> parse with the lalr generated parser, then if that rejects your string because the program was ill formed, call the hand rolled one for guesswork/diagnostics

This feels like a recipe for disaster. If the hand-rolled parser won't match a formal grammar, why would it match the generated parser?

The poor programmer will be debugging the wrong thing.

It reminds me of my short stint writing C++ where I'd read undefined memory in release mode, but when I ran it under debug mode it just worked.

> It reminds me of my short stint writing C++ where I'd read undefined memory in release mode, but when I ran it under debug mode it just worked.

I assume it’s far too late at this point, but that almost always means that you’re invoking UB. Your next step should be enabling UBSan.

The generated parser will match the grammar.

The hand rolled parser might do, but also might not, what with software being difficult and testing being boring and so forth.

There's risk, but it seems like you could run both parsers against the same unit tests to help mitigate.