Related issue: #4062.

This fails with a -1 index as noted in #4062. Also, a vector is not any more compact than a stack array because a vector is just an array with more c++ isms around it

The old code caused an illegal memory r/w operation with index -1, while the new code raises an exception. That's a clear improvement, even if it does not fix your issue.

And std::vector<bool> "is a possibly space-efficient specialization of std::vector for the type bool", see documentation.

it only throws an exception if you use c++ exceptions which are often disabled due to perf and general usability problems (which is why we do it). A bool vector could be a bitmask underneath, but if it is, you take a perf hit instead - probably not a great tradeoff for a small stack var - especially since the compiler can optimize bool[] any way it sees fit