bug: strange type class member accepted

[Bernard James POPE]

Hi,

It appears that GHC 6.0 and 5.04.3 accept this code:

   module Main where

   class Foo a where
      bug :: ()

   -- main is not involved in the problem, just here for completeness
   main :: IO ()
   main = undefined

However, "bug" violates the requirement in the Report:

   section 4.3.1 Class Declarations

   class cx => C u where cdecls

   The type of the top-level class method vi is:
   vi :: forall u,w. (C u, cxi) =>ti
   The ti must mention u; 
          ^^^^

The type of "bug" does not mention the variable "a".

ghc/ghci accepts this code without error, or even warning, and it is even possible
to make instances of this class.

However, there does not seem to be any way to use "bug" without getting
a type error, so I doubt this is a big problem in practice. 

For comparison hugs reports a type error message.

Cheers,
Bernie.