DefaultSignatures and MultiParamTypeClasses

[Reiner Pope]

Hi all,

I just tried, and it appears that the new DefaultSignatures extension doesn't work with multi parameter type classes. For example, when I compile this file:

----
{-# LANGUAGE DefaultSignatures, MultiParamTypeClasses #-}
module Test where

class C a b where
  conv :: a -> b

  default conv :: D a b => a -> b
  conv = dconv

class D a b where
  dconv :: a -> b
-----

I get the error message:

-----
/tmp/Test.hs:4:1:
    The multi-parameter class `C' cannot have generic methods
    In the class declaration for `C'
Failed, modules loaded: none.
-----

Is there a reason for this restriction, or is it merely an accident?

Cheers,
Reiner