`clang-scan-deps` is not using clang's default config files

[hanickadot]

I found out clang-scan-deps is not loading default's clang's config files. They need to be passed explicitly.

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More details:

When building a modularized code in cmake on mac (latest brew's LLVM and cmake 4) when I try to build a modularized code I get error that some random file is missing:

/opt/homebrew/opt/llvm/bin/../include/c++/v1/__locale_dir/support/bsd_like.h:21:10: fatal error: 'time.h' file not found

If I pass --config=/Users/hanka/.config/clang it will work correctly.

clang 20 from homebrew on mac is now using a bunch of configuration files (per each platform and target + user custom config file) clang-scan-deps and other tools should be able to load clang's default configuration files in same manner as clang.

[tcbrindle]

I had the same problem with clang-scan-deps and Homebrew-build LLVM, and I found that adding -v to the command line gave me a clue...

It looks like Homebrew has recently started incorrectly setting the system configuration file directory to the non-existent /usr/etc/clang (on Intel Macs) rather than the correct /usr/local/etc/clang. Overriding with --config-system-dir seems to work. (On Apple Silicon these paths are /opt/etc/clang and /opt/homebrew/etc/clang instead I think?)

Possibly related to Homebrew/homebrew-core#220757

[tcbrindle]

I filed Homebrew/homebrew-core#221782