[GHC] #11642: Typechecker doesn't use evidence available from pattern synonym?

[GHC]

#11642: Typechecker doesn't use evidence available from pattern synonym?
-------------------------------------+-------------------------------------
           Reporter:  bgamari        |             Owner:
               Type:  bug            |            Status:  new
           Priority:  high           |         Milestone:  8.0.1
          Component:  Compiler       |           Version:  7.10.3
  (Type checker)                     |
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  GHC rejects
  Unknown/Multiple                   |  valid program
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 Either that or I am missing something...

 Consider the following,
 {{{#!hs
 {-# LANGUAGE GADTs, PolyKinds, PatternSynonyms, RankNTypes,
              TypeOperators, ViewPatterns #-}

 module Test where

 data TypeRep (a :: k) where
   TypeCon :: String -> TypeRep a
   TypeApp :: TypeRep f -> TypeRep x -> TypeRep (f x)

 data TypeRepX where
   TypeRepX :: TypeRep a -> TypeRepX

 data (a :: k1) :~~: (b :: k2) where
    HRefl :: a :~~: a

 trArrow :: TypeRep (->)
 trArrow = undefined

 eqTypeRep :: TypeRep (a :: k1) -> TypeRep (b :: k2) -> Maybe (a :~~: b)
 eqTypeRep = undefined

 typeRepKind :: forall (k :: *). forall (a :: k). TypeRep a -> TypeRep k
 typeRepKind = undefined

 pattern TRFun :: forall fun. ()
               => forall arg res. (fun ~ (arg -> res))
               => TypeRep arg
               -> TypeRep res
               -> TypeRep fun
 pattern TRFun arg res <- TypeApp (TypeApp (eqTypeRep trArrow -> Just
 HRefl) arg) res

 buildApp :: TypeRepX -> TypeRepX -> TypeRepX
 buildApp (TypeRepX f) (TypeRepX x) =
   case typeRepKind f of
     TRFun arg _ ->
       case eqTypeRep arg x of
         Just HRefl ->
           TypeRepX $ TypeApp f x
 }}}

 This fails with,
 {{{
 $ ghc Test.hs -fprint-explicit-kinds
 [1 of 1] Compiling Test             ( Test.hs, Test.o )

 Test.hs:38:30: error:
     • Expected kind ‘TypeRep (k1 -> res) a’,
         but ‘f’ has kind ‘TypeRep k a’
     • In the first argument of ‘TypeApp’, namely ‘f’
       In the second argument of ‘($)’, namely ‘TypeApp f x’
       In the expression: TypeRepX $ TypeApp f x
     • Relevant bindings include
         arg :: TypeRep * arg (bound at Test.hs:35:11)
 }}}

 That is, the typechecker doesn't believe that `f`'s type (why is it saying
 "kind" here?), `TypeRep k a`, will unify with `TypeRep (k1 -> res) a`,
 despite the `TRFun` pattern match, which should have brought into scope
 that `k ~ (arg -> res)`.

 This was tested with a recent snapshot from `ghc-8.0`
 (23baff798aca5856650508ad0f7727045efe3680).

 Am I missing something here or is this a bug?

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11642>
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