"lookupIdSubst" warnings when compiling haddock

I'm getting warnings of the form

| Run Ghc CompileHs Stage1: utils/haddock/haddock-api/src/Haddock/Types.hs => _build/stage1/utils/haddock/build/Haddock/Types.o
...
WARNING: file compiler/GHC/Core/Subst.hs, line 256
  GHC.Core.Subst.lookupIdSubst exprIsConApp1
                               subst_expr $cp1Exception_skvd
  InScope {r_ahOg a_ahOh w1_ahOk $fExceptionHaddockException}
  Call stack:
      CallStack (from HasCallStack):
        callStackDoc, called at compiler/GHC/Utils/Outputable.hs:1282:29 in ghc:GHC.Utils.Outputable
        warnPprTrace, called at compiler/GHC/Core/Subst.hs:255:18 in ghc:GHC.Core.Subst
...

Might not lead to miscompilations, but we should look into where we construct the invalid in-scope set. Judging from the size of the reported in-scope set, this should be relatively easy to pin down once reproduced locally.