Error GHC-91510: illegal polymorphic type

If you extract it to a type synonym you get a more informative error message: type Foo :: (k -> k -> Type) -> (k -> Type) -> Constraint type Foo cat f = forall x. Objects cat (f x) T2.hs:28:1: error: [GHC-02550] • Quantified predicate must have a class or type variable head: forall (x :: k). Objects cat (f x) • In the quantified constraint ‘forall (x :: k). Objects cat (f x)’ In the type synonym declaration for ‘Foo’ | 28 | type Foo cat f = forall x. Objects cat (f x) | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^