               String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"Coercion:" SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> Type -> Type -> SDoc
pprEq Type
tv_ty Type
final_rhs,
               String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"Left Kind is:" SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> Type -> SDoc
forall a. Outputable a => a -> SDoc
ppr (HasDebugCallStack => Type -> Type
Type -> Type
typeKind Type
tv_ty),
               String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"Right Kind is:" SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> Type -> SDoc
forall a. Outputable a => a -> SDoc
ppr (HasDebugCallStack => Type -> Type
Type -> Type
typeKind Type
final_rhs) ]

       -- Update the unification variable itself
       ; TyVar -> Type -> TcS ()
unifyTyVar TyVar
tv Type
final_rhs

       -- Provide Refl evidence for the constraint
       -- Ignore 'swapped' because it's Refl!
       ; CtEvidence -> Bool -> EvTerm -> TcS ()
setEvBindIfWanted CtEvidence
new_ev Bool
True (EvTerm -> TcS ()) -> EvTerm -> TcS ()
forall a b. (a -> b) -> a -> b
$
         TcCoercion -> EvTerm
evCoercion (Type -> TcCoercion
mkNomReflCo Type
final_rhs)

       -- Kick out any constraints that can now be rewritten
       ; [TyVar] -> TcS ()
kickOutAfterUnification [TyVar
tv]

       ; StopOrContinue (Either IrredCt EqCt)
-> TcS (StopOrContinue (Either IrredCt EqCt))
forall a. a -> TcS a
forall (m :: * -> *) a. Monad m => a -> m a
return (CtEvidence -> SDoc -> StopOrContinue (Either IrredCt EqCt)
forall a. CtEvidence -> SDoc -> StopOrContinue a
Stop CtEvidence
new_ev (String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"Solved by unification")) }}}}

  -- Otherwise unification is off the table
  | Bool
otherwise
  = CtEvidence
-> EqRel
-> SwapFlag
-> CanEqLHS
-> Type
-> TcS (StopOrContinue (Either IrredCt EqCt))
canEqCanLHSFinish_no_unification CtEvidence
ev EqRel
eq_rel SwapFlag
swapped CanEqLHS
lhs Type
rhs

  where
    -- 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

---------------------------
-- Unification is off the table
canEqCanLHSFinish_no_unification :: CtEvidence
-> EqRel
-> SwapFlag
-> CanEqLHS
-> Type
-> TcS (StopOrContinue (Either IrredCt EqCt))
canEqCanLHSFinish_no_unification CtEvidence
ev EqRel
eq_rel SwapFlag
swapped CanEqLHS
lhs Type
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.
       ; PuResult () Reduction
check_result <- CtEvidence
-> EqRel -> CanEqLHS -> Type -> TcS (PuResult () Reduction)
checkTypeEq CtEvidence
ev EqRel
eq_rel CanEqLHS
lhs Type
rhs

       ; let lhs_ty :: Type
lhs_ty = CanEqLHS -> Type
canEqLHSType CanEqLHS
lhs
       ; case PuResult () Reduction
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
-> CtEvidence
-> EqRel
-> SwapFlag
-> CanEqLHS
-> Type
-> TcS (StopOrContinue (Either IrredCt EqCt))
forall unused.
CheckTyEqResult
-> CtEvidence
-> EqRel
-> SwapFlag
-> CanEqLHS
-> Type
-> TcS (StopOrContinue (Either IrredCt unused))
tryIrredInstead CheckTyEqResult
reason CtEvidence
ev EqRel
eq_rel SwapFlag
swapped CanEqLHS
lhs Type
rhs

            PuOK Bag ()
_ Reduction
rhs_redn
              -> do { CtEvidence
new_ev <- RewriterSet
-> CtEvidence
-> SwapFlag
-> Reduction
-> Reduction
-> TcS CtEvidence
rewriteEqEvidence RewriterSet
emptyRewriterSet CtEvidence
ev SwapFlag
swapped
                                   (Role -> Type -> Reduction
mkReflRedn (EqRel -> Role
eqRelRole EqRel
eq_rel) Type
lhs_ty)
                                   Reduction
rhs_redn

                    -- 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
                    ; Either IrredCt EqCt -> TcS (StopOrContinue (Either IrredCt EqCt))
forall a. a -> TcS (StopOrContinue a)
continueWith (Either IrredCt EqCt -> TcS (StopOrContinue (Either IrredCt EqCt)))
-> Either IrredCt EqCt
-> TcS (StopOrContinue (Either IrredCt EqCt))
forall a b. (a -> b) -> a -> b
$ EqCt -> Either IrredCt EqCt
forall a b. b -> Either a b
Right (EqCt -> Either IrredCt EqCt) -> EqCt -> Either IrredCt EqCt
forall a b. (a -> b) -> a -> b
$
                      EqCt { eq_ev :: CtEvidence
eq_ev  = CtEvidence
new_ev, eq_eq_rel :: EqRel
eq_eq_rel = EqRel
eq_rel
                           , eq_lhs :: CanEqLHS
eq_lhs = CanEqLHS
lhs
                           , eq_rhs :: Type
eq_rhs = Reduction -> Type
reductionReducedType Reduction
rhs_redn } } }

----------------------
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
-> Type
-> CanEqLHS
-> TcS (StopOrContinue (Either unused EqCt))
swapAndFinish CtEvidence
ev EqRel
eq_rel SwapFlag
swapped Type
lhs_ty CanEqLHS
can_rhs
  = do { let role :: Role
role = EqRel -> Role
eqRelRole EqRel
eq_rel
       ; CtEvidence
new_ev <- RewriterSet
-> CtEvidence
-> SwapFlag
-> Reduction
-> Reduction
-> TcS CtEvidence
rewriteEqEvidence RewriterSet
emptyRewriterSet CtEvidence
ev (SwapFlag -> SwapFlag
flipSwap SwapFlag
swapped)
                       (Role -> Type -> Reduction
mkReflRedn Role
role (CanEqLHS -> Type
canEqLHSType CanEqLHS
can_rhs))
                       (Role -> Type -> Reduction
mkReflRedn Role
role Type
lhs_ty)
       ; Either unused EqCt -> TcS (StopOrContinue (Either unused EqCt))
forall a. a -> TcS (StopOrContinue a)
continueWith (Either unused EqCt -> TcS (StopOrContinue (Either unused EqCt)))
-> Either unused EqCt -> TcS (StopOrContinue (Either unused EqCt))
forall a b. (a -> b) -> a -> b
$ EqCt -> Either unused EqCt
forall a b. b -> Either a b
Right (EqCt -> Either unused EqCt) -> EqCt -> Either unused EqCt
forall a b. (a -> b) -> a -> b
$
         EqCt { eq_ev :: CtEvidence
eq_ev  = CtEvidence
new_ev, eq_eq_rel :: EqRel
eq_eq_rel = EqRel
eq_rel
              , eq_lhs :: CanEqLHS
eq_lhs = CanEqLHS
can_rhs, eq_rhs :: Type
eq_rhs = Type
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
-> Type
-> TcS (StopOrContinue (Either IrredCt unused))
tryIrredInstead CheckTyEqResult
reason CtEvidence
ev EqRel
eq_rel SwapFlag
swapped CanEqLHS
lhs Type
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
$$ CanEqLHS -> SDoc
forall a. Outputable a => a -> SDoc
ppr CanEqLHS
lhs SDoc -> SDoc -> SDoc
forall doc. IsDoc doc => doc -> doc -> doc
$$ Type -> SDoc
forall a. Outputable a => a -> SDoc
ppr Type
rhs)
       ; let role :: Role
role = EqRel -> Role
eqRelRole EqRel
eq_rel
       ; CtEvidence
new_ev <- RewriterSet
-> CtEvidence
-> SwapFlag
-> Reduction
-> Reduction
-> TcS CtEvidence
rewriteEqEvidence RewriterSet
emptyRewriterSet CtEvidence
ev SwapFlag
swapped
                       (Role -> Type -> Reduction
mkReflRedn Role
role (CanEqLHS -> Type
canEqLHSType CanEqLHS
lhs))
                       (Role -> Type -> Reduction
mkReflRedn Role
role Type
rhs)
       ; CtIrredReason
-> CtEvidence -> TcS (StopOrContinue (Either IrredCt unused))
forall a.
CtIrredReason
-> CtEvidence -> TcS (StopOrContinue (Either IrredCt a))
finishCanWithIrred (CheckTyEqResult -> CtIrredReason
NonCanonicalReason CheckTyEqResult
reason) CtEvidence
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
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 -> Type -> TcS (StopOrContinue a)
canEqReflexive CtEvidence
ev EqRel
eq_rel Type
ty
  = do { CtEvidence -> Bool -> EvTerm -> TcS ()
setEvBindIfWanted CtEvidence
ev Bool
True (EvTerm -> TcS ()) -> EvTerm -> TcS ()
forall a b. (a -> b) -> a -> b
$
         TcCoercion -> EvTerm
evCoercion (Role -> Type -> TcCoercion
mkReflCo (EqRel -> Role
eqRelRole EqRel
eq_rel) Type
ty)
       ; CtEvidence -> String -> TcS (StopOrContinue a)
forall a. CtEvidence -> String -> TcS (StopOrContinue a)
stopWith CtEvidence
ev String
"Solved by reflexivity" }

