C++26: std::is_within_lifetime
Summary
The article introduces C++26s std::is_within_lifetime, a consteval function to check whether a pointer points to an object within its lifetime during constant evaluation. It explains its primary use for determining the active union member, discusses design decisions such as being consteval-only and taking a pointer, and covers the original motivation from implementing a compact Optional<bool}, noting that compiler support as of February 2026 is still forthcoming.