getC: Type signature needed when existential types are used but not in the type signature

This module:

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

import Data.Typeable

data ExTypeable = forall a. Typeable a => ExTypeable a

-- unExTypeable :: Typeable h => ExTypeable -> Maybe h
unExTypeable (ExTypeable a) = cast a

with the type signature commented out gives this in the HEAD:

q.hs:10:30:
    Could not deduce (Typeable b) from the context (Typeable a)
      arising from a use of `cast' at q.hs:10:30-35
    Possible fix:
      add (Typeable b) to the context of the constructor `ExTypeable'
    In the expression: cast a
    In the definition of `unExTypeable':
        unExTypeable (ExTypeable a) = cast a

It works in 6.6.

It's part of the getC test.

Trac metadata

Trac field	Value
Version	6.6.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	high
Resolution	Unresolved
Component	Compiler (Type checker)
Test case	getC
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system	Unknown
Architecture	Unknown