Compiler panic with unused existential type variable and -O2

Summary

The following file makes GHC panic:

-- B.hs
{-# LANGUAGE
  DeriveDataTypeable,
  StandaloneDeriving,
  GADTs,
  ScopedTypeVariables #-}

module B where

import Data.Data (Data(..))

data T where
  C :: forall k. T

deriving instance Data T

Steps to reproduce

ghc -O B.hs

Output:

[1 of 1] Compiling B                ( test/B.hs, test/B.o )
ghc: panic! (the 'impossible' happened)
  (GHC version 8.10.4:
	setBndrsDemandInfo
  [k_a15F]
  Call stack:
      CallStack (from HasCallStack):
        callStackDoc, called at compiler/utils/Outputable.hs:1179:37 in ghc:Outputable
        pprPanic, called at compiler/stranal/DmdAnal.hs:964:28 in ghc:DmdAnal

Expected behavior

Should compile successfully.

Environment

• GHC version used: 8.10, 9.0.1