{- Note [Equalities with incompatible 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] (tv :: k1) ~ (rhs :: k2)

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

  [X] co :: k1 ~ k2
  [X] (tv :: k1) ~ ((rhs |> sym co) :: k1)

Wrinkles:

(EIK1) When X is W, the new type-level wanted is effectively rewritten by the
     kind-level one. We thus include the kind-level wanted in the RewriterSet
     for the type-level one. See Note [Wanteds rewrite Wanteds] 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   : a ~ (b |> kw)
        [W] kw : Type ~ (Type->Type)

     But we do /not/ want to regard `w` as canonical, and use it for rewriting
     other constraints: `kw` is insoluble, and replacing something of kind
     `Type` with something of kind `Type->Type` (even wrapped in an insouluble
     cast) does not help, and doing so turns out to lead to much worse error
     messages.  (In particular, if 'a' is a unification variable, we might
     unify, losing the tracking info that it depends on solving `kw`.)

     Conclusion: if a RHS contains a corecion hole arising from fixing a hetero-kinded
     equality, treat the equality (`w` in this case) as non-canonical, so that
       * It will not be used for unification
       * It will not be used for rewriting
     Instead, it lands in the inert_irreds in the inert set, awaiting solution of
     that `kw`.

     (EIK2a) We must later indeed unify if/when the kind-level wanted, `kw` gets
     solved. This is done in kickOutAfterFillingCoercionHole, which kicks out
     all equalities whose RHS mentions the filled-in coercion hole.  Note that
     it looks for type family equalities, too, because of the use of unifyTest
     in canEqTyVarFunEq.

     (EIK2b) What if the RHS mentions /other/ coercion holes?  How can that happen?  The
     main way is like this. Assume F :: forall k. k -> Type
        [W] kw : k  ~ Type
        [W] w  : a ~ F k t
     We can rewrite `w` with `kw` like this:
        [W] w' : 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 tests JuanLopez only typechecks if we do.  So we'd like to treat
     this kind of equality as canonical.

     Hence the ch_hetero_kind field in CoercionHole: it is True of constraints
     created by `canEqCanLHSHetero` to fix up hetero-kinded equalities; and False otherwise:

     * An equality constraint is non-canonical if it mentions a hetero-kind
       CoercionHole on the RHS.  See the `hasCoercionHoleCo` test in GHC.Tc.Utils.checkCo.

     * Hetero-kind CoercionHoles are created when the parent's CtOrigin is
       KindEqOrigin: see GHC.Tc.Utils.TcMType.newCoercionHole and friends.  We
       set this origin, via `mkKindLoc`, in `mk_kind_eq` in `canEqCanLHSHetero`.

(EIK3) Suppose we have [W] (a :: k1) ~ (rhs :: k2). We duly follow the
     algorithm detailed here, producing [W] co :: k1 ~ k2, and adding
     [W] (a :: k1) ~ ((rhs |> sym co) :: k1) to the irreducibles. Some time
     later, we solve co, and fill in co's coercion hole. This kicks out
     the irreducible as described in (2).

     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 (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. This is
     implemented in the cast case of can_eq_nc.

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]d. 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 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 by `break_wanted`
in `GHC.Tc.Solver.Monad.checkTouchableTyVarEq`.

Why TauTvs? See [Why TauTvs] below.

Critically, we emit the two new constraints (the last two above)
directly instead of calling wrapUnifierTcS. (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
checkTouchableTyVarEq, 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 [Flattening type-family applications when matching instances] 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 :: RewriterSet        -- New rewriters
                                        -- See GHC.Tc.Types.Constraint
                                        -- Note [Wanteds rewrite Wanteds]
                  -> CtEvidence         -- Old evidence :: olhs ~ orhs (not swapped)
                                        --              or orhs ~ olhs (swapped)
                  -> SwapFlag
                  -> Reduction          -- lhs_co :: olhs ~ nlhs
                  -> Reduction          -- rhs_co :: orhs ~ nrhs
                  -> 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 :: RewriterSet
-> CtEvidence
-> SwapFlag
-> Reduction
-> Reduction
-> TcS CtEvidence
rewriteEqEvidence RewriterSet
new_rewriters CtEvidence
old_ev SwapFlag
swapped (Reduction TcCoercion
lhs_co Type
nlhs) (Reduction TcCoercion
rhs_co Type
nrhs)
  | 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 -> Type -> CtEvidence
CtEvidence -> Type -> CtEvidence
setCtEvPredType CtEvidence
old_ev Type
new_pred)

  | CtGiven { ctev_evar :: CtEvidence -> 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
                                  TcCoercion -> TcCoercion -> TcCoercion
`mkTransCo` SwapFlag -> TcCoercion -> TcCoercion
maybeSymCo SwapFlag
swapped (TyVar -> TcCoercion
mkCoVarCo TyVar
old_evar)
                                  TcCoercion -> TcCoercion -> TcCoercion
`mkTransCo` TcCoercion
rhs_co)
       ; CtLoc -> (Type, EvTerm) -> TcS CtEvidence
newGivenEvVar CtLoc
loc (Type
new_pred, EvTerm
new_tm) }

  | CtWanted { ctev_dest :: CtEvidence -> TcEvDest
ctev_dest = TcEvDest
dest
             , ctev_rewriters :: CtEvidence -> RewriterSet
ctev_rewriters = RewriterSet
rewriters } <- CtEvidence
old_ev
  , let rewriters' :: RewriterSet
rewriters' = RewriterSet
rewriters RewriterSet -> RewriterSet -> RewriterSet
forall a. Semigroup a => a -> a -> a
S.<> RewriterSet
new_rewriters
  = do { (CtEvidence
new_ev, TcCoercion
hole_co) <- CtLoc
-> RewriterSet
-> Role
-> Type
-> Type
-> TcS (CtEvidence, TcCoercion)
newWantedEq CtLoc
loc RewriterSet
rewriters' (CtEvidence -> Role
ctEvRole CtEvidence
old_ev) Type
nlhs Type
nrhs
       ; let co :: TcCoercion
co = SwapFlag -> TcCoercion -> TcCoercion
maybeSymCo SwapFlag
swapped (TcCoercion -> TcCoercion) -> TcCoercion -> TcCoercion
forall a b. (a -> b) -> a -> b
$
                  TcCoercion
lhs_co TcCoercion -> TcCoercion -> TcCoercion
`mkTransCo` TcCoercion
hole_co TcCoercion -> TcCoercion -> TcCoercion
`mkTransCo` TcCoercion -> TcCoercion
mkSymCo TcCoercion
rhs_co
       ; HasDebugCallStack => TcEvDest -> TcCoercion -> TcS ()
TcEvDest -> TcCoercion -> TcS ()
setWantedEq TcEvDest
dest TcCoercion
co
       ; String -> SDoc -> TcS ()
traceTcS String
"rewriteEqEvidence" ([SDoc] -> SDoc
forall doc. IsDoc doc => [doc] -> doc
vcat [ CtEvidence -> SDoc
forall a. Outputable a => a -> SDoc
ppr CtEvidence
old_ev
                                            , Type -> SDoc
forall a. Outputable a => a -> SDoc
ppr Type
nlhs
                                            , Type -> SDoc
forall a. Outputable a => a -> SDoc
ppr Type
nrhs
                                            , TcCoercion -> SDoc
forall a. Outputable a => a -> SDoc
ppr TcCoercion
co
                                            , RewriterSet -> SDoc
forall a. Outputable a => a -> SDoc
ppr RewriterSet
new_rewriters ])
       ; CtEvidence -> TcS CtEvidence
forall a. a -> TcS a
forall (m :: * -> *) a. Monad m => a -> m a
return CtEvidence
new_ev }

#if __GLASGOW_HASKELL__ <= 810
  | otherwise
  = panic "rewriteEvidence"
#endif
  where
    new_pred :: Type
new_pred = CtEvidence -> Type -> Type -> Type
mkTcEqPredLikeEv CtEvidence
old_ev Type
nlhs Type
nrhs
    loc :: CtLoc
loc      = CtEvidence -> CtLoc
ctEvLoc CtEvidence
old_ev

{-
**********************************************************************
*                                                                    *
                   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:

* 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.

