[GHC] #12763: Incorrect behavior with empty functional dependencies

[GHC]

#12763: Incorrect behavior with empty functional dependencies
-------------------------------------+-------------------------------------
           Reporter:  diatchki       |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
  (Type checker)                     |
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 This is a corner case, but it would appear that GHC doesn't do proper
 improvements for classes with "empty" functional dependencies.  Consider
 the following example:

 {{{#!hs
 class C a | -> a where
    m :: a -> ()

 instance C Int

 f x = m x
 }}}

 The inferred type for `f` is `C a => a -> ()`, however I was expecting it
 to infer `Int -> ()`.

 The reasoning is as follows:  the use of `m` generates a `C a` constraint
 (`a` is a unification variable).  This should have interacted with the `C
 Int` instance (in a rather odd vacuous sort of way) to generate a derived
 constraint `a ~ Int`, which should have then instantiated `a` to `Int`.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12763>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler