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

[igloo]

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.

[simonpj]

Good report. It was a real bug, due to the (fairly new) implication constraints. I think I have fixed it, and I've added a test.

Simon

Tue Jun 19 17:26:13 BST 2007  simonpj@microsoft.com
  * Fix a bug in the handling of implication constraints (Trac #1430)