[GHC] #7123: Dictionary method should be applicable even in existential context

[GHC]

#7123: Dictionary method should be applicable even in existential context
------------------------------+---------------------------------------------
 Reporter:  heisenbug         |          Owner:                  
     Type:  bug               |         Status:  new             
 Priority:  normal            |      Component:  Compiler        
  Version:  7.4.1             |       Keywords:                  
       Os:  Unknown/Multiple  |   Architecture:  Unknown/Multiple
  Failure:  None/Unknown      |       Testcase:                  
Blockedby:                    |       Blocking:                  
  Related:                    |  
------------------------------+---------------------------------------------
 {{{
 {-# LANGUAGE GADTs, TypeFamilies #-}

 class Foo a where
   type Bar a
   baz :: Bar a -> Int


 instance Foo Int where
   type Bar Int = (Int, Int)
   baz (i, _) = i


 data Quux where
   Quux :: Foo a => a -> Bar a -> Quux

 q = Quux (42 :: Int) (1, 2)

 i = case q of Quux _ b -> baz b
 }}}


   repro.hs:18:31:
     Could not deduce (Bar a0 ~ Bar a)
     from the context (Foo a)
       bound by a pattern with constructor
                  Quux :: forall a. Foo a => a -> Bar a -> Quux,
                in a case alternative
       at repro.hs:18:15-22
     NB: `Bar' is a type function, and may not be injective
     The type variable `a0' is ambiguous
     Possible fix: add a type signature that fixes these type variable(s)
     In the first argument of `baz', namely `b'
     In the expression: baz b
     In a case alternative: Quux _ b -> baz b
   Failed, modules loaded: none.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7123>
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