Infinite loop/panic with an existential type.

The symptoms of this bug are quite similar to #14723 (closed), but I don't know if the cause is exactly the same, ergo a new report.

To reproduce:

1. Make T.hs

{-# LANGUAGE DataKinds, ExistentialQuantification, GADTs, PolyKinds, TypeOperators #-}
module T where

import Data.Kind

data Elem :: k -> [k] -> Type where
  Here :: Elem x (x : xs)
  There :: Elem x xs -> Elem x (y : xs)

data EntryOfVal (v :: Type) (kvs :: [Type]) = forall (k :: Type). EntryOfVal (Elem (k, v) kvs)

1. Compile it with ghc T.hs -ddump-tc-trace

# etc.
checkExpectedKind
  *
  TYPE t_aXd[tau:1]
  <*>_N
kcLHsQTyVars: not-cusk
  EntryOfVal
  []
  [(k_aW0 :: Type)]
  []
  [k_aW0[sk:1]]
  *ghc: panic! (the 'impossible' happened)
  (GHC version 8.4.3 for x86_64-apple-darwin):
	kcConDecl

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

1. Append the following to T.hs (not minimized, sorry)

type family EntryOfValKey (eov :: EntryOfVal v kvs) :: Type where
  EntryOfValKey ('EntryOfVal (_ :: Elem (k, v) kvs)) = k
type family GetEntryOfVal (eov :: EntryOfVal v kvs) :: Elem (EntryOfValKey eov, v) kvs where
  GetEntryOfVal ('EntryOfVal e) = e

type family FirstEntryOfVal (v :: Type) (kvs :: [Type]) :: EntryOfVal v kvs where
  FirstEntryOfVal v ((k, v) : _) = 'EntryOfVal Here
  FirstEntryOfVal v (_ : kvs) = 'EntryOfVal (There (GetEntryOfVal (FirstEntryOfVal v kvs)))

1. Compile with a plain ghc T.hs
2. Wait until bored, then knock the compiler out of its infinite loop by killing it.
3. Compile again with ghc T.hs -ddump-tc-trace

# etc.
checkExpectedKind
  *
  TYPE t_aYg[tau:1]
  <*>_N
kcLHsQTyVars: not-cuskghc: panic! (the 'impossible' happened)
  (GHC version 8.4.3 for x86_64-apple-darwin):
	kcConDecl

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

Found while trying to answer this SO question.

Trac metadata

Trac field	Value
Version	8.4.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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