Warning on pattern synonyms in ghci

Defining and using a pattern synonym in ghci shows a warning in the development version

ghci> :set -XPatternSynonyms 
ghci> pattern P = True
ghci> P
WARNING: file compiler/GHC/Core/Opt/Simplify/Env.hs, line 686
  $bP
  Call stack:
      CallStack (from HasCallStack):
        callStackDoc, called at compiler/GHC/Driver/Ppr.hs:137:29 in ghc:GHC.Driver.Ppr
        warnPprTrace, called at compiler/GHC/Core/Opt/Simplify/Env.hs:686:31 in ghc:GHC.Core.Opt.Simplify.Env

The warning comes from

refineFromInScope :: InScopeSet -> Var -> Var
refineFromInScope in_scope v
  | isLocalId v = case lookupInScope in_scope v of
                  Just v' -> v'
                  Nothing -> WARN( True, ppr v ) v  -- This is an error!
  | otherwise = v

I have checked that if this WARN is changed to a panic, there are two testsuite failures and both concern defining a pattern synonym in ghci https://gitlab.haskell.org/tweag/ghc/-/jobs/525160. Somehow the in-scope set does not contain the builder and the matcher in that case.