Google is _big_ and C++ is used for many projects that build with disparate toolchains (even if they are all based on Clang/LLVM); each project can have different needs to adopt parts of C++20 sooner than the others, and different costs for doing so. Moving from Clang modules to C++ standard modules is an effort that, for some of our projects, lives at the nexus of "Are Standard Modules sufficiently done upstream to be able to replace our use of Clang modules?" and "What is the cost of doing that?" and "What is the benefit of making that switch?" For other projects, adopting standard modules is a more straightforward calculus.
E.g. for standard concepts in libc++:
std::derived_from (D74292) std::convertible_to (D77961) Arithmetic concepts (D88131) std::move_constructible (D96230) std::copy_constructible (D96232) std::invocable (D96235) std::regular_invocable (D96235) etc.
Google is _big_ and C++ is used for many projects that build with disparate toolchains (even if they are all based on Clang/LLVM); each project can have different needs to adopt parts of C++20 sooner than the others, and different costs for doing so. Moving from Clang modules to C++ standard modules is an effort that, for some of our projects, lives at the nexus of "Are Standard Modules sufficiently done upstream to be able to replace our use of Clang modules?" and "What is the cost of doing that?" and "What is the benefit of making that switch?" For other projects, adopting standard modules is a more straightforward calculus.