Failed kind inference in class declaration (regression error)

[goldfire]

Consider the following code:

{-# LANGUAGE MultiParamTypeClasses
 #-}

class Foo a b

class Foo a b => Bar a b where
    baz :: a b

This compiles fine in 7.2.1 but fails in 7.4.1-rc1, with the following error:

Sandbox.hs:7:12:
    `a' is applied to too many type arguments
    In the type `a b'
    In the class declaration for `Bar'

[dreixel]

This is not really a regression; 7.2 was too liberal. The Haskell report specifies that unconstrained type variables should default to kind *. So Foo now correctly gets the kind * -> * -> Constraint, which means you cannot later use a with kind * -> *. If you know that a should have kind * -> *, then you should declare Foo as:

class Foo (a :: * -> *) b