One metric is the amount of red in cppreference. Sometimes one implementation's red box is better than another implementation's green box. Clang supports the

One metric is the amount of red in cppreference. Sometimes one implementation's red box is better than another implementation's green box. Clang supports the majority of C++20. The biggest blocker on coroutines is a Windows ABI issue and concepts will be ready after a few DRs are polished off. Basically, other than some minor CTAD and non-type template parameter enhancements, the biggest feature lagging behind are modules. IMO, I'm not ready for modules, my dependencies are not ready for modules, neither is my IDE nor my build system. So, for my interests, C++20 support in Clang is more than sufficient. Apple and Google are perfectly fine with C++17 for their purposes, ... Well, yes, and they will likely be using C++17 for quite a bit longer. In particular, not only is C++20 a massive update to the language, but the rate of development in the llvm-project has far exceeded the rate at which maintenance capacity can be increased. Not to mention a myriad of other issues, notably the architectural problems in Clang.