Oddity with ConstraintKinds and TypeFamilies

See the attached program (not sure how to reduce it further - I don't know whether the relevant interaction is really with !TypeFamilies, Rank2Types, or what).

When compiled with -DWORKAROUND, it compiles, when without, it doesn't. The difference:

#ifndef WORKAROUND
    ap :: (forall a. C e a => a -> r) -> e -> r
#else
    ap :: c ~ C e => (forall a. c a => a -> r) -> e -> r
#endif

(Where 'C e' is an associated type of 'e' of kind * -> Constraint.)

I sort of assumed that 'Some Type' and 'a ~ Some Type => a' would always be equivalent, but maybe I'm wrong. Feel free to mark as invalid if I am.

Trac metadata

Trac field	Value
Version	7.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture