[P1020R1](https://wg21.link/P1020R1) Smart Pointer Creation With Default Initialization [P1973R1](https://wg21.link/P1973R1) Renaming `default_init` To `for_overwrite` Final feature-test macro in WG21-N4861: `#define __cpp_lib_smart_ptr_for_overwrite 202002L`