ccls says erroneously "ccls [3]: use of undeclared identifier 'std'"

[gav451]

brew doctor output

Your system is ready to brew.

Verification

• I ran brew update twice and am still able to reproduce my issue.
• My "brew doctor output" above says Your system is ready to brew or a definitely unrelated Tier message.
• This issue's title and/or description do not reference a single formula e.g. brew install wget. If they do, open an issue at https://github.com/Homebrew/homebrew-core/issues/new/choose instead.

brew config output

HOMEBREW_VERSION: 5.0.13
ORIGIN: https://github.com/Homebrew/brew
HEAD: 2a48e23bc9dc24a78bcb982167574985f5bd6784
Last commit: 6 days ago
Branch: stable
Core tap JSON: 08 Feb 12:09 UTC
Core cask tap JSON: 08 Feb 12:09 UTC
HOMEBREW_PREFIX: /opt/homebrew
HOMEBREW_CASK_OPTS: []
HOMEBREW_DISPLAY: /private/tmp/com.apple.launchd.5oFOiu3UTC/org.xquartz:0
HOMEBREW_DOWNLOAD_CONCURRENCY: 20
HOMEBREW_EDITOR: emacsclient -q -a ""
HOMEBREW_FORBID_PACKAGES_FROM_PATHS: set
HOMEBREW_MAKE_JOBS: 10
Homebrew Ruby: 3.4.8 => /opt/homebrew/Library/Homebrew/vendor/portable-ruby/3.4.8/bin/ruby
CPU: deca-core 64-bit arm_donan
Clang: 17.0.0 build 1700
Git: 2.52.0 => /opt/homebrew/bin/git
Curl: 8.7.1 => /usr/bin/curl
macOS: 26.2-arm64
CLT: 26.2.0.0.1.1764812424
Xcode: 26.2
Metal Toolchain: N/A
Rosetta 2: false

What were you trying to do (and why)?

I was trying to eliminate errors and warnings from C++ code by means of the ccls language server.
I was using the built-in eglot package of emacs-plus as front-end of ccls, but the front-end should not matter.

What happened (include all command output)?

An minimal example is:

#include <string>

int main() {
  std::string hello = "Hello World";
  return 0;
}

where ccls returns the following error:

    4   2 error    e-f-b    ccls [3]: use of undeclared identifier 'std'

where the 4 and 2 are the position of the error. I have the following .ccls configuration file

clang
-isystem/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include
-isystem/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/c++/v1/__cxx03
%c -std=c11
%cpp -std=c++20

What did you expect to happen?

I expected that ccls told me that the minimal example is correct (as compiling the example as well as using clangd confirm).

Step-by-step reproduction instructions (by running brew commands)

brew install emacs-plus
brew install ccls