The simplest program

The current HEAD is not able to compile the simplest of all Haskell 
programs... but I don't think the bug affects anyone.
Compiling

> module Main where
> main = return ()

yields

Main.hs:1:
    Ambiguous type variable `m' in these top-level constraint
      `Monad m' arising from use of `return' at Main.hs:3
    Possible cause: the monomorphism restriction applied to the 
following:
      main :: m () (bound at Main.hs:3)
    Probable fix: give these definition(s) an explicit type signature

Giving the type signature helps, of course. The same program is 
accepted by ghc-5.04.2.

Cheers,

Wolfgang