[GHC] #3440: Improve error message for GADT failures

[GHC]

#3440: Improve error message for GADT failures
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    Reporter:  simonpj                  |        Owner:              
        Type:  bug                      |       Status:  new         
    Priority:  low                      |    Milestone:  7.0.1       
   Component:  Compiler (Type checker)  |      Version:  6.10.4      
    Keywords:                           |     Testcase:              
   Blockedby:                           |   Difficulty:  Unknown     
          Os:  Unknown/Multiple         |     Blocking:              
Architecture:  Unknown/Multiple         |      Failure:  None/Unknown
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Changes (by michalt):

 * cc: michal.terepeta@… (added)


Comment:

 Reproducible with current HEAD (7.1.20101029):
 {{{
 Test.hs:12:22:
     Couldn't match type `a1' with `a'
       `a1' is a rigid type variable bound by
            a pattern with constructor
            GADT :: forall a. a -> Fam a -> GADT (Fam a)
              at Test.hs:12:9
       `a' is a rigid type variable bound by
           the type signature for `unwrap' at Test.hs:11:21
     In the expression: x
     In the expression: (x, y)
     In an equation for `unwrap': unwrap (GADT x y) = (x, y)
 }}}

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/3440#comment:4>
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