* 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.

* 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_visible 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.
-}

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 ()
forall a. TcS (StopOrContinue a) -> SolverStage a
Stage (TcS (StopOrContinue ()) -> SolverStage ())
-> TcS (StopOrContinue ()) -> SolverStage ()
forall a b. (a -> b) -> a -> b
$
    do { InertCans
inerts <- TcS InertCans
getInertCans
       ; if | Just (CtEvidence
ev_i, SwapFlag
swapped) <- InertCans -> EqCt -> Maybe (CtEvidence, SwapFlag)
inertsCanDischarge InertCans
inerts EqCt
work_item
            -> do { CtEvidence -> Bool -> EvTerm -> TcS ()
setEvBindIfWanted CtEvidence
ev Bool
True (EvTerm -> TcS ()) -> EvTerm -> TcS ()
forall a b. (a -> b) -> a -> b
$
                    TcCoercion -> EvTerm
evCoercion (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)
                                              (CtEvidence -> Role
ctEvRole CtEvidence
ev_i)
                                              (HasDebugCallStack => CtEvidence -> TcCoercion
CtEvidence -> TcCoercion
ctEvCoercion CtEvidence
ev_i))
                  ; CtEvidence -> String -> TcS (StopOrContinue ())
forall a. CtEvidence -> String -> TcS (StopOrContinue a)
stopWith CtEvidence
ev String
"Solved from inert" }

            | Bool
otherwise
            -> () -> TcS (StopOrContinue ())
forall a. a -> TcS (StopOrContinue a)
continueWith () }

inertsCanDischarge :: InertCans -> EqCt
                   -> Maybe ( CtEvidence  -- The evidence for the inert
                            , SwapFlag )  -- Whether we need mkSymCo
inertsCanDischarge :: InertCans -> EqCt -> Maybe (CtEvidence, SwapFlag)
inertsCanDischarge InertCans
inerts (EqCt { eq_lhs :: EqCt -> CanEqLHS
eq_lhs = CanEqLHS
lhs_w, eq_rhs :: EqCt -> Type
eq_rhs = Type
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 -> Type
eq_rhs = Type
rhs_i
                                , eq_eq_rel :: EqCt -> EqRel
eq_eq_rel = EqRel
eq_rel }
                                  <- InertCans -> CanEqLHS -> [EqCt]
findEq InertCans
inerts CanEqLHS
lhs_w
                         , Type
rhs_i HasDebugCallStack => Type -> Type -> Bool
Type -> Type -> Bool
`tcEqType` Type
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 <- Type -> Maybe CanEqLHS
canEqLHS_maybe Type
rhs_w
  , (CtEvidence
ev_i : [CtEvidence]
_) <- [ CtEvidence
ev_i | EqCt { eq_ev :: EqCt -> CtEvidence
eq_ev = CtEvidence
ev_i, eq_rhs :: EqCt -> Type
eq_rhs = Type
rhs_i
                                , eq_eq_rel :: EqCt -> EqRel
eq_eq_rel = EqRel
eq_rel }
                             <- InertCans -> CanEqLHS -> [EqCt]
findEq InertCans
inerts CanEqLHS
rhs_lhs
                         , Type
rhs_i HasDebugCallStack => Type -> Type -> Bool
Type -> Type -> Bool
`tcEqType` CanEqLHS -> Type
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)

    inert_beats_wanted :: CtEvidence -> EqRel -> Bool
inert_beats_wanted CtEvidence
ev_i EqRel
eq_rel
      = -- eqCanRewriteFR:        see second bullet of Note [Combining equalities]
        -- strictly_more_visible: see last bullet of Note [Combining equalities]
        CtFlavourRole
fr_i CtFlavourRole -> CtFlavourRole -> Bool
`eqCanRewriteFR` CtFlavourRole
fr_w
        Bool -> Bool -> Bool
&& Bool -> Bool
not ((CtLoc
loc_w CtLoc -> CtLoc -> Bool
`strictly_more_visible` CtEvidence -> CtLoc
ctEvLoc 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 Note [Combining equalities], final bullet
    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)

inertsCanDischarge 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 [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 [The Unification Level Flag] in GHC.Tc.Solver.Monad

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 irred :: IrredCt
irred@(IrredCt { ir_ev :: IrredCt -> CtEvidence
ir_ev = CtEvidence
ev })
  | EqPred EqRel
eq_rel Type
t1 Type
t2 <- Type -> Pred
classifyPredType (CtEvidence -> Type
ctEvPred CtEvidence
ev)
  = Ct -> EqRel -> Type -> Type -> SolverStage ()
lookup_eq_in_qcis (IrredCt -> Ct
CIrredCan IrredCt
irred) EqRel
eq_rel Type
t1 Type
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 work_item :: EqCt
work_item@(EqCt { eq_lhs :: EqCt -> CanEqLHS
eq_lhs = CanEqLHS
lhs, eq_rhs :: EqCt -> Type
eq_rhs = Type
rhs, eq_eq_rel :: EqCt -> EqRel
eq_eq_rel = EqRel
eq_rel })
  = Ct -> EqRel -> Type -> Type -> SolverStage ()
lookup_eq_in_qcis (EqCt -> Ct
CEqCan EqCt
work_item) EqRel
eq_rel (CanEqLHS -> Type
canEqLHSType CanEqLHS
lhs) Type
rhs

--------------------
lookup_eq_in_qcis :: Ct -> 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 :: Ct -> EqRel -> Type -> Type -> SolverStage ()
lookup_eq_in_qcis Ct
work_ct EqRel
eq_rel Type
lhs Type
rhs
  = 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 { EvBindsVar
ev_binds_var <- TcS EvBindsVar
getTcEvBindsVar
       ; InertCans
ics <- TcS InertCans
getInertCans
       ; if CtEvidence -> Bool
isWanted CtEvidence
ev                       -- Never look up Givens in quantified constraints
         Bool -> Bool -> Bool
&& Bool -> Bool
not ([QCInst] -> Bool
forall a. [a] -> Bool
forall (t :: * -> *) a. Foldable t => t a -> Bool
null (InertCans -> [QCInst]
inert_insts InertCans
ics))      -- Shortcut common case
         Bool -> Bool -> Bool
&& Bool -> Bool
not (EvBindsVar -> Bool
isCoEvBindsVar EvBindsVar
ev_binds_var) -- See Note [Instances in no-evidence implications]
         then TcS (StopOrContinue ())
try_for_qci
         else () -> TcS (StopOrContinue ())
forall a. a -> TcS (StopOrContinue a)
continueWith () }
  where
    ev :: CtEvidence
ev  = Ct -> CtEvidence
ctEvidence Ct
work_ct
    loc :: CtLoc
loc = CtEvidence -> CtLoc
ctEvLoc CtEvidence
ev
    role :: Role
role = EqRel -> Role
eqRelRole EqRel
eq_rel

    try_for_qci :: TcS (StopOrContinue ())
try_for_qci  -- First try looking for (lhs ~ rhs)
       | Just (Class
cls, [Type]
tys) <- EqRel -> Type -> Type -> Maybe (Class, [Type])
boxEqPred EqRel
eq_rel Type
lhs Type
rhs
       = do { ClsInstResult
res <- Type -> CtLoc -> TcS ClsInstResult
matchLocalInst (Class -> [Type] -> Type
mkClassPred Class
cls [Type]
tys) CtLoc
loc
            ; String -> SDoc -> TcS ()
traceTcS String
"lookup_irred_in_qcis:1" (Type -> SDoc
forall a. Outputable a => a -> SDoc
ppr (Class -> [Type] -> Type
mkClassPred Class
cls [Type]
tys))
            ; case ClsInstResult
res of
                OneInst { cir_mk_ev :: ClsInstResult -> [EvExpr] -> EvTerm
cir_mk_ev = [EvExpr] -> EvTerm
mk_ev }
                  -> CtEvidence -> ClsInstResult -> TcS (StopOrContinue ())
forall a. CtEvidence -> ClsInstResult -> TcS (StopOrContinue a)
chooseInstance CtEvidence
ev (ClsInstResult
res { cir_mk_ev = mk_eq_ev cls tys mk_ev })
                ClsInstResult
_ -> TcS (StopOrContinue ())
try_swapping }
       | Bool
otherwise
       = () -> TcS (StopOrContinue ())
forall a. a -> TcS (StopOrContinue a)
continueWith ()

