[GHC] #15351: QuantifiedConstraints ignore FunctionalDependencies

[GHC]

#15351: QuantifiedConstraints ignore FunctionalDependencies
-------------------------------------+-------------------------------------
        Reporter:  aaronvargo        |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.5
  checker)                           |             Keywords:
      Resolution:                    |  QuantifiedConstraints
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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Description changed by aaronvargo:

Old description:

> 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
> }}}

New description:

 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


 • Could not deduce (C (f a) a0) arising from a use of ‘foo’
 ...
 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
 }}}

--

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15351#comment:1>
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