[GHC] #5516: Universally quantified GADT context leads to overlapping instance

[GHC]

#5516: Universally quantified GADT context leads to overlapping instance
---------------------------------+------------------------------------------
    Reporter:  andersk           |       Owner:                           
        Type:  bug               |      Status:  new                      
    Priority:  normal            |   Component:  Compiler                 
     Version:  7.3               |    Keywords:                           
    Testcase:                    |   Blockedby:                           
          Os:  Unknown/Multiple  |    Blocking:                           
Architecture:  Unknown/Multiple  |     Failure:  GHC rejects valid program
---------------------------------+------------------------------------------

Comment(by batterseapower):

 FYI Simon pointed out that my example from 2893 wasn't actually making use
 of the GADT-carried dictionary. Instead, it was just using the top level
 instance decl.

 The story is that GHC can infer polymorphic evidence but not make use of
 it, even with this sort of GADT trick. If you explicitly scrutinise the
 polymorphic GADT at a monomorphic type GHC *can* make use of the evidence,
 however.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/5516#comment:1>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler