Can't put quantified constraint in type synonym

Summary

Can't put a quantified constraint in a type synonym.

Steps to reproduce

Put the following into a file test.hs, then run ghci test.hs:

{-# LANGUAGE
    RankNTypes,
    QuantifiedConstraints,
    ConstraintKinds #-}

module Foo where

type Foo f = (forall s. Semigroup s => Semigroup (f s))

You get the error:

[1 of 1] Compiling Foo              ( test.hs, interpreted )

test.hs:8:40: error:
    • Expected a type, but ‘Semigroup (f s)’ has kind ‘Constraint’
    • In the type ‘(forall s. Semigroup s => Semigroup (f s))’
      In the type declaration for ‘Foo’
  |
8 | type Foo f = (forall s. Semigroup s => Semigroup (f s))
  |                                        ^^^^^^^^^^^^^^^
Failed, no modules loaded.

Expected behavior

It should compile, and the type synonym Foo should have kind Foo :: (* -> *) -> Constraint.

Environment

• GHC version used: 8.6.5