    try_swapping :: TcS (StopOrContinue ())
try_swapping  -- Now try looking for (rhs ~ lhs)  (see #23333)
       | Just (Class
cls, [Type]
tys) <- EqRel -> Type -> Type -> Maybe (Class, [Type])
boxEqPred EqRel
eq_rel Type
rhs Type
lhs
       = do { ClsInstResult
res <- Type -> CtLoc -> TcS ClsInstResult
matchLocalInst (Class -> [Type] -> Type
mkClassPred Class
cls [Type]
tys) CtLoc
loc
            ; String -> SDoc -> TcS ()
traceTcS String
"lookup_irred_in_qcis:2" (Type -> SDoc
forall a. Outputable a => a -> SDoc
ppr (Class -> [Type] -> Type
mkClassPred Class
cls [Type]
tys))
            ; case ClsInstResult
res of
                OneInst { cir_mk_ev :: ClsInstResult -> [EvExpr] -> EvTerm
cir_mk_ev = [EvExpr] -> EvTerm
mk_ev }
                  -> do { CtEvidence
ev' <- RewriterSet
-> CtEvidence
-> SwapFlag
-> Reduction
-> Reduction
-> TcS CtEvidence
rewriteEqEvidence RewriterSet
emptyRewriterSet CtEvidence
ev SwapFlag
IsSwapped
                                      (Role -> Type -> Reduction
mkReflRedn Role
role Type
rhs) (Role -> Type -> Reduction
mkReflRedn Role
role Type
lhs)
                        ; CtEvidence -> ClsInstResult -> TcS (StopOrContinue ())
forall a. CtEvidence -> ClsInstResult -> TcS (StopOrContinue a)
chooseInstance CtEvidence
ev' (ClsInstResult
res { cir_mk_ev = mk_eq_ev cls tys mk_ev }) }
                ClsInstResult
_ -> do { String -> SDoc -> TcS ()
traceTcS String
"lookup_irred_in_qcis:3" (Ct -> SDoc
forall a. Outputable a => a -> SDoc
ppr Ct
work_ct)
                        ; () -> TcS (StopOrContinue ())
forall a. a -> TcS (StopOrContinue a)
continueWith () }}
       | Bool
otherwise
       = () -> TcS (StopOrContinue ())
forall a. a -> TcS (StopOrContinue a)
continueWith ()

    mk_eq_ev :: Class -> [Type] -> ([EvExpr] -> EvTerm) -> [EvExpr] -> EvTerm
mk_eq_ev Class
cls [Type]
tys [EvExpr] -> EvTerm
mk_ev [EvExpr]
evs
      | TyVar
sc_id : [TyVar]
rest <- Class -> [TyVar]
classSCSelIds Class
cls  -- Just one superclass for this
      = Bool -> EvTerm -> EvTerm
forall a. HasCallStack => Bool -> a -> a
assert ([TyVar] -> Bool
forall a. [a] -> Bool
forall (t :: * -> *) a. Foldable t => t a -> Bool
null [TyVar]
rest) (EvTerm -> EvTerm) -> EvTerm -> EvTerm
forall a b. (a -> b) -> a -> b
$ case ([EvExpr] -> EvTerm
mk_ev [EvExpr]
evs) of
          EvExpr EvExpr
e -> EvExpr -> EvTerm
EvExpr (TyVar -> EvExpr
forall b. TyVar -> Expr b
Var TyVar
sc_id EvExpr -> [Type] -> EvExpr
forall b. Expr b -> [Type] -> Expr b
`mkTyApps` [Type]
tys EvExpr -> EvExpr -> EvExpr
forall b. Expr b -> Expr b -> Expr b
`App` EvExpr
e)
          EvTerm
ev       -> String -> SDoc -> EvTerm
forall a. HasCallStack => String -> SDoc -> a
pprPanic String
"mk_eq_ev" (EvTerm -> SDoc
forall a. Outputable a => a -> SDoc
ppr EvTerm
ev)
      | Bool
otherwise = String -> SDoc -> EvTerm
forall a. HasCallStack => String -> SDoc -> a
pprPanic String
"finishEqCt" (Ct -> SDoc
forall a. Outputable a => a -> SDoc
ppr Ct
work_ct)

{- 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.
-}


{-
**********************************************************************
*                                                                    *
    Functional dependencies for type families
*                                                                    *
**********************************************************************

Note [Reverse order of fundep equations]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider this scenario (from dependent/should_fail/T13135_simple):

  type Sig :: Type -> Type
  data Sig a = SigFun a (Sig a)

  type SmartFun :: forall (t :: Type). Sig t -> Type
  type family SmartFun sig = r | r -> sig where
    SmartFun @Type (SigFun @Type a sig) = a -> SmartFun @Type sig

  [W] SmartFun @kappa sigma ~ (Int -> Bool)

The injectivity of SmartFun allows us to produce two new equalities:

  [W] w1 :: Type ~ kappa
  [W] w2 :: SigFun @Type Int beta ~ sigma

for some fresh (beta :: SigType). The second Wanted here is actually
heterogeneous: the LHS has type Sig Type while the RHS has type Sig kappa.
Of course, if we solve the first wanted first, the second becomes homogeneous.

When looking for injectivity-inspired equalities, we work left-to-right,
producing the two equalities in the order written above. However, these
equalities are then passed into wrapUnifierTcS, which will fail, adding these
to the work list. However, crucially, the work list operates like a *stack*.
So, because we add w1 and then w2, we process w2 first. This is silly: solving
w1 would unlock w2. So we make sure to add equalities to the work
list in left-to-right order, which requires a few key calls to 'reverse'.

This treatment is also used for class-based functional dependencies, although
we do not have a program yet known to exhibit a loop there. It just seems
like the right thing to do.

When this was originally conceived, it was necessary to avoid a loop in T13135.
That loop is now avoided by continuing with the kind equality (not the type
equality) in canEqCanLHSHetero (see Note [Equalities with incompatible kinds]).
However, the idea of working left-to-right still seems worthwhile, and so the calls
to 'reverse' remain.

Note [Improvement orientation]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
See also Note [Fundeps with instances, and equality orientation], which describes
the Exact Same Problem, with the same solution, but for functional dependencies.

A very delicate point is the orientation of equalities
arising from injectivity improvement (#12522).  Suppose we have
  type family F x = t | t -> x
  type instance F (a, Int) = (Int, G a)
where G is injective; and wanted constraints

  [W] TF (alpha, beta) ~ fuv
  [W] fuv ~ (Int, <some type>)

The injectivity will give rise to constraints

  [W] gamma1 ~ alpha
  [W] Int ~ beta

The fresh unification variable gamma1 comes from the fact that we
can only do "partial improvement" here; see Section 5.2 of
"Injective type families for Haskell" (HS'15).

Now, it's very important to orient the equations this way round,
so that the fresh unification variable will be eliminated in
favour of alpha.  If we instead had
   [W] alpha ~ gamma1
then we would unify alpha := gamma1; and kick out the wanted
constraint.  But when we grough it back in, it'd look like
   [W] TF (gamma1, beta) ~ fuv
and exactly the same thing would happen again!  Infinite loop.

This all seems fragile, and it might seem more robust to avoid
introducing gamma1 in the first place, in the case where the
actual argument (alpha, beta) partly matches the improvement
template.  But that's a bit tricky, esp when we remember that the
kinds much match too; so it's easier to let the normal machinery
handle it.  Instead we are careful to orient the new
equality with the template on the left.  Delicate, but it works.

-}

tryFamFamInjectivity :: CtEvidence -> EqRel
                     -> TyCon -> [TcType] -> TyCon -> [TcType] -> MCoercion
                     -> TcS Bool  -- True <=> some unification happened
tryFamFamInjectivity :: CtEvidence
-> EqRel
-> TyCon
-> [Type]
-> TyCon
-> [Type]
-> MCoercion
-> TcS Bool
tryFamFamInjectivity CtEvidence
ev EqRel
eq_rel TyCon
fun_tc1 [Type]
fun_args1 TyCon
fun_tc2 [Type]
fun_args2 MCoercion
mco
  | EqRel
ReprEq <- EqRel
eq_rel
  = Bool -> TcS Bool
forall a. a -> TcS a
forall (m :: * -> *) a. Monad m => a -> m a
return Bool
False   -- Injectivity applies only for Nominal equalities
  | TyCon
fun_tc1 TyCon -> TyCon -> Bool
forall a. Eq a => a -> a -> Bool
/= TyCon
fun_tc2
  = Bool -> TcS Bool
forall a. a -> TcS a
forall (m :: * -> *) a. Monad m => a -> m a
return Bool
False   -- If the families don't match, stop.
  | CtEvidence -> Bool
isGiven CtEvidence
ev
  = Bool -> TcS Bool
forall a. a -> TcS a
forall (m :: * -> *) a. Monad m => a -> m a
return Bool
False   -- See Note [No Given/Given fundeps] in GHC.Tc.Solver.Dict

  -- So this is a [W] (F tys1 ~N# F tys2)

  -- Is F an injective type family
  | Injective [Bool]
inj <- TyCon -> Injectivity
tyConInjectivityInfo TyCon
fun_tc1
  = CtEvidence -> Role -> (UnifyEnv -> TcM ()) -> TcS Bool
unifyFunDeps CtEvidence
ev Role
Nominal ((UnifyEnv -> TcM ()) -> TcS Bool)
-> (UnifyEnv -> TcM ()) -> TcS Bool
forall a b. (a -> b) -> a -> b
$ \UnifyEnv
uenv ->
    UnifyEnv -> [TypeEqn] -> TcM ()
uPairsTcM UnifyEnv
uenv [ Type -> Type -> TypeEqn
forall a. a -> a -> Pair a
Pair Type
ty1 Type
ty2
                   | (Type
ty1,Type
ty2,Bool
True) <- [Type] -> [Type] -> [Bool] -> [(Type, Type, Bool)]
forall a b c. [a] -> [b] -> [c] -> [(a, b, c)]
zip3 [Type]
fun_args1 [Type]
fun_args2 [Bool]
inj ]

    -- Built-in synonym families don't have an entry point for this
    -- use case. So, we just use sfInteractInert and pass two equal
    -- RHSs. We *could* add another entry point, but then there would
    -- be a burden to make sure the new entry point and existing ones
    -- were internally consistent. This is slightly distasteful, but
    -- it works well in practice and localises the problem.  Ugh.
  | Just BuiltInSynFamily
ops <- TyCon -> Maybe BuiltInSynFamily
isBuiltInSynFamTyCon_maybe TyCon
fun_tc1
  = let tc_kind :: Type
tc_kind = TyCon -> Type
tyConKind TyCon
fun_tc1
        ki1 :: Type
ki1 = HasDebugCallStack => Type -> [Type] -> Type
Type -> [Type] -> Type
piResultTys Type
tc_kind [Type]
fun_args1
        ki2 :: Type
ki2 | MCoercion
MRefl <- MCoercion
mco
            = Type
ki1   -- just a small optimisation
            | Bool
otherwise
            = HasDebugCallStack => Type -> [Type] -> Type
Type -> [Type] -> Type
piResultTys Type
tc_kind [Type]
fun_args2

        fake_rhs1 :: Type
fake_rhs1 = Type -> Type
anyTypeOfKind Type
ki1
        fake_rhs2 :: Type
fake_rhs2 = Type -> Type
anyTypeOfKind Type
ki2

        eqs :: [TypeEqn]
        eqs :: [TypeEqn]
eqs = BuiltInSynFamily -> [Type] -> Type -> [Type] -> Type -> [TypeEqn]
sfInteractInert BuiltInSynFamily
ops [Type]
fun_args1 Type
fake_rhs1 [Type]
fun_args2 Type
fake_rhs2
    in
    CtEvidence -> Role -> (UnifyEnv -> TcM ()) -> TcS Bool
unifyFunDeps CtEvidence
ev Role
Nominal ((UnifyEnv -> TcM ()) -> TcS Bool)
-> (UnifyEnv -> TcM ()) -> TcS Bool
forall a b. (a -> b) -> a -> b
$ \UnifyEnv
uenv ->
    UnifyEnv -> [TypeEqn] -> TcM ()
uPairsTcM UnifyEnv
uenv [TypeEqn]
eqs

  | Bool
otherwise  -- ordinary, non-injective type family
  = Bool -> TcS Bool
forall a. a -> TcS a
forall (m :: * -> *) a. Monad m => a -> m a
return Bool
False

--------------------
tryFunDeps :: EqCt -> SolverStage ()
tryFunDeps :: EqCt -> SolverStage ()
tryFunDeps work_item :: EqCt
work_item@(EqCt { eq_lhs :: EqCt -> CanEqLHS
eq_lhs = CanEqLHS
lhs, eq_ev :: EqCt -> CtEvidence
eq_ev = CtEvidence
ev })
  = TcS (StopOrContinue ()) -> SolverStage ()
forall a. TcS (StopOrContinue a) -> SolverStage a
Stage (TcS (StopOrContinue ()) -> SolverStage ())
-> TcS (StopOrContinue ()) -> SolverStage ()
forall a b. (a -> b) -> a -> b
$
    case CanEqLHS
lhs of
       TyFamLHS TyCon
tc [Type]
args -> do { InertCans
inerts <- TcS InertCans
getInertCans
                              ; Bool
imp1 <- InertCans -> TyCon -> [Type] -> EqCt -> TcS Bool
improveLocalFunEqs InertCans
inerts TyCon
tc [Type]
args EqCt
work_item
                              ; Bool
imp2 <- TyCon -> [Type] -> EqCt -> TcS Bool
improveTopFunEqs TyCon
tc [Type]
args EqCt
work_item
                              ; if (Bool
imp1 Bool -> Bool -> Bool
|| Bool
imp2)
                                then Ct -> TcS (StopOrContinue ())
forall a. Ct -> TcS (StopOrContinue a)
startAgainWith (CtEvidence -> Ct
mkNonCanonical CtEvidence
ev)
                                else () -> TcS (StopOrContinue ())
forall a. a -> TcS (StopOrContinue a)
continueWith () }
       TyVarLHS {} -> () -> TcS (StopOrContinue ())
forall a. a -> TcS (StopOrContinue a)
continueWith ()

--------------------
improveTopFunEqs :: TyCon -> [TcType] -> EqCt -> TcS Bool
-- See Note [FunDep and implicit parameter reactions]
improveTopFunEqs :: TyCon -> [Type] -> EqCt -> TcS Bool
improveTopFunEqs TyCon
fam_tc [Type]
args (EqCt { eq_ev :: EqCt -> CtEvidence
eq_ev = CtEvidence
ev, eq_rhs :: EqCt -> Type
eq_rhs = Type
rhs })
  | CtEvidence -> Bool
isGiven CtEvidence
ev
  = Bool -> TcS Bool
forall a. a -> TcS a
forall (m :: * -> *) a. Monad m => a -> m a
return Bool
False  -- See Note [No Given/Given fundeps]

  | Bool
otherwise
  = do { (FamInstEnv, FamInstEnv)
fam_envs <- TcS (FamInstEnv, FamInstEnv)
getFamInstEnvs
       ; [TypeEqn]
eqns <- (FamInstEnv, FamInstEnv)
-> TyCon -> [Type] -> Type -> TcS [TypeEqn]
improve_top_fun_eqs (FamInstEnv, FamInstEnv)
fam_envs TyCon
fam_tc [Type]
args Type
rhs
       ; String -> SDoc -> TcS ()
traceTcS String
"improveTopFunEqs" ([SDoc] -> SDoc
forall doc. IsDoc doc => [doc] -> doc
vcat [ TyCon -> SDoc
forall a. Outputable a => a -> SDoc
ppr TyCon
fam_tc SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> [Type] -> SDoc
forall a. Outputable a => a -> SDoc
ppr [Type]
args SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> Type -> SDoc
forall a. Outputable a => a -> SDoc
ppr Type
rhs
                                           , [TypeEqn] -> SDoc
forall a. Outputable a => a -> SDoc
ppr [TypeEqn]
eqns ])
       ; CtEvidence -> Role -> (UnifyEnv -> TcM ()) -> TcS Bool
unifyFunDeps CtEvidence
ev Role
Nominal ((UnifyEnv -> TcM ()) -> TcS Bool)
-> (UnifyEnv -> TcM ()) -> TcS Bool
forall a b. (a -> b) -> a -> b
$ \UnifyEnv
uenv ->
         UnifyEnv -> [TypeEqn] -> TcM ()
uPairsTcM (UnifyEnv -> UnifyEnv
bump_depth UnifyEnv
uenv) ([TypeEqn] -> [TypeEqn]
forall a. [a] -> [a]
reverse [TypeEqn]
eqns) }
         -- Missing that `reverse` causes T13135 and T13135_simple to loop.
         -- See Note [Reverse order of fundep equations]

  where
    bump_depth :: UnifyEnv -> UnifyEnv
bump_depth UnifyEnv
env = UnifyEnv
env { u_loc = bumpCtLocDepth (u_loc env) }
        -- ToDo: this location is wrong; it should be FunDepOrigin2
        -- See #14778

improve_top_fun_eqs :: FamInstEnvs
                    -> TyCon -> [TcType] -> TcType
                    -> TcS [TypeEqn]
improve_top_fun_eqs :: (FamInstEnv, FamInstEnv)
-> TyCon -> [Type] -> Type -> TcS [TypeEqn]
improve_top_fun_eqs (FamInstEnv, FamInstEnv)
fam_envs TyCon
fam_tc [Type]
args Type
rhs_ty
  | Just BuiltInSynFamily
ops <- TyCon -> Maybe BuiltInSynFamily
isBuiltInSynFamTyCon_maybe TyCon
fam_tc
  = [TypeEqn] -> TcS [TypeEqn]
forall a. a -> TcS a
forall (m :: * -> *) a. Monad m => a -> m a
return (BuiltInSynFamily -> [Type] -> Type -> [TypeEqn]
sfInteractTop BuiltInSynFamily
ops [Type]
args Type
rhs_ty)

