LLVM is a big part of it. GCC is more opaque (and monolithic) in comparison. Having an intermediate language and a modular architecture makes it easier to build specialized compilers.
Another selling point is the Clang CFE, which amortizes the complexity of C++ parsing and semantic analysis, much to the chagrin of the Edison Design Group.
Meanwhile EDG are the ones coming up with an usable reflection proposal for C++26, and since Apple and Google stepped out, not many of those profiting from clang are that eager to contribute to upstream fronted.
I was surprised to see they were even still active, much less leading the way on reflection. Hopefully they will manage to stick around.
Anyway, such a pessimistic view of Clang/LLVM is unwarranted IMO. I have yet to see any metrics that imply their abandonment. Also, considering that Google is likely closer to 300 million LOC than 200 million, they really don't have a choice. Likewise for Apple unless they’ve given up on WebKit and LLVM for Swift.
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.
