[clangd] `PrerequisiteModulesTests` Generating Unexpected Error when Scanning for Dependencies on macOS

[qiongsiwu]

clangd's PrerequisiteModulesTests are passing on macOS, but produces unexpected diagnostic errors. This is reproducible using commit bb6e94a05d15d289e3685c5599f0eb905dc46925.

For example

$ ./tools/clang/tools/extra/clangd/unittests/ClangdTests --gtest_filter="PrerequisiteModulesTests.*"                                                        
Note: Google Test filter = PrerequisiteModulesTests.*
[==========] Running 8 tests from 1 test suite.
[----------] Global test environment set-up.
[----------] 8 tests from PrerequisiteModulesTests
[ RUN      ] PrerequisiteModulesTests.NonModularTest
[       OK ] PrerequisiteModulesTests.NonModularTest (10 ms)
[ RUN      ] PrerequisiteModulesTests.ModuleWithoutDepTest
[       OK ] PrerequisiteModulesTests.ModuleWithoutDepTest (2 ms)
[ RUN      ] PrerequisiteModulesTests.ModuleWithDepTest
Scanning modules dependencies for foo.h failed: error: invalid argument '-std=c++20' not allowed with 'C'

Scanning modules dependencies for foo.h failed: error: invalid argument '-std=c++20' not allowed with 'C'

Ignored diagnostic. argument unused during compilation: '-c'
Built module N:Part to /var/folders/50/tcq20qrs2wj563qz76hymwlc0000gn/T/clangd/module_files/N-part.cppm2503448646646106135-79-eb-1a-03-e9-78/N-Part.pcm
Scanning modules dependencies for foo.h failed: error: invalid argument '-std=c++20' not allowed with 'C'

Scanning modules dependencies for foo.h failed: error: invalid argument '-std=c++20' not allowed with 'C'

Ignored diagnostic. argument unused during compilation: '-c'
Built module M to /var/folders/50/tcq20qrs2wj563qz76hymwlc0000gn/T/clangd/module_files/M.cppm5130802603315192334-a5-3f-5b-38-18-91/M.pcm
...

@ChuanqiXu9 pointed out that the errors should be fixed in clangd (#66462 (comment)) instead of updating the tests.

[llvmbot]

@llvm/issue-subscribers-clangd

Author: Qiongsi Wu (qiongsiwu)

`clangd`'s `PrerequisiteModulesTests` are passing on macOS, but produces unexpected diagnostic errors. This is reproducible using commit `bb6e94a05d15d289e3685c5599f0eb905dc46925`.

For example

$ ./tools/clang/tools/extra/clangd/unittests/ClangdTests --gtest_filter="PrerequisiteModulesTests.*"                                                        
Note: Google Test filter = PrerequisiteModulesTests.*
[==========] Running 8 tests from 1 test suite.
[----------] Global test environment set-up.
[----------] 8 tests from PrerequisiteModulesTests
[ RUN      ] PrerequisiteModulesTests.NonModularTest
[       OK ] PrerequisiteModulesTests.NonModularTest (10 ms)
[ RUN      ] PrerequisiteModulesTests.ModuleWithoutDepTest
[       OK ] PrerequisiteModulesTests.ModuleWithoutDepTest (2 ms)
[ RUN      ] PrerequisiteModulesTests.ModuleWithDepTest
Scanning modules dependencies for foo.h failed: error: invalid argument '-std=c++20' not allowed with 'C'

Scanning modules dependencies for foo.h failed: error: invalid argument '-std=c++20' not allowed with 'C'

Ignored diagnostic. argument unused during compilation: '-c'
Built module N:Part to /var/folders/50/tcq20qrs2wj563qz76hymwlc0000gn/T/clangd/module_files/N-part.cppm2503448646646106135-79-eb-1a-03-e9-78/N-Part.pcm
Scanning modules dependencies for foo.h failed: error: invalid argument '-std=c++20' not allowed with 'C'

Scanning modules dependencies for foo.h failed: error: invalid argument '-std=c++20' not allowed with 'C'

Ignored diagnostic. argument unused during compilation: '-c'
Built module M to /var/folders/50/tcq20qrs2wj563qz76hymwlc0000gn/T/clangd/module_files/M.cppm5130802603315192334-a5-3f-5b-38-18-91/M.pcm
...

@ChuanqiXu9 pointed out that the errors should be fixed in clangd (#66462 (comment)) instead of updating the tests.