[GHC] #12473: Ambiguous type var with DefaultSignatures and FunctionalDependencies

[GHC]

#12473: Ambiguous type var with DefaultSignatures and FunctionalDependencies
-------------------------------------+-------------------------------------
        Reporter:  dylex             |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.0.1
  checker)                           |
      Resolution:                    |             Keywords:
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:                    |
-------------------------------------+-------------------------------------

Comment (by goldfire):

 There is a fine workaround: `-XAllowAmbiguousTypes`, as the error message
 suggests. But I tend to agree that this is incorrect behavior by GHC:
 should we be doing an ambiguity check on generic-default signatures? I
 think not. Why not? Because we never call a function with the type given
 by a generic-default signature.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12473#comment:1>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler