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The following code works in GHC 6.8.2:

{-# OPTIONS -fglasgow-exts #-}
module Test where

data Existential = forall a. Ex (GADT a)

data GADT :: * -> * where
        GCon :: Int -> GADT ()

-- g :: Existential -> Int
g (Ex (GCon x)) = x

The compiler correctly infers the type of x in the definition of g, and correctly infers the type of g.

In GHC 6.10.1, however, this fails with:

Test.hs:10:12:
    GADT pattern match with non-rigid result type `t'
      Solution: add a type signature
    In the definition of `g': g (Ex (GCon x)) = x
Failed, modules loaded: none.

The solution, to include the type signature of g (that I have commented out), works, but why can't GHC figure out the type of g? GHC 6.8.2 could!

An equivalent version with a non-GADT instead, works correctly:

data GADT a = GCon Int

Ticket #2206 (closed) might be related.

Trac metadata

Trac field	Value
Version	6.10.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
Test case
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