  -- see Note [Type inference for type families with injectivity]
  | TyCon -> Bool
isOpenTypeFamilyTyCon TyCon
fam_tc
  , Injective [Bool]
injective_args <- TyCon -> Injectivity
tyConInjectivityInfo TyCon
fam_tc
  , let fam_insts :: [FamInst]
fam_insts = (FamInstEnv, FamInstEnv) -> TyCon -> [FamInst]
lookupFamInstEnvByTyCon (FamInstEnv, FamInstEnv)
fam_envs TyCon
fam_tc
  = -- it is possible to have several compatible equations in an open type
    -- family but we only want to derive equalities from one such equation.
    do { let improvs :: [([Type], Subst, [TyVar], Maybe CoAxBranch)]
improvs = [FamInst]
-> (FamInst -> [TyVar])
-> (FamInst -> [Type])
-> (FamInst -> Type)
-> (FamInst -> Maybe CoAxBranch)
-> [([Type], Subst, [TyVar], Maybe CoAxBranch)]
forall a.
[a]
-> (a -> [TyVar])
-> (a -> [Type])
-> (a -> Type)
-> (a -> Maybe CoAxBranch)
-> [([Type], Subst, [TyVar], Maybe CoAxBranch)]
buildImprovementData [FamInst]
fam_insts
                           FamInst -> [TyVar]
fi_tvs FamInst -> [Type]
fi_tys FamInst -> Type
fi_rhs (Maybe CoAxBranch -> FamInst -> Maybe CoAxBranch
forall a b. a -> b -> a
const Maybe CoAxBranch
forall a. Maybe a
Nothing)

       ; String -> SDoc -> TcS ()
traceTcS String
"improve_top_fun_eqs2" ([([Type], Subst, [TyVar], Maybe CoAxBranch)] -> SDoc
forall a. Outputable a => a -> SDoc
ppr [([Type], Subst, [TyVar], Maybe CoAxBranch)]
improvs)
       ; (([Type], Subst, [TyVar], Maybe CoAxBranch) -> TcS [TypeEqn])
-> [([Type], Subst, [TyVar], Maybe CoAxBranch)] -> TcS [TypeEqn]
forall (m :: * -> *) (f :: * -> *) a b.
(Monad m, Traversable f) =>
(a -> m [b]) -> f a -> m [b]
concatMapM ([Bool]
-> ([Type], Subst, [TyVar], Maybe CoAxBranch) -> TcS [TypeEqn]
injImproveEqns [Bool]
injective_args) ([([Type], Subst, [TyVar], Maybe CoAxBranch)] -> TcS [TypeEqn])
-> [([Type], Subst, [TyVar], Maybe CoAxBranch)] -> TcS [TypeEqn]
forall a b. (a -> b) -> a -> b
$
         Int
-> [([Type], Subst, [TyVar], Maybe CoAxBranch)]
-> [([Type], Subst, [TyVar], Maybe CoAxBranch)]
forall a. Int -> [a] -> [a]
take Int
1 [([Type], Subst, [TyVar], Maybe CoAxBranch)]
improvs }

  | Just CoAxiom Branched
ax <- TyCon -> Maybe (CoAxiom Branched)
isClosedSynFamilyTyConWithAxiom_maybe TyCon
fam_tc
  , Injective [Bool]
injective_args <- TyCon -> Injectivity
tyConInjectivityInfo TyCon
fam_tc
  = (([Type], Subst, [TyVar], Maybe CoAxBranch) -> TcS [TypeEqn])
-> [([Type], Subst, [TyVar], Maybe CoAxBranch)] -> TcS [TypeEqn]
forall (m :: * -> *) (f :: * -> *) a b.
(Monad m, Traversable f) =>
(a -> m [b]) -> f a -> m [b]
concatMapM ([Bool]
-> ([Type], Subst, [TyVar], Maybe CoAxBranch) -> TcS [TypeEqn]
injImproveEqns [Bool]
injective_args) ([([Type], Subst, [TyVar], Maybe CoAxBranch)] -> TcS [TypeEqn])
-> [([Type], Subst, [TyVar], Maybe CoAxBranch)] -> TcS [TypeEqn]
forall a b. (a -> b) -> a -> b
$
    [CoAxBranch]
-> (CoAxBranch -> [TyVar])
-> (CoAxBranch -> [Type])
-> (CoAxBranch -> Type)
-> (CoAxBranch -> Maybe CoAxBranch)
-> [([Type], Subst, [TyVar], Maybe CoAxBranch)]
forall a.
[a]
-> (a -> [TyVar])
-> (a -> [Type])
-> (a -> Type)
-> (a -> Maybe CoAxBranch)
-> [([Type], Subst, [TyVar], Maybe CoAxBranch)]
buildImprovementData (Branches Branched -> [CoAxBranch]
forall (br :: BranchFlag). Branches br -> [CoAxBranch]
fromBranches (CoAxiom Branched -> Branches Branched
forall (br :: BranchFlag). CoAxiom br -> Branches br
co_ax_branches CoAxiom Branched
ax))
                         CoAxBranch -> [TyVar]
cab_tvs CoAxBranch -> [Type]
cab_lhs CoAxBranch -> Type
cab_rhs CoAxBranch -> Maybe CoAxBranch
forall a. a -> Maybe a
Just

  | Bool
otherwise
  = [TypeEqn] -> TcS [TypeEqn]
forall a. a -> TcS a
forall (m :: * -> *) a. Monad m => a -> m a
return []

  where
      in_scope :: InScopeSet
in_scope = TyCoVarSet -> InScopeSet
mkInScopeSet (Type -> TyCoVarSet
tyCoVarsOfType Type
rhs_ty)

      buildImprovementData
          :: [a]                     -- axioms for a TF (FamInst or CoAxBranch)
          -> (a -> [TyVar])          -- get bound tyvars of an axiom
          -> (a -> [Type])           -- get LHS of an axiom
          -> (a -> Type)             -- get RHS of an axiom
          -> (a -> Maybe CoAxBranch) -- Just => apartness check required
          -> [( [Type], Subst, [TyVar], Maybe CoAxBranch )]
             -- Result:
             -- ( [arguments of a matching axiom]
             -- , RHS-unifying substitution
             -- , axiom variables without substitution
             -- , Maybe matching axiom [Nothing - open TF, Just - closed TF ] )
      buildImprovementData :: forall a.
[a]
-> (a -> [TyVar])
-> (a -> [Type])
-> (a -> Type)
-> (a -> Maybe CoAxBranch)
-> [([Type], Subst, [TyVar], Maybe CoAxBranch)]
buildImprovementData [a]
axioms a -> [TyVar]
axiomTVs a -> [Type]
axiomLHS a -> Type
axiomRHS a -> Maybe CoAxBranch
wrap =
          [ ([Type]
ax_args, Subst
subst, [TyVar]
unsubstTvs, a -> Maybe CoAxBranch
wrap a
axiom)
          | a
axiom <- [a]
axioms
          , let ax_args :: [Type]
ax_args = a -> [Type]
axiomLHS a
axiom
                ax_rhs :: Type
ax_rhs  = a -> Type
axiomRHS a
axiom
                ax_tvs :: [TyVar]
ax_tvs  = a -> [TyVar]
axiomTVs a
axiom
                in_scope1 :: InScopeSet
in_scope1 = InScopeSet
in_scope InScopeSet -> [TyVar] -> InScopeSet
`extendInScopeSetList` [TyVar]
ax_tvs
          , Just Subst
subst <- [Bool -> InScopeSet -> Type -> Type -> Maybe Subst
tcUnifyTyWithTFs Bool
False InScopeSet
in_scope1 Type
ax_rhs Type
rhs_ty]
          , let notInSubst :: TyVar -> Bool
notInSubst TyVar
tv = Bool -> Bool
not (TyVar
tv TyVar -> VarEnv Type -> Bool
forall a. TyVar -> VarEnv a -> Bool
`elemVarEnv` Subst -> VarEnv Type
getTvSubstEnv Subst
subst)
                unsubstTvs :: [TyVar]
unsubstTvs    = (TyVar -> Bool) -> [TyVar] -> [TyVar]
forall a. (a -> Bool) -> [a] -> [a]
filter (TyVar -> Bool
notInSubst (TyVar -> Bool) -> (TyVar -> Bool) -> TyVar -> Bool
forall (f :: * -> *). Applicative f => f Bool -> f Bool -> f Bool
<&&> TyVar -> Bool
isTyVar) [TyVar]
ax_tvs ]
                   -- The order of unsubstTvs is important; it must be
                   -- in telescope order e.g. (k:*) (a:k)

