Missing definitions when using zig drop-in c/c++ compiler and clangd