llvm (version 20.1.1): clang-scan-deps is unable to find system headers

[therealthingy]

Output of brew config

HOMEBREW_VERSION: 4.4.27
ORIGIN: https://github.com/Homebrew/brew
HEAD: 9b1efcd944408db0bd85f82fe4724ccae04a812f
Last commit: 5 days ago
Branch: stable
Core tap JSON: 05 Apr 12:25 UTC
Core cask tap JSON: 05 Apr 12:25 UTC
HOMEBREW_PREFIX: /opt/homebrew
HOMEBREW_CASK_OPTS: []
HOMEBREW_DISPLAY: /private/tmp/com.apple.launchd.rZZmuHSYes/org.xquartz:0
HOMEBREW_MAKE_JOBS: 8
Homebrew Ruby: 3.3.7 => /opt/homebrew/Library/Homebrew/vendor/portable-ruby/3.3.7/bin/ruby
CPU: octa-core 64-bit arm_firestorm_icestorm
Clang: 17.0.0 build 1700
Git: 2.49.0 => /opt/homebrew/bin/git
Curl: 8.7.1 => /usr/bin/curl
macOS: 15.4-arm64
CLT: 16.3.0.0.1.1742442376
Xcode: N/A
Rosetta 2: false

Output of brew doctor

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Warning: Some installed casks are deprecated or disabled.
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  vmware-fusion

Warning: Some installed formulae are deprecated or disabled.
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Description of issue

I think the latest llvm "Formulae" "broke" my CMake projects.

When I compile this demo project using llvm@19 everything builds fine:

cmake --workflow --preset llvm19
Executing workflow step 1 of 2: configure preset "llvm19"

-- Configuring done (0.1s)
-- Generating done (0.0s)
-- Build files have been written to: /Users/gary/Downloads/llvm-test/build-llvm19

Executing workflow step 2 of 2: build preset "llvm19"

[1/3] Generating CXX dyndep file CMakeFiles/test.dir/CXX.dd
+ cmake --workflow --preset llvm20
Executing workflow step 1 of 2: configure preset "llvm20"

-- Configuring done (0.1s)
-- Generating done (0.0s)
-- Build files have been written to: /Users/gary/Downloads/llvm-test/build-llvm20

However, using llvm (20.1.1):

cmake --workflow --preset llvm20
Executing workflow step 1 of 2: configure preset "llvm20"

-- Configuring done (0.1s)
-- Generating done (0.0s)
-- Build files have been written to: /Users/gary/Downloads/llvm-test/build-llvm20

Executing workflow step 2 of 2: build preset "llvm20"

[1/4] Scanning /Users/gary/Downloads/llvm-test/test.cpp for CXX dependencies
FAILED: CMakeFiles/test.dir/test.cpp.o.ddi
"/opt/homebrew/Cellar/llvm/20.1.1/bin/clang-scan-deps" -format=p1689 -- /opt/homebrew/opt/llvm/bin/clang++   -g -std=gnu++23 -arch arm64 -x c++ /Users/gary/Downloads/llvm-test/test.cpp -c -o CMakeFiles/test.dir/test.cpp.o -resource-dir "/opt/homebrew/Cellar/llvm/20.1.1/lib/clang/20" -MT CMakeFiles/test.dir/test.cpp.o.ddi -MD -MF CMakeFiles/test.dir/test.cpp.o.ddi.d > CMakeFiles/test.dir/test.cpp.o.ddi.tmp && mv CMakeFiles/test.dir/test.cpp.o.ddi.tmp CMakeFiles/test.dir/test.cpp.o.ddi
Error while scanning dependencies for /Users/gary/Downloads/llvm-test/test.cpp:
In file included from /Users/gary/Downloads/llvm-test/test.cpp:1:
In file included from /opt/homebrew/opt/llvm/bin/../include/c++/v1/iostream:45:
In file included from /opt/homebrew/opt/llvm/bin/../include/c++/v1/ios:223:
In file included from /opt/homebrew/opt/llvm/bin/../include/c++/v1/__locale:14:
In file included from /opt/homebrew/opt/llvm/bin/../include/c++/v1/__locale_dir/locale_base_api.h:115:
In file included from /opt/homebrew/opt/llvm/bin/../include/c++/v1/__locale_dir/support/apple.h:18:
/opt/homebrew/opt/llvm/bin/../include/c++/v1/__locale_dir/support/bsd_like.h:21:10: fatal error: 'time.h' file not found
ninja: build stopped: subcommand failed.

In both cases, I used Ninja as CMake generator and C++ 23.

The issue doesn't occur when compiling w/ Unix Makefiles as generator or when setting the CMake flag CMAKE_CXX_SCAN_FOR_MODULES to 0.

Relevant casks

llvm

[therealthingy]

Presumably related bug(s):

• Incorrect LLVM system config directory when using clang-scan-deps homebrew-core#221782
• LLVM / Clang system config file setting breaks with -no-canonical-prefixes homebrew-core#220757