It's part of C++23, in form of an attribute https://en.cppreference.com/w/cpp/language/attributes/assume https://en.cppreference.com/w/cpp/language/attributes/a

It's part of C++23, in form of an attribute https://en.cppreference.com/w/cpp/language/attributes/assume https://en.cppreference.com/w/cpp/language/attributes/assume.