Assertion failure with ExistentialQuantification and ApplicativeDo

{-# LANGUAGE ExistentialQuantification, ApplicativeDo #-}

module Bug where

data T f = forall a. MkT (f a)

runf :: forall f. Functor f => IO (T f)
runf = do
    return ()
    MkT fa <- runf
    return $ MkT fa

Bug.hs:11:18: error:ghc: panic! (the 'impossible' happened)
  (GHC version 8.6.3 for x86_64-unknown-linux):
	No skolem info:
  [a_a1nb[ssk:2]]
  Call stack:
      CallStack (from HasCallStack):
        callStackDoc, called at compiler/utils/Outputable.hs:1160:37 in ghc:Outputable
        pprPanic, called at compiler/typecheck/TcErrors.hs:2891:5 in ghc:TcErrors

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