ConstraintKind inconsistency?

Question

I want to use custom constraints to help curb combinatorial explosion:

{-# LANGUAGE ConstraintKinds #-}
{-# LANGUAGE MultiParamTypeClasses #-}

class DifferentTypes a b
type DifferentTypes3 a b c = (DifferentTypes a b, DifferentTypes b c, DifferentTypes a c)

---

When I use my custom constraint, this code fails to compile:

class Foo a
class Bar a
instance (DifferentTypes3 a b c, Bar a, Bar b, Bar c) => Foo (a, b, c)

---

But when I instead manually write out my constraints, it compiles:

class Foo a
class Bar a
instance (DifferentTypes a b, DifferentTypes b c, DifferentTypes a c, Bar a, Bar b, Bar c) => Foo (a, b, c)

Is this a bug, or am I missing something?

Answer 1

With GHC ticket #8359 fixed, your original code (using ConstraintKinds) now typechecks as expected without UndecidableInstances.