Core Lint error with representation polymorphism and partial type sigs

The following program slips through the representation polymorphism check and creates a polymorphic binder, causing a panic or a Lint error

{-# LANGUAGE PartialTypeSignatures #-}
module M where

import GHC.Exts

f x = x :: (_ :: (TYPE (_ _)))

I've tried pinpointing the problem. The call to newInferExpTypeFRR in Tc/Utils/Unify.hs creates an ExpType with ir_frr containing the fixed representation constraint. But apparently that information is not acted upon.