GHC panic! when unpacking a GADT in GHC >=9.6

Summary

GHC version >=9.6 incorrectly unpack GADTs, leading to some variant of the following GHC panic!

<no location info>: error:
    panic! (the 'impossible' happened)
  GHC version 9.12.1:
        refineFromInScope
  InScope {wild_00 $krep_ayi $krep_ayj $krep_ayk $krep_ayl $krep_aym
           $krep_ayn $krep_ayo $krep_ayp $krep_ayq $krep_ayr $krep_ays
           $krep_ayt $krep_ayu $krep_ayv boom $tc'Int $tc'Word $tcIntOrWord
           $tc'WrapIntOrWord $tcWrapIntOrWord $trModule boom_syA boom_syB
           $trModule_syC $trModule_syD $trModule_syE $trModule_syF $krep_syG
           $tcIntOrWord_syH $tcIntOrWord_syI $krep_syJ $krep_syK $tc'Int_syL
           $tc'Int_syM $krep_syN $tc'Word_syO $tc'Word_syP
           $tcWrapIntOrWord_syQ $tcWrapIntOrWord_syR $krep_syS
           $tc'WrapIntOrWord_syT $tc'WrapIntOrWord_syU}
  unbx_axs
  Call stack:
      CallStack (from HasCallStack):
        callStackDoc, called at compiler/GHC/Utils/Panic.hs:190:37 in ghc-9.12.1-623c:GHC.Utils.Panic
        pprPanic, called at compiler/GHC/Core/Opt/Simplify/Env.hs:960:30 in ghc-9.12.1-623c:GHC.Core.Opt.Simplify.Env

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

Steps to reproduce

Create MyLib.hs with the following contents and compile it with optimisations, i.e., with ghc -O MyLib.hs.

-- file: MyLib.hs
{-# LANGUAGE KindSignatures #-}
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE GADTs #-}

module MyLib where

data IntOrWord (isInt :: Bool) where
    Int :: !Int -> IntOrWord True
    Word :: !Word -> IntOrWord False

data WrapIntOrWord (isInt :: Bool)
    = WrapIntOrWord {-# UNPACK #-} !(IntOrWord isInt)

boom :: WrapIntOrWord True
boom = WrapIntOrWord (Int 1)

Expected behavior

I expect GHC to ignore the unusable UNPACK pragma and emit a warning, as GHC 9.4.8 did.

src/MyLib.hs:12:7: warning:
    • Ignoring unusable UNPACK pragma
        on the first argument of ‘WrapIntOrWord’
    • In the definition of data constructor ‘WrapIntOrWord’
      In the data type declaration for ‘WrapIntOrWord’
   |
12 |     = WrapIntOrWord {-# UNPACK #-} !(IntOrWord isInt)
   |       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Environment

GHC 9.4.8 has the correct behaviour (ignore the UNPACK pragma, emit a warning).

GHC versions 9.6.6, 9.8.4, 9.10.1, and 9.12.1 panic.

Optional:

• Operating System: macOS 10.15
• System Architecture: aarch64