Dear Cafe, and thanks, Georgi, for answering my previous question here's another difference I noticed (based on real code, but reduced to a minimal test case) GHCi, version 9.0.0.20200925: https://www.haskell.org/ghc/ :? for help Prelude> :set -XRankNTypes Prelude> let f :: (forall a . a -> ()) -> () ; f = g ; g x = undefined <interactive>:2:43: error: • Couldn't match type ‘p0’ with ‘forall a. a -> ()’ Expected: (forall a. a -> ()) -> () Actual: p0 -> () Cannot instantiate unification variable ‘p0’ with a type involving polytypes: forall a. a -> () • In the expression: g In an equation for ‘f’: f = g The declaration is accepted by ghc-8. For ghc-9, I have to give a type signature for g (copy that of f), or replace `f = g` with `f x = g x`. - J.W.