Boost C++ Libraries

...one of the most highly regarded and expertly designed C++ library projects in the world. — Herb Sutter and Andrei Alexandrescu, C++ Coding Standards

This is an older version of Boost and was released in 2020. The current version is 1.91.0.

test::handler::handler (3 of 4 overloads)

handler(
    boost::none_t);