std::ignore's behavior outside of use with std::tie is not specified in any finalized standard. https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2023/p2968r

std::ignore's behavior outside of use with std::tie is not specified in any finalized standard. https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2023/p2968r2.html https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2023/p29... aims to address that, but that won't be included until C++26 (which also includes _ as a sibling commenter mentions).