std::memory_order is an unscoped enum, and so does not need its members to be prefixed with std::memory_order::, just std::. This used to work, but in C++20 it no longer does. Use the standard way to name these constants, which works in both C++17 and C++20. Reviewed-by: Rafael Ávila de Espíndola <espindola@scylladb.com> Message-Id: <20200512092408.115649-1-avi@scylladb.com>