[Haskell-cafe] Weird issue with ConstraintKinds

Consider the following (minimal, for illustration purposes) code: {-# LANGUAGE ConstraintKinds #-} module Weird where class A a where class B b where class C c where data X a = X a data Y a = Y a -- works fine, but can be verbose when things multiply instance (A a, B a) => C (X a) where -- So use ConstraintKinds: type D a = (A a, B a) instance D a => C (Y a) where ===== and now I get Variable `a' occurs more often than in the instance head in the constraint: D a (Use -XUndecidableInstances to permit this) In the instance declaration for `C (Y a)' Why?? Since D is an abbreviation, why would it behave differently than when I hand expand it? [This is with GHC 7.6.3] Jacques