Clangd apply different c++ standard to diffrent file

Question

When I used VSCode+XMake+Clangd+MSVC, clangd confused me.

If I wrote this in xmake.lua:

set_language("c++23")

I could not use std::filesystem. Then I found Clangd expanded the marco __cplusplus to 202400L in main.cpp and to 201402L in other any files, which made me confusing.

Then I wrote this in xmake.lua:

set_language("c++20")

The marco __cplusplus was expanded to 202002L in all files. So that my Clangd could not recognize std::println, althought it is still able to compile.

I thought all the problem were caused by Clangd. But how to solve it?

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VSCode 1.90.2, clangd 18.1.3, xmake v2.9.2+HEAD.6b6557c54, VS 17.10.1

• HolyBlackCat: Does your build system generate compile_commands.json? Among other things it should contain -std=c++?? for all files, assuming the build system passes it to the compiler in the first place.
• Awaaaaawwwwa: It did. And there are three scenarios int the compile_commands.json: 1. The argument was /std:c++latest. It was the generation result of "c++23" in xmake.lua. 2. The argument was /std:c++20. It was the generation result of "c++20" in xmake.lua. 3. I replaced the argument with /std:c++23. Then the macro __cplusplus was expand to 201402L in all the file, including main.cpp
• HolyBlackCat: There's this old bug about Clang not understanding /std:c++latest in compile_commands.json. Also Clang seems to not understand MSVC's /std:c++23 (only 20 and below and latest).