Take the following program:

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

data A (a :: k) where
  MkA :: A MkA

On GHC 8.4.2, this is rejected with a sensible error message:

$ /opt/ghc/8.4.2/bin/ghc Bug.hs
[1 of 1] Compiling Bug              ( Bug.hs, Bug.o )

Bug.hs:6:12: error:
    • Data constructor ‘MkA’ cannot be used here
        (it is defined and used in the same recursive group)
    • In the first argument of ‘A’, namely ‘MkA’
      In the type ‘A MkA’
      In the definition of data constructor ‘MkA’
  |
6 |   MkA :: A MkA
  |            ^^^

On GHC HEAD, however, this causes a GHC internal error:

$ ghc2/inplace/bin/ghc-stage2 Bug.hs
[1 of 1] Compiling Bug              ( Bug.hs, Bug.o )

Bug.hs:6:12: error:
    • GHC internal error: ‘MkA’ is not in scope during type checking, but it passed the renamer
      tcl_env of environment: [asv :-> Type variable ‘k’ = k :: *,
                               asw :-> Type variable ‘a’ = a :: k,
                               rqs :-> ATcTyCon A :: forall k. k -> *]
    • In the first argument of ‘A’, namely ‘MkA’
      In the type ‘A MkA’
      In the definition of data constructor ‘MkA’
  |
6 |   MkA :: A MkA
  |            ^^^