GHC panic: tcLookupGlobalOnly

{-# LANGUAGE ExplicitForAll, KindSignatures, StandaloneKindSignatures,
             DataKinds, GADTs #-}

module T18725 where

import Data.Kind (Type)

type U :: Type
data U where P :: forall u. E u -> U
data E (u :: U)

This program results in a panic with GHC HEAD:

GHCi, version 9.1.0.20200918: https://www.haskell.org/ghc/  :? for help
Prelude> :load T18725.hs
[1 of 1] Compiling T18725           ( T18725.hs, interpreted )
ghc: panic! (the 'impossible' happened)
  (GHC version 9.1.0.20200918:
        tcLookupGlobalOnly
  U
  Call stack:
      CallStack (from HasCallStack):
        callStackDoc, called at compiler/GHC/Utils/Panic.hs:185:37 in ghc:GHC.Utils.Pa
nic
        pprPanic, called at compiler/GHC/Tc/Utils/Env.hs:256:35 in ghc:GHC.Tc.Utils.En
v

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

However, it gives a reasonable error message with GHC 8.10:

GHCi, version 8.10.1: https://www.haskell.org/ghc/  :? for help
Prelude> :load T18725.hs
[1 of 1] Compiling T18725           ( T18725.hs, interpreted )

T18725.hs:10:14: error:
    • Type constructor ‘U’ cannot be used here
        (it is defined and used in the same recursive group)
    • In the kind ‘U’
   |
10 | data E (u :: U)
   |              ^
Failed, no modules loaded.

The regression appears related to StandaloneKindSignatures, as removing the type U :: Type line makes the issue go away.