using GADT syntax for an associated datatype causes GHC to panic

I am aware that this is probably not supposed to be supported. But even if using a GADT as an associated datatype instance does not directly work, it would be nice if

• the GADT syntax was supported for "normal"

  associated datatypes,

• the error message was more informative.

Example program:

{-# LANGUAGE TypeFamilies, GADTs #-}

class X a where
  data T a

instance X Int where
  data T Int where
    C :: T Int

Error message:

ghc-6.9.20080615: panic! (the 'impossible' happened)
  (GHC version 6.9.20080615 for x86_64-unknown-linux):
        Maybe.fromJust: Nothing

Occurs with recent snapshot, but also with 6.8.2 and 6.8.3.

Andres

Trac metadata

Trac field	Value
Version	6.9
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
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Operating system	Unknown
Architecture	Unknown