mkGADTVars panic with a GADT data family instance and unused variable in forall

{-# LANGUAGE DataKinds                  #-}
{-# LANGUAGE GADTs                      #-}
{-# LANGUAGE PolyKinds                  #-}
{-# LANGUAGE ScopedTypeVariables        #-}
{-# LANGUAGE StandaloneKindSignatures   #-}
{-# LANGUAGE TypeApplications           #-}
{-# LANGUAGE TypeFamilies               #-}

module T23784 where

import Data.Kind

type Sing :: forall k. k -> Type
data family Sing a

data instance forall a b. Sing a where
  S :: Sing '()

<no location info>: error:
    panic! (the 'impossible' happened)
  GHC version 9.9.20230724:
        mkGADTVars
  [k_awT[sk:1], b_awF[sk:1], k_awU[sk:1], a_awE[sk:1]]
  <InScope = {a_awE[sk:1] k_awU[sk:1]}
   IdSubst   = []
   TvSubst   = [awE :-> '(), awU :-> ()]
   CvSubst   = []>
  Call stack:
      CallStack (from HasCallStack):
        callStackDoc, called at compiler\GHC\Utils\Panic.hs:191:37 in ghc-9.9-inplace:GHC.Utils.Panic
        pprPanic, called at compiler\GHC\Tc\TyCl.hs:4244:9 in ghc-9.9-inplace:GHC.Tc.TyCl
  CallStack (from HasCallStack):
    panic, called at compiler\GHC\Utils\Error.hs:503:29 in ghc-9.9-inplace:GHC.Utils.Error


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

I think this is to do with the way we create a representation TyCon for a data family instance using the full set of bound variables in the forall (see the use of ty_binders in mkAlgTyCon in GHC.Tc.TyCl.Instance.tcDataFamInstDecl and Note [Data type families] in GHC.Core.TyCon).

I ran into this when working on error messages for unused type variables in family instances in !11032 (closed). I will investigate, but pinging @rae as the authority on this code (in case anything obvious springs to mind).