Representation-polymorphic unboxed sum causes a panic

The existing representation-polymorphism checks allow some unboxed sums to slip through the cracks:

{-# LANGUAGE MagicHash #-}
{-# LANGUAGE PolyKinds #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE UnboxedTuples #-}

module RepPolySum where

import GHC.Exts

baz :: forall (rep :: RuntimeRep) (a :: TYPE rep). () -> (# Int# | a #)
baz _ = (# 17# | #)

ghc: panic! (the 'impossible' happened)
  (GHC version 9.0.1:
        runtimeRepPrimRep
  typePrimRep (a_aCt :: TYPE rep_aCs)
  rep_aCs
  Call stack:
      CallStack (from HasCallStack):
        callStackDoc, called at compiler\GHC\Utils\Outputable.hs:1230:37 in ghc:GHC.Utils.Outputable
        pprPanic, called at compiler\\GHC\\Types\\RepType.hs:527:5 in ghc:GHC.Types.RepType

(That's with GHC 9.0; the same error exists on HEAD.)

It wouldn't be hard to fix with the existing mechanisms (in the desugarer), but I'm currently fixing it in the typechecker as part of !6164 (closed).