Unexpected cryptic GADT type error

Consider the following module:

{-# LANGUAGE GADTs #-}

module Bug where

import Control.Monad (void)

data D = D

data G a where
  G :: G Int

ok :: (Functor m, Monad m) => m ()
ok = void $ case D of D -> return "xyz"

bad :: (Functor m, Monad m) => m ()
bad = void $ case G of G -> return "xyz"

Function "ok" is accepted by GHC, but function "bad" triggers a cryptic type error:

Bug.hs:16:24:
    Couldn't match type `a0' with `[Char]'
      `a0' is untouchable
           inside the constraints (Int ~ Int)
           bound at a pattern with constructor
                      G :: G Int,
                    in a case alternative
    In the pattern: G
    In a case alternative: G -> return "xyz"
    In the second argument of `($)', namely
      `case G of { G -> return "xyz" }'

I hope this is a bug in typechecker. Tested on 7.2.2 and 7.3.20111130.

Trac metadata

Trac field	Value
Version	7.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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