update: thanks to Asumu’s help, I was able to downgrade to v8.3 and am no longer blocked by this change. I’m still curious about the change here (looks like @mflatt wrote it?) — happy to follow up offline in a less-urgent manner :slightly_smiling_face:

What would I run to see the error? I haven’t been able to piece it together from the discussion.

I wonder whether some lexer maybe fails to obey the lexer/c contract, which requires the second result value to be a symbol. I think the contract is not checked, and probably a non-symbol result was formerly passed through. If that’s it, then module-lexer could be changed to invert its test and use hash? to detect a new-style result instead of symbol? to detect an old-style result. (Or maybe it’s just some bug due to changes in module-lexer.)

I’ll email you the files.