      injImproveEqns :: [Bool]
                     -> ([Type], Subst, [TyCoVar], Maybe CoAxBranch)
                     -> TcS [TypeEqn]
      injImproveEqns :: [Bool]
-> ([Type], Subst, [TyVar], Maybe CoAxBranch) -> TcS [TypeEqn]
injImproveEqns [Bool]
inj_args ([Type]
ax_args, Subst
subst, [TyVar]
unsubstTvs, Maybe CoAxBranch
cabr)
        = do { Subst
subst <- Subst -> [TyVar] -> TcS Subst
instFlexiX Subst
subst [TyVar]
unsubstTvs
                  -- If the current substitution bind [k -> *], and
                  -- one of the un-substituted tyvars is (a::k), we'd better
                  -- be sure to apply the current substitution to a's kind.
                  -- Hence instFlexiX.   #13135 was an example.

             ; [TypeEqn] -> TcS [TypeEqn]
forall a. a -> TcS a
forall (m :: * -> *) a. Monad m => a -> m a
return [ Type -> Type -> TypeEqn
forall a. a -> a -> Pair a
Pair (HasDebugCallStack => Subst -> Type -> Type
Subst -> Type -> Type
substTy Subst
subst Type
ax_arg) Type
arg
                        -- NB: the ax_arg part is on the left
                        -- see Note [Improvement orientation]
                      | case Maybe CoAxBranch
cabr of
                          Just CoAxBranch
cabr' -> [Type] -> CoAxBranch -> Bool
apartnessCheck (HasDebugCallStack => Subst -> [Type] -> [Type]
Subst -> [Type] -> [Type]
substTys Subst
subst [Type]
ax_args) CoAxBranch
cabr'
                          Maybe CoAxBranch
_          -> Bool
True
                      , (Type
ax_arg, Type
arg, Bool
True) <- [Type] -> [Type] -> [Bool] -> [(Type, Type, Bool)]
forall a b c. [a] -> [b] -> [c] -> [(a, b, c)]
zip3 [Type]
ax_args [Type]
args [Bool]
inj_args ] }


improveLocalFunEqs :: InertCans -> TyCon -> [TcType] -> EqCt -> TcS Bool
-- Generate improvement equalities, by comparing
-- the current work item with inert CFunEqs
-- E.g.   x + y ~ z,   x + y' ~ z   =>   [W] y ~ y'
--
-- See Note [FunDep and implicit parameter reactions]
improveLocalFunEqs :: InertCans -> TyCon -> [Type] -> EqCt -> TcS Bool
improveLocalFunEqs InertCans
inerts TyCon
fam_tc [Type]
args (EqCt { eq_ev :: EqCt -> CtEvidence
eq_ev = CtEvidence
work_ev, eq_rhs :: EqCt -> Type
eq_rhs = Type
rhs })
  | [FunDepEqn (CtLoc, RewriterSet)] -> Bool
forall a. [a] -> Bool
forall (t :: * -> *) a. Foldable t => t a -> Bool
null [FunDepEqn (CtLoc, RewriterSet)]
improvement_eqns
  = Bool -> TcS Bool
forall a. a -> TcS a
forall (m :: * -> *) a. Monad m => a -> m a
return Bool
False
  | Bool
otherwise
  = do { String -> SDoc -> TcS ()
traceTcS String
"interactFunEq improvements: " (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
"Eqns:" SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> [FunDepEqn (CtLoc, RewriterSet)] -> SDoc
forall a. Outputable a => a -> SDoc
ppr [FunDepEqn (CtLoc, RewriterSet)]
improvement_eqns
                        , String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"Candidates:" SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> [EqCt] -> SDoc
forall a. Outputable a => a -> SDoc
ppr [EqCt]
funeqs_for_tc
                        , String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"Inert eqs:" SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> InertEqs -> SDoc
forall a. Outputable a => a -> SDoc
ppr (InertCans -> InertEqs
inert_eqs InertCans
inerts) ]
       ; CtEvidence -> [FunDepEqn (CtLoc, RewriterSet)] -> TcS Bool
emitFunDepWanteds CtEvidence
work_ev [FunDepEqn (CtLoc, RewriterSet)]
improvement_eqns }
  where
    funeqs :: InertFunEqs
funeqs        = InertCans -> InertFunEqs
inert_funeqs InertCans
inerts
    funeqs_for_tc :: [EqCt]
    funeqs_for_tc :: [EqCt]
funeqs_for_tc = [ EqCt
funeq_ct | [EqCt]
equal_ct_list <- InertFunEqs -> TyCon -> [[EqCt]]
forall a. FunEqMap a -> TyCon -> [a]
findFunEqsByTyCon InertFunEqs
funeqs TyCon
fam_tc
                               , EqCt
funeq_ct <- [EqCt]
equal_ct_list
                               , EqRel
NomEq EqRel -> EqRel -> Bool
forall a. Eq a => a -> a -> Bool
== EqCt -> EqRel
eq_eq_rel EqCt
funeq_ct ]
                                  -- representational equalities don't interact
                                  -- with type family dependencies
    work_loc :: CtLoc
work_loc      = CtEvidence -> CtLoc
ctEvLoc CtEvidence
work_ev
    work_pred :: Type
work_pred     = CtEvidence -> Type
ctEvPred CtEvidence
work_ev
    fam_inj_info :: Injectivity
fam_inj_info  = TyCon -> Injectivity
tyConInjectivityInfo TyCon
fam_tc

    --------------------
    improvement_eqns :: [FunDepEqn (CtLoc, RewriterSet)]
    improvement_eqns :: [FunDepEqn (CtLoc, RewriterSet)]
improvement_eqns
      | Just BuiltInSynFamily
ops <- TyCon -> Maybe BuiltInSynFamily
isBuiltInSynFamTyCon_maybe TyCon
fam_tc
      =    -- Try built-in families, notably for arithmethic
        (EqCt -> [FunDepEqn (CtLoc, RewriterSet)])
-> [EqCt] -> [FunDepEqn (CtLoc, RewriterSet)]
forall (t :: * -> *) a b. Foldable t => (a -> [b]) -> t a -> [b]
concatMap (BuiltInSynFamily
-> Type -> EqCt -> [FunDepEqn (CtLoc, RewriterSet)]
do_one_built_in BuiltInSynFamily
ops Type
rhs) [EqCt]
funeqs_for_tc

      | Injective [Bool]
injective_args <- Injectivity
fam_inj_info
      =    -- Try improvement from type families with injectivity annotations
        (EqCt -> [FunDepEqn (CtLoc, RewriterSet)])
-> [EqCt] -> [FunDepEqn (CtLoc, RewriterSet)]
forall (t :: * -> *) a b. Foldable t => (a -> [b]) -> t a -> [b]
concatMap ([Bool] -> Type -> EqCt -> [FunDepEqn (CtLoc, RewriterSet)]
do_one_injective [Bool]
injective_args Type
rhs) [EqCt]
funeqs_for_tc

      | Bool
otherwise
      = []

    --------------------
    do_one_built_in :: BuiltInSynFamily
-> Type -> EqCt -> [FunDepEqn (CtLoc, RewriterSet)]
do_one_built_in BuiltInSynFamily
ops Type
rhs (EqCt { eq_lhs :: EqCt -> CanEqLHS
eq_lhs = TyFamLHS TyCon
_ [Type]
iargs, eq_rhs :: EqCt -> Type
eq_rhs = Type
irhs, eq_ev :: EqCt -> CtEvidence
eq_ev = CtEvidence
inert_ev })
      | Bool -> Bool
not (CtEvidence -> Bool
isGiven CtEvidence
inert_ev Bool -> Bool -> Bool
&& CtEvidence -> Bool
isGiven CtEvidence
work_ev)  -- See Note [No Given/Given fundeps]
      = CtEvidence -> [TypeEqn] -> [FunDepEqn (CtLoc, RewriterSet)]
mk_fd_eqns CtEvidence
inert_ev (BuiltInSynFamily -> [Type] -> Type -> [Type] -> Type -> [TypeEqn]
sfInteractInert BuiltInSynFamily
ops [Type]
args Type
rhs [Type]
iargs Type
irhs)

      | Bool
otherwise
      = []

    do_one_built_in BuiltInSynFamily
_ Type
_ EqCt
_ = String -> SDoc -> [FunDepEqn (CtLoc, RewriterSet)]
forall a. HasCallStack => String -> SDoc -> a
pprPanic String
"interactFunEq 1" (TyCon -> SDoc
forall a. Outputable a => a -> SDoc
ppr TyCon
fam_tc) -- TyVarLHS

