GHC panic when using DeriveAnyClass with functor-like class and datatype with a type variable in a contravariant position

The following code

{-# LANGUAGE DeriveAnyClass #-}
{-# LANGUAGE KindSignatures #-}
module Example where

class C (a :: * -> *)
data T a = MkT (a -> Int) deriving C

fails to compile with the following GHC panic:

$ /opt/ghc/8.0.1/bin/ghc Example.hs
[1 of 1] Compiling Example          ( Example.hs, Example.o )
ghc: panic! (the 'impossible' happened)
  (GHC version 8.0.1 for x86_64-unknown-linux):
	contravariant

Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug

The culprit is that when DeriveAnyClass comes up with the constraints for typeclass instances whose argument is of kind * -> *, it calls deepSubtypesContaining, which immediately bails out when it sees a type variable in a contravariant or "bad" position. I'm less sure of what the fix should be.

Trac metadata

Trac field	Value
Version	8.0.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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