Recursive superclass check is defeated by quantified constraints

This is accepted:

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

module Bug where

import Data.Kind

class ((() :: Constraint) => C a) => C a

The vacuous constraint () hides the recursive superclass C a. Note that I do not have RecursiveSuperclasses enabled.

The culprit is TcTyDecls.checkClassCycles.

This is not a "real" bug report, in that I found the problem only by reading the source code, not by observing the misbehavior. Still, it shouldn't be hard to fix. Should we just look through quantified constraints always?