[nhc-bugs] Type checker bug

[Simon Marlow]

The following module should elicit a type error:

  type T a =3D Int

  class C a where
   f :: a -> T a
   g :: T a -> a

  instance C Char

  main =3D print (g (f 'a'))

but nhc98 compiles it without complaint.  GHC & Hugs both emit an
unresolved overloading error:

test187.hs:9:
    Ambiguous type variable(s) `a' in the constraint `C a'
    arising from use of `g' at test187.hs:9
    In the first argument of `print', namely `(g (f 'a'))'
    In the definition of `main': print (g (f 'a'))

test187.hs:9:
    Ambiguous type variable(s) `a' in the constraint `Show a'
    arising from use of `print' at test187.hs:9
    In the definition of `main': print (g (f 'a'))

I believe nhc98's Binary library makes use of the same trick in its
definition of the Bin type.  The type synonym in the example should
really be a newtype.

Cheers,
	Simon