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 aThe 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?