[GHC] #3743: type checker fails to infer an implicit parameter constraint in the presence of existential types.

[GHC]

#3743: type checker fails to infer an implicit parameter constraint in the
presence of existential types.
---------------------------------+------------------------------------------
    Reporter:  int-e             |        Owner:              
        Type:  bug               |       Status:  new         
    Priority:  normal            |    Milestone:              
   Component:  Compiler          |      Version:  6.13        
    Keywords:                    |   Difficulty:              
          Os:  Unknown/Multiple  |     Testcase:              
Architecture:  Unknown/Multiple  |      Failure:  None/Unknown
---------------------------------+------------------------------------------
 In this code,

 {{{
 {-# LANGUAGE ImplicitParams, GADTs #-}

 class Foo a

 data M where M :: Foo a => a -> M

 x :: (?x :: ()) => ()
 x = undefined

 -- foo :: (?x :: ()) => M -> ()
 foo y = case y of
     M _ -> x
 }}}

 I'd expect ghc to infer the commented out type for {{{foo}}}. However, it
 fails to do that:

 {{{
 foo.hs:12:12:
     Unbound implicit parameter (?x::())
       arising from a use of `x' at foo.hs:12:12
     In the expression: x
     In a case alternative: M _ -> x
     In the expression: case y of { M _ -> x }
 }}}

 Is this a bug or an expected shortcoming of the type checker? The program
 compiles fine with the explicit type signature. (In the original program
 the type signature was big and completely unenlightening, so I'd rather
 not have to write it.)

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/3743>
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