Absurd "Could not specialise imported function ‘$WSTKScalar’"

Summary

I will try to minimized if asked.

I'm getting

example/MnistFcnnRanked1.hs: warning: [-Wmissed-specialisations]
    Could not specialise imported function ‘HordeAd.Core.TensorKind.$WSTKScalar’
      when specialising ‘HordeAd.Core.OpsAst.$fBaseTensorAstNoSimplify_$s$csdot1’
    Probable fix: add INLINABLE pragma on ‘HordeAd.Core.TensorKind.$WSTKScalar’

where STKScalar is a constructor

data STensorKind y where
  STKScalar :: GoodScalar r
            => STensorKind (TKScalar r)
  ...

and sdot0 is a method in a class (and has no definition in the AstNoSimplify instance mentioned in the message)

  sdot0 :: forall r sh. (GoodScalar r, KnownShS sh)
        => target (TKS sh r) -> target (TKS sh r) -> target (TKS '[] r)
  {-# INLINE sdot0 #-}  -- this doesn't want to specialize
  sdot0 t u | SNat <- shsProduct (knownShS @sh) = ssum (sflatten (t * u))

My guesses what's wrong with the message (which comes from -Wmissed-specialisations, which was very reliable to date):

1. sdot0 has INLINE, so should not be specialized, should it?
2. but if specialization is requested anyway, I don't see a reason it should fail
3. I don't think one can add INLINABLE to a constructor
4. in any case, -fexpose-all-unfoldings is in effect, so INLINABLE should never be needed

Steps to reproduce

git clone git@github.com:Mikolaj/horde-ad.git
cd horde-ad
git checkout repro-spec-scalar
curl https://ghc.gitlab.haskell.org/head.hackage/cabal.project > cabal.project.local
rm -rf ~/.cabal/packages/head.hackage.ghc.haskell.org
cabal update
cabal build exampleLibrary -w /home/mikolaj/r/ghc.HEAD/ghc/_build/stage1/bin/ghc --allow-newer --enable-optimization

Expected behavior

Emits a sensible warning message. Even better, inlines, as requested. Ideally, specializes without inlining (I have -fexpose-all-unfoldings, -fspecialise-aggressively and -fdicts-cheap (and I add -fpolymorphic-specialisation as often as I can) so it doesn't feel right I need to modify code and add dozens of pragmas to specialize things).

Environment

• GHC version used: ghc-9.13.20250212

Optional:

• Operating System: Ubuntu
• System Architecture: AMD64