HEAD crash with "Template variable unbound in rewrite rule"

Edit: a greatly minimized version is at https://github.com/sheaf/ghc-26682 and a slightly minimized version is on branch repro-UNPACK2.

Summary

I'm getting many alike errors, the last of which reads

[11 of 12] Compiling TestMnistRNNS    ( test/simplified/TestMnistRNNS.hs, dist-newstyle/build/x86_64-linux/ghc-9.15.20251212/horde-ad-0.3.0.0/l/testCommonLibrary/build/testCommonLibrary/TestMnistRNNS.o, dist-newstyle/build/x86_64-linux/ghc-9.15.20251212/horde-ad-0.3.0.0/l/testCommonLibrary/build/testCommonLibrary/TestMnistRNNS.dyn_o )
<no location info>: error:
    panic! (the 'impossible' happened)
  GHC version 9.15.20251212:
	Template variable unbound in rewrite rule
  Variable: $dKnownShS_s1cQdq :: ShS
                                   ('(:) @Nat width_a1cMgj ('(:) @Nat SizeMnistHeight ('[] @Nat)))
  Rule bndrs: [width_a1cMgj, $dKnownShS_s1cQdq, $dKnownShS_s1cTVC,
               $dNumScalar_s1cTVB, $dBaseTensor_s1cTVA]
  LHS args: [TYPE: '(:)
                     @Nat width_a1cMgj ('(:) @Nat SizeMnistHeight ('[] @Nat)),
             TYPE: Double, TYPE: Concrete, $dKnownShS_s1cTVC,
             $dNumScalar_s1cTVB, $dBaseTensor_s1cTVA]
  Actual args: [TYPE: '(:)
                        @Nat width_a1cMvH ('(:) @Nat SizeMnistHeight ('[] @Nat)),
                TYPE: Double, TYPE: Concrete,
                C:KnownShS
                  @('(:) @Nat width_a1cMvH ('(:) @Nat SizeMnistHeight ('[] @Nat)))
                  $dKnownShS_s1cQ5E,
                $dNumScalar_s1cPQO, $fBaseTensorConcrete, range_s1cVXr,
                lvl_s1cVXq
                `cast` (Sym N:StdGen :: (SMGen :: Type) ~R# (StdGen :: Type))]
  Call stack:
      CallStack (from HasCallStack):
        pprPanic, called at compiler/GHC/Core/Rules.hs:794:10 in ghc-9.15-inplace:GHC.Core.Rules

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

Interestingly, removing the only UNPACK pragma from package ox-arrays is a workaround for this issue (a workaround which I use successfully for some days now, but it lowers performance of horde-ad a bit, unfortunately). Here's how the UNPACK looks like (it unpacks a SNat; I hope this does not interfere with how typing plugins work).

  ConsKnown :: forall n sh i. {-# UNPACK #-} SNat n -> ListH sh i -> ListH (Just n : sh) i

BTW, the error message looks similar to the one in #13882.

Steps to reproduce

git clone git@github.com:Mikolaj/horde-ad.git
cd horde-ad
git checkout repro-UNPACK
cabal update
cabal build testCommonLibrary --enable-optimization --allow-newer -w /home/mikolaj/r/ghc.Andreas3/ghc/_build/stage1/bin/ghc --constraint=ghc-typelits-natnormalise==0.9.1

This is not minimized in any way so far, except that -fpolymorphic-specialisation is removed and it still repros.

Expected behavior

Compiles.

Environment

• GHC version used: 9.15.20251212 and others

Optional:

• Operating System: Ubuntu
• System Architecture: amd64