Improve error message for GADT failures

If you write

 type family Fam a :: *

 data GADT :: * -> * where
   GADT :: a -> Fam a -> GADT (Fam a)

 unwrap :: GADT (Fam a) -> (a, Fam a)
 unwrap (GADT x y) = (x, y)

then typechecking unwrap should certainly fail. And it does, but with a horrible message:

 Main.hs:9:21:
   Couldn't match expected type `a' against inferred type `a1'
     `a' is a rigid type variable bound by
         the type signature for `unwrap' at Main.hs:8:20
     `a1' is a rigid type variable bound by
          the constructor `GADT' at Main.hs:9:8
   In the expression: x
   In the expression: (x, y)
   In the definition of `unwrap': unwrap (GADT x y) = (x, y)

It would be better to say something more like:

	Cannot deduce (a ~ a1) from (Fam a ~ Fam a1)

See the thread at http://thread.gmane.org/gmane.comp.lang.haskell.cafe/62322

Trac metadata

Trac field	Value
Version	6.10.4
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
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