Panic on COMPLETE pragma with mismatched type variable order

The motivating example is to allow patterns with phantom types to interact nicely with TypeApplications, so in the below we can do L @Int :: a -> L a Int to specify the phantom.

{-# LANGUAGE PatternSynonyms #-}
{-# LANGUAGE ExplicitForAll #-}
data E a b = L' a | R b
pattern L :: forall b a. a -> E a b
pattern L a = L' a
{-# COMPLETE L, R #-}

Issues occur when nesting cases

{-# OPTIONS_GHC -fwarn-incomplete-patterns #-}
testMono :: E (E Int Int) Int -> Int
testMono x = case x of
  L (L _) -> 0
  L (R _) -> 1
  R _ -> 2

And GHC panics when polymorphic

{-# OPTIONS_GHC -fwarn-incomplete-patterns #-}
testPoly :: E (E a b) c -> Int
testPoly x = case x of
  L (L _) -> 0
  L (R _) -> 1
  R _ -> 2

The compiler balks

ghc: panic! (the 'impossible' happened)
  (GHC version 8.2.1 for x86_64-unknown-linux):
	mkOneConFull: Not TyConApp:
  c_a50V
  Call stack:
      CallStack (from HasCallStack):
        prettyCurrentCallStack, called at compiler/utils/Outputable.hs:1133:58 in ghc:Outputable
        callStackDoc, called at compiler/utils/Outputable.hs:1137:37 in ghc:Outputable
        pprPanic, called at compiler/deSugar/Check.hs:976:30 in ghc:Check

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

So it seems like there's two related issues here:

1. The COMPLETE pragma does not work in nesting for out-of-order type variables
2. Some printing logic is missing for the error in this polymorphic case

Note that if we define

pattern L :: forall a b. a -> E a b
pattern L a = L' a

Note that we do not get the incomplete warning or the panic, so it seems directly related to the order of the variables, rather than explicit quantification in general