Why Fedora and GCC only?

Fully agreed regarding Gentoo, their Clang compilation efforts, and the contributions from Sam James and others. They have been really helpful, especially with their focus on contributing patches upstream were feasible. It's also great that their approach to detect silent miscompilation is quite different, which adds additional verification.

There must have been similar efforts going on for Homebrew and Macports and the various BSDs, to increase Clang compatibility, but compared to the Gentoo effort, I have seen fewer upstream contributions. Personally, I found that rather disappointing. I'm not aware of the Clang upstream project making similar assessments like we did for GCC regarding overall impact, and taking active steps to manage that. Or Apple when they switched Xcode to more errors well before upstream Clang, as I understand it.

The Clang change, along with Fedora's express desire to offer Clang as a fully supported compiler to package maintainers, certainly provided some justification for tackling these issues, and opened up even some limited additional resources (and every bit helps for this). But were it not for Gentoo's contributions, I think the practical impact of the earlier Clang change would have been pretty limited unfortunately.