ImpredicativeTypes and ConstraintKinds don't interact as expected

Given this code:

{-# LANGUAGE MultiParamTypeClasses, FlexibleInstances, ImpredicativeTypes, ConstraintKinds #-}

class P a b
instance P a b

type Test a = forall b. P a b

I would expect to be able to actually use the Test Constraint in a function, like so:

foo :: Test a => a -> a
foo = id

But GHC complains that:

    Illegal polymorphic or qualified type: Test a
    In the type signature for ‘foo’: foo :: Test a => a -> a

I get the exact same error message if I replace ImpredicativeTypes with RankNTypes. If this is intended behavior under ImpredicativeTypes, how difficult would it be to add another extension that allows this type of polymorphism?

Trac metadata

Trac field	Value
Version	7.8.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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