[GHC] #15351: QuantifiedConstraints ignore FunctionalDependencies

[GHC]

#15351: QuantifiedConstraints ignore FunctionalDependencies
-------------------------------------+-------------------------------------
           Reporter:  aaronvargo     |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.5
  (Type checker)                     |
           Keywords:                 |  Operating System:  Unknown/Multiple
  QuantifiedConstraints              |
       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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 The following code fails to compile:

 {{{#!hs
 {-# LANGUAGE QuantifiedConstraints #-}
 {-# LANGUAGE FunctionalDependencies #-}

 class C a b | a -> b where
   foo :: a -> b

 bar :: (forall a. C (f a) Int) => f a -> String
 bar = show . foo
 }}}

 {{{
 • Could not deduce (Show a0) arising from a use of ‘show’
 ...
 The type variable ‘a0’ is ambiguous
 }}}

 Yet it ought to work, since this is perfectly fine with top-level
 instances:

 {{{#!hs
 instance C [a] Int where ...

 baz :: [a] -> String
 baz = show . foo
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15351>
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