Cannot create type synonym for quantified constraint without ImpredicativeTypes

{-# LANGUAGE KindSignatures, RankNTypes, ConstraintKinds, QuantifiedConstraints, FlexibleInstances, UndecidableInstances #-}
module Bug where

type F1 (f :: * -> *) = forall a. Eq (f a)
class (Functor f, F1 f) => C f
instance (Functor f, F1 f) => C f
type F2 f = (Functor f, F1 f)

Bug.hs:7:1: error:
    • Illegal polymorphic type: F1 f
      GHC doesn't yet support impredicative polymorphism
    • In the type synonym declaration for ‘F2’
  |
7 | type F2 f = (Functor f, F1 f)
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

(GHC accepts the program with ImpredicativeTypes.)

(Functor f, F1 f) is allowed as a superclass constraint, and as an instance constraint, but a type synonym cannot be made for it.

Not sure if this really counts as a bug ("just switch on ImpredicativeTypes"), but I think it's worth discussing. I prefer to keep ImpredicativeTypes switched off, but if something can be a constraint, shouldn't I be able to create a type synonym of it?

Trac metadata

Trac field	Value
Version	8.6.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture