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The following program typechecks on GHC 8.0.2 through 8.6.3, but panics on HEAD:

{-# LANGUAGE GADTs      #-}
{-# LANGUAGE TypeInType #-}
module Bug where

import Data.Kind

data T f :: f Type -> Type where
  MkT :: T f a

$ ~/Software/ghc4/inplace/bin/ghc-stage2 Bug.hs
[1 of 1] Compiling Bug              ( Bug.hs, Bug.o )
ghc-stage2: panic! (the 'impossible' happened)
  (GHC version 8.7.20190219 for x86_64-unknown-linux):
        piResultTys1
  k_a10k[tau:1]
  [*]
  Call stack:
      CallStack (from HasCallStack):
        callStackDoc, called at compiler/utils/Outputable.hs:1159:37 in ghc:Outputable
        pprPanic, called at compiler/types/Type.hs:1063:5 in ghc:Type