[clang] [C++20][Modules] Implement P1857R3 Modules Dependency Discovery (PR #107168)

[via cfe-commits]

yronglin wrote:

err_invalid_module_or_import_directive currently seems not good enough, clang should give more clear suggestions, I'd prefer to improve this in a separate patch, WDYT?

https://github.com/llvm/llvm-project/pull/107168