Warning for unused existentially quantified types

I found a bug in my code that boils down to:

{-# LANGUAGE ExistentialQuantification #-}

data X = forall a b. X a

Obviously the type 'b' is not used. It didn't hurt, but it could have been, that I intended to do something with the 'b' and then forgot it. I would like to get a warning in this situation like "Unused type variable 'b'".

It might even be, that no existential quantification is needed at all, as in the following case:

data Y = forall a. Y

Trac metadata

Trac field	Value
Version	6.12.3
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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CC	ghc@henning-thielemann.de
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