Panic in occurence analysis phase (?), getRuntimeRep

Compiling this:

{-# LANGUAGE DataKinds, GADTs, KindSignatures #-}
module Holo ()
where

data ADT (p :: Integer) where
  ADT ::
    { a :: a
    , b :: Integer
    } -> ADT p

foo = undefined {b=undefined}

1. .yields:

ghc: panic! (the 'impossible' happened)
  (GHC version 8.4.3 for x86_64-unknown-linux):
        getRuntimeRep
  p_a29z :: Integer
  Call stack:
      CallStack (from HasCallStack):
        callStackDoc, called at compiler/utils/Outputable.hs:1150:37 in ghc:Outputable
        pprPanic, called at compiler/types/Type.hs:1967:18 in ghc:Type

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

-v4 suggests this happend during the occurence analysis phase (log attached).

Trac metadata

Trac field	Value
Version	8.4.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	high
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC	goldfire, simonpj
Operating system	Linux
Architecture