ccls does not recognize C attributes

[computerscience-person]

Observed behavior

ccls gives an error on C attributes (feature was standardized in C23 and is supported by clang 9 and gcc 10).

Minimal example:

[[nodiscard]] void* identity(void* item) {
  return item;
}

Expected behavior

No error (feature standardized in C23).

Steps to reproduce

Minimal example:

[[nodiscard]] void* identity(void* item) {
  return item;
}

System information

• ccls version (git describe --tags --long): 0.20240202
• clang version: 14.0.6
• OS: Linux nixos 6.6.32
• Editor: Neovim
• Language client (and version):

[rherilier]

Did you use -std=c2x or -std=c23 as compilation flag?

As your ccls seems to be build against clang 14.0.6 (which has been released in 2022 while C23 is only official since February), I suppose you are compiling with the later when you should use the former to make ccls use the correct C version.

As my box runs Debian testing (which rely on llvm 19.x), I did some tests using a c27 project with ccls and clangd:

• ccls does not complain at all (surely because it fell back to c23);
• clangd-16 complains about the wrong compilation flag and the C attribute (surely because it fell back to c17);
• clangd-19 complains about the wrong compilation flag but not about the C attribute (surely because it fell back to c23).

[thepigeongenerator]

To add to this note, you may need to generate compile_commands.json for it to be recognised properly. (the language-server won't know your compilation flags by just looking at your code)
Generating this can be done through a utility such as bear, when using GNU Makefiles, and likely others.

Hope this helps!
-Quinn