[Note: What I originally wrote in my top-level comment was inaccurate; I edited that comment, but later posted another update: https://news.ycombinator.com/item

[Note: What I originally wrote in my top-level comment was inaccurate; I edited that comment, but later posted another update: https://news.ycombinator.com/item?id=33773043#33775630 https://news.ycombinator.com/item?id=33773043#33775630.] The purpose of this particular naming rule is to allow adding new typedefs such as int128_t. The "undefined behaviour" part is for declaration of any reserved identifier (not specifically for this naming rule). I don't know why the standard uses "undefined behaviour" instead of the other classes (https://en.cppreference.com/w/cpp/language/ub); https://en.cppreference.com/w/cpp/language/ub); I suspect because it gives compilers the most flexibility.