Type families, `TypeError`, and `-fdefer-type-errors` cause "opt_univ fell into a hole"

If I create a type family – open or closed – with a case that evaluates to a TypeError, and define a top-level binding with this type, loading the file with -fdefer-type-errors enabled (or via :load!/:reload!) panics GHC with "opt_univ fell into a hole". (And if I used :load! or :reload!, -fdefer-type-errors doesn't get unset.)

A minimal example:

{-# LANGUAGE TypeFamilies, DataKinds, UndecidableInstances #-}

module T12104 where

import GHC.TypeLits

type family F a where
  F a = TypeError (Text "error")

err :: F ()
err = ()

results in the panic

….hs:9:7: warning: [-Wdeferred-type-errors]
    • error
    • In the expression: ()
      In an equation for ‘err’: err = ()
ghc: panic! (the 'impossible' happened)
  (GHC version 8.0.1 for x86_64-apple-darwin):
	opt_univ fell into a hole {a4Va}

Adding more cases to the type family, or making it open, still cause the crash. This holds whether the error case is a final catch-all case, or something more like

type family F a where
  F () = TypeError (Text "error")
  F a  = ()

Just using a type synonym for F doesn't cause a panic, however, and nor does giving err the type TypeError (Text "error") directly.