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 https://gitlab.haskell.org/ghc/ghc/-/issues/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