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When we initially implemented the C++20 synchronization library, we reluctantly accepted for the implementation to be backported to C++03 upon request from the person who provided the patch. This was when we were only starting to have experience with the issues this can create, so we flinched. Nowadays, we have a much stricter stance about not backporting features to previous standards. We have recently started fixing several bugs (and near bugs) in our implementation of the synchronization library. A recurring theme during these reviews has been how difficult to understand the current code is, and upon inspection it becomes clear that being able to use a few recent C++ features (in particular lambdas) would help a great deal. The code would still be pretty intricate, but it would be a lot easier to reason about the flow of callbacks through things like __thread_poll_with_backoff. As a result, this patch drops support for the synchronization library before C++20. This makes us more strictly conforming and opens the door to major simplifications, in particular around atomic_wait which was supported all the way to C++03. This change will probably have some impact on downstream users, however since the C++20 synchronization library was added only in LLVM 10 (~3 years ago) and it's quite a niche feature, the set of people trying to use this part of the library before C++20 should be reasonably small.