    --------------------
    -- See Note [Type inference for type families with injectivity]
    do_one_injective :: [Bool] -> Type -> EqCt -> [FunDepEqn (CtLoc, RewriterSet)]
do_one_injective [Bool]
inj_args Type
rhs (EqCt { eq_lhs :: EqCt -> CanEqLHS
eq_lhs = TyFamLHS TyCon
_ [Type]
inert_args
                                        , eq_rhs :: EqCt -> Type
eq_rhs = Type
irhs, eq_ev :: EqCt -> CtEvidence
eq_ev = CtEvidence
inert_ev })
      | Bool -> Bool
not (CtEvidence -> Bool
isGiven CtEvidence
inert_ev Bool -> Bool -> Bool
&& CtEvidence -> Bool
isGiven CtEvidence
work_ev) -- See Note [No Given/Given fundeps]
      , Type
rhs HasDebugCallStack => Type -> Type -> Bool
Type -> Type -> Bool
`tcEqType` Type
irhs
      = CtEvidence -> [TypeEqn] -> [FunDepEqn (CtLoc, RewriterSet)]
mk_fd_eqns CtEvidence
inert_ev ([TypeEqn] -> [FunDepEqn (CtLoc, RewriterSet)])
-> [TypeEqn] -> [FunDepEqn (CtLoc, RewriterSet)]
forall a b. (a -> b) -> a -> b
$ [ Type -> Type -> TypeEqn
forall a. a -> a -> Pair a
Pair Type
arg Type
iarg
                              | (Type
arg, Type
iarg, Bool
True) <- [Type] -> [Type] -> [Bool] -> [(Type, Type, Bool)]
forall a b c. [a] -> [b] -> [c] -> [(a, b, c)]
zip3 [Type]
args [Type]
inert_args [Bool]
inj_args ]
      | Bool
otherwise
      = []

    do_one_injective [Bool]
_ Type
_ EqCt
_ = String -> SDoc -> [FunDepEqn (CtLoc, RewriterSet)]
forall a. HasCallStack => String -> SDoc -> a
pprPanic String
"interactFunEq 2" (TyCon -> SDoc
forall a. Outputable a => a -> SDoc
ppr TyCon
fam_tc)  -- TyVarLHS

    --------------------
    mk_fd_eqns :: CtEvidence -> [TypeEqn] -> [FunDepEqn (CtLoc, RewriterSet)]
    mk_fd_eqns :: CtEvidence -> [TypeEqn] -> [FunDepEqn (CtLoc, RewriterSet)]
mk_fd_eqns CtEvidence
inert_ev [TypeEqn]
eqns
      | [TypeEqn] -> Bool
forall a. [a] -> Bool
forall (t :: * -> *) a. Foldable t => t a -> Bool
null [TypeEqn]
eqns  = []
      | Bool
otherwise  = [ FDEqn { fd_qtvs :: [TyVar]
fd_qtvs = [], fd_eqs :: [TypeEqn]
fd_eqs = [TypeEqn]
eqns
                             , fd_pred1 :: Type
fd_pred1 = Type
work_pred
                             , fd_pred2 :: Type
fd_pred2 = Type
inert_pred
                             , fd_loc :: (CtLoc, RewriterSet)
fd_loc   = (CtLoc
loc, RewriterSet
inert_rewriters) } ]
      where
        initial_loc :: CtLoc
initial_loc  -- start with the location of the Wanted involved
          | CtEvidence -> Bool
isGiven CtEvidence
work_ev = CtLoc
inert_loc
          | Bool
otherwise       = CtLoc
work_loc
        eqn_orig :: CtOrigin
eqn_orig        = Type
-> CtOrigin
-> RealSrcSpan
-> Type
-> CtOrigin
-> RealSrcSpan
-> CtOrigin
InjTFOrigin1 Type
work_pred (CtLoc -> CtOrigin
ctLocOrigin CtLoc
work_loc) (CtLoc -> RealSrcSpan
ctLocSpan CtLoc
work_loc)
                                       Type
inert_pred (CtLoc -> CtOrigin
ctLocOrigin CtLoc
inert_loc) (CtLoc -> RealSrcSpan
ctLocSpan CtLoc
inert_loc)
        eqn_loc :: CtLoc
eqn_loc         = CtLoc -> CtOrigin -> CtLoc
setCtLocOrigin CtLoc
initial_loc CtOrigin
eqn_orig
        inert_pred :: Type
inert_pred      = CtEvidence -> Type
ctEvPred CtEvidence
inert_ev
        inert_loc :: CtLoc
inert_loc       = CtEvidence -> CtLoc
ctEvLoc CtEvidence
inert_ev
        inert_rewriters :: RewriterSet
inert_rewriters = CtEvidence -> RewriterSet
ctEvRewriters CtEvidence
inert_ev
        loc :: CtLoc
loc = CtLoc
eqn_loc { ctl_depth = ctl_depth inert_loc `maxSubGoalDepth`
                                    ctl_depth work_loc }

{- Note [Type inference for type families with injectivity]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Suppose we have a type family with an injectivity annotation:
    type family F a b = r | r -> b

Then if we have an equality like F s1 t1 ~ F s2 t2,
we can use the injectivity to get a new Wanted constraint on
the injective argument
  [W] t1 ~ t2

That in turn can help GHC solve constraints that would otherwise require
guessing.  For example, consider the ambiguity check for
   f :: F Int b -> Int
We get the constraint
   [W] F Int b ~ F Int beta
where beta is a unification variable.  Injectivity lets us pick beta ~ b.

Injectivity information is also used at the call sites. For example:
   g = f True
gives rise to
   [W] F Int b ~ Bool
from which we can derive b.  This requires looking at the defining equations of
a type family, ie. finding equation with a matching RHS (Bool in this example)
and inferring values of type variables (b in this example) from the LHS patterns
of the matching equation.  For closed type families we have to perform
additional apartness check for the selected equation to check that the selected
is guaranteed to fire for given LHS arguments.

These new constraints are Wanted constraints, but we will not use the evidence.
We could go further and offer evidence from decomposing injective type-function
applications, but that would require new evidence forms, and an extension to
FC, so we don't do that right now (Dec 14).

We generate these Wanteds in three places, depending on how we notice the
injectivity.

1. When we have a [W] F tys1 ~ F tys2. This is handled in canEqCanLHS2, and
described in Note [Decomposing type family applications] in GHC.Tc.Solver.Equality

2. When we have [W] F tys1 ~ T and [W] F tys2 ~ T. Note that neither of these
constraints rewrites the other, as they have different LHSs. This is done
in improveLocalFunEqs, called during the interactWithInertsStage.

3. When we have [W] F tys ~ T and an equation for F that looks like F tys' = T.
This is done in improve_top_fun_eqs, called from the top-level reactions stage.

See also Note [Injective type families] in GHC.Core.TyCon

Note [Cache-caused loops]
~~~~~~~~~~~~~~~~~~~~~~~~~
It is very dangerous to cache a rewritten wanted family equation as 'solved' in our
solved cache (which is the default behaviour or xCtEvidence), because the interaction
may not be contributing towards a solution. Here is an example:

Initial inert set:
  [W] g1 : F a ~ beta1
Work item:
  [W] g2 : F a ~ beta2
The work item will react with the inert yielding the _same_ inert set plus:
    (i)   Will set g2 := g1 `cast` g3
    (ii)  Will add to our solved cache that [S] g2 : F a ~ beta2
    (iii) Will emit [W] g3 : beta1 ~ beta2
Now, the g3 work item will be spontaneously solved to [G] g3 : beta1 ~ beta2
and then it will react the item in the inert ([W] g1 : F a ~ beta1). So it
will set
      g1 := g ; sym g3
and what is g? Well it would ideally be a new goal of type (F a ~ beta2) but
remember that we have this in our solved cache, and it is ... g2! In short we
created the evidence loop:

        g2 := g1 ; g3
        g3 := refl
        g1 := g2 ; sym g3

To avoid this situation we do not cache as solved any workitems (or inert)
which did not really made a 'step' towards proving some goal. Solved's are
just an optimization so we don't lose anything in terms of completeness of
solving.
-}