Panic with bad representation polymorphism + hs-boot

Given

X.hs-boot

{-# LANGUAGE TypeFamilies #-}
module X where
import GHC.Exts
type family T :: RuntimeRep
f :: forall (a :: TYPE T). a

X.hs

module X where
import Y

Y.hs

module Y where
import {-# SOURCE #-} X
g () = f

compiling ghc X crashes:

[2 of 3] Compiling Y                ( Y.hs, Y.o )

<no location info>: error:
    panic! (the 'impossible' happened)
  GHC version 9.7.20230615:
	isUnliftedType
  forall (a :: TYPE T). a :: TYPE T
  Call stack:
      CallStack (from HasCallStack):
        callStackDoc, called at compiler/GHC/Utils/Panic.hs:191:37 in ghc-9.7-inplace:GHC.Utils.Panic
        pprPanic, called at compiler/GHC/Core/Type.hs:2295:22 in ghc-9.7-inplace:GHC.Core.Type
        isUnliftedType, called at compiler/GHC/StgToCmm/Closure.hs:211:5 in ghc-9.7-inplace:GHC.StgToCmm.Closure

I suspect this will be fixed by #22109, when we check that top-level binds are lifted in the typechecker rather than the desugarer.