atomic_wait() and atomic_notify_{one,all}() are available in C++20, which we can't depend on right now. So we implement our own fallback implementation. This is a simple implementation for systems which don't have either futexes or the C++20 atomic wait API. That means it's not very efficient, just simple. The unit test tests the fallback implementation only: it is not Qt's business to test the Standard Library. The fallback implementation and the Standard Library's are not binary-compatible and cannot be mixed. Callers must ensure that all sides use the same implementation and the easiest way to do that is to only use this in non-inline code, or at worst inline code that isn't shared across libraries/modules/plugins. Change-Id: Ib5ce7a497e034ebabb2cfffd1761a0e497dd17d4 Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>