Intellisense does not assume C23 when using CMake with c_std_23

[robindegen]

Environment

• OS and Version: Arch
• VS Code Version: C/C++ prerelease (1.33.2?)
• C/C++ Extension Version: C23

Bug Summary and Steps to Reproduce

Bug Summary:

Steps to reproduce:

1. Create a basic Cmake file using c_std_23 as the language version. Then generate with compile commands.
2. In VSCode, things like nullptr, bool etc are not at all recognized.

What is really happening is that CMake correctly assumes that C23 is the standard for GCC and completely omits -std=c23 (or gnu23) from the command-line in compile-commands.json. Hence, the language server does not know that C23 is used and just falls back to C99 (likely). Clangd shows this exact same behavior. C99 is no longer a correct default to fall back to. Hasn't been for a while.

Expected behavior:
Intellisense should be compiler version aware and know that C23 is implicit if no flags are specified.

Configuration and Logs

The fix is just to specify C23 in c_cpp_properties.json. If this file is not provided; intellisense defaults are incorrect:

{
    "version": 4,
    "configurations": [
        {
            "name": "Linux",
            "compilerPath": "/usr/bin/cc",
            "cStandard": "c23",
            "intelliSenseMode": "linux-gcc-x64",
            "compileCommands": "${workspaceFolder}/build/compile_commands.json"
        }
    ]
}

And then suddenly intellisense has basic C23 support (somewhat.. at least it doesn't complain anymore)

Other Extensions

No response

Additional context

No response

[sean-mcmanus]

@robindegen It appears that gcc 15 switched the default standard to C23, but not gcc 14 or earlier or with the latest clang or msvc compilers.

IntelliSense is using c17 by default because gcc is reporting __STDC_ISO_10646__ 201706L as the define.

[sean-mcmanus]

@robindegen It appears the gcc default is actually C17, but it's silently accepting C23 code: __STDC_ISO_10646__ is still 201706L.

[sean-mcmanus]

@robindegen It appears to use C23 because gcc is implementing those particular C23 features you're using as a language extension for C17, which our EDG compiler doesn't properly take into account yet.