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This bug is a variant of #19628 (closed).

To reproduce, load the following file into ghci:

{-# LANGUAGE UnliftedDatatypes #-}

import GHC.Exts

type Strict :: forall r. * -> TYPE ('BoxedRep r)
data Strict a where
  Force :: a -> Strict @r a

x :: Int
x = 10
  where
    test :: Strict @Unlifted Int
    test = Force undefined

and evaluate x:

ghci> x
Segmentation fault (core dumped)

The code is the same as in #19628 (closed), except that I defined Force to be levity-polymorphic (and instantiated to Unlifted at the call site).

I believe the problem is in schemeT:

                if isUnliftedTypeKind (tyConResKind (dataConTyCon con))
                then RETURN_UNLIFTED P
                else ENTER)

This attempts to check for liftedness by looking at the type of the datacon, but this is not enough: the kind is TYPE (BoxedRep r), and isUnliftedTypeKind conservatively returns False. The correct thing to do would be to check arguments, but in STG the type arguments are erased. So I'm not sure how to proceed.