Why isn't this Overlapping?

[Anthony Clayden]

--ghc 7.10 or 8.0.1

{-# LANGUAGE DataKinds, KindSignatures, GADTs,
                          MultiParamTypeClasses,
FunctionalDependencies, FlexibleInstances,
                          UndecidableInstances,
NoOverlappingInstances   #-}

class TypeEq a a' (b :: Bool) | a a' -> b

instance (b ~ True) => TypeEq a a b
instance (b ~ False) => TypeEq a a' b

Those two instance heads are nearly identical, surely they
overlap?
And for a type-level type equality test, they must be
unifiable.
But GHC doesn't complain.

If I take off the FunDep, then GHC complains.

AFAICT none of those extensions imply Overlaps,
but to be sure I've put NoOverlapping.


AntC