ScopedTypeVariables binds a non-existent variable

GHC accepts this definition:

f :: (forall a. a -> a) -> ()
f (x :: b -> b) = x (undefined :: b) `seq` ()

If a scoped type variable is bound always to a variable, then what variable is b bound to? In the desugared Core, it looks like b is bound to Any. Note that I couldn't find any valid argument for x here other than undefined.

I think this definition should be rejected.

In terms of implementation, the zonkTcTypeToType function should never encounter a unfilled-in SigTv. If it does, error. I don't like putting the error there, but I'm not sure I know another place to do it.

Trac metadata

Trac field	Value
Version	8.5
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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