ASSERT failure with visible dependent quantification

If I say

{-# LANGUAGE ScopedTypeVariables #-}

module Bug where

x :: forall a -> a -> a
x = x

I get

ghc-stage2: panic! (the 'impossible' happened)
  (GHC version 8.11.0.20200108:
	ASSERT failed! file compiler/GHC/Hs/Types.hs, line 970

Of course, my program should be rejected, but not with an ASSERTion failure.

Tagging @RyanGlScott, Mr. VDQ.