Foo.hs

-- this is the input file we're processing
module Foo where

{-# ANN module () #-}

TcSplice.hs

-- ... many things ...

convertAnnotationWrapper :: ForeignHValue -> TcM (Either MsgDoc Serialized)
convertAnnotationWrapper fhv = do
  liftIO (putStrLn "§§§ in convertAnnotationWrapper")
  dflags <- getDynFlags
  liftIO (putStrLn "§§§ got the dynflag")
  if gopt Opt_ExternalInterpreter dflags
    then do
      liftIO (putStrLn "§§§ we have an external interpreter") >> (Right <$> runTH THAnnWrapper fhv)
    else do
      liftIO (putStrLn "§§§ no external interpreter!")
      annotation_wrapper <- liftIO $ wormhole dflags fhv
      liftIO (putStrLn "§§§ after wormhole, about to return")
      let serialized_data = case unsafeCoerce# annotation_wrapper of
           AnnotationWrapper value | let serialized = toSerialized serializeWithData value ->
               -- Got the value and dictionaries: build the serialized value and                                                        
               -- call it a day. We ensure that we seq the entire serialized value                                                      
               -- in order that any errors in the user-written code for the                                                             
               -- annotation are exposed at this point.  This is also why we are                                                        
               -- doing all this stuff inside the context of runMeta: it has the                                                        
               -- facilities to deal with user error in a meta-level expression                                                         
               seqSerialized serialized `seq` serialized
      liftIO (evaluate serialized_data)
      liftIO (putStrLn "§§§ unsafeCoerce# went fine, leaving convertAnnotationWrapper")
      return $ Right serialized_data
 
 -- ... many other things ...

trace

[... a lot of things before ...]
§§§ in convertAnnotationWrapper
§§§ got the dynflag
§§§ no external interpreter!
§§§ after wormhole, about to return
Segmentation fault (core dumped)