Backpack + PolyKinds panic

While working on !4853 (closed) I could trigger the following panic:

• Add {-# LANGUAGE PolyKinds #-} to ./testsuite/tests/backpack/should_compile/bkp40.bkp
• Run ghc-8.10 --backpack bkp40.bkp

With ghc-8.10, one gets:

ghc: panic! (the 'impossible' happened)
  (GHC version 8.10.2:
	flatten_args wandered into deeper water than usual
  Call stack:
      CallStack (from HasCallStack):
        callStackDoc, called at compiler/utils/Outputable.hs:1179:37 in ghc:Outputable
        pprPanic, called at compiler/typecheck/TcFlatten.hs:1029:21 in ghc:TcFlatten

With HEAD one gets:

ghc: panic! (the 'impossible' happened)
  (GHC version 9.1.20210119:
	funResultTy
  Eq (*)
  Call stack:
      CallStack (from HasCallStack):
        callStackDoc, called at compiler/GHC/Utils/Panic.hs:181:37 in ghc:GHC.Utils.Panic
        pprPanic, called at compiler/GHC/Core/Type.hs:1183:49 in ghc:GHC.Core.Type

I am not blocked on this, but obviously it’s a bug.