The codebase had quite a few instances of .size() being called and then checked to see if it was 0 or not (or less than 1, or greater-than-or-equal-to 1, which are basically equivalent), when .empty() could be called instead (negated or not, depending on what the original condition was).
This introduces a new mechanism for handling bit flags. Instead of using an int (or two, or three) to write bits into, this uses the std::bitset in a wrapper class that accepts an enum class as input. This means that certain classes of errors we've had a few of in the engine, where a coder used the wrong set of flags or checked them against the wrong variable (SF_STEALTH vs SIF_STEALTH, for example) can now be detected at a compiler level rather than not at all.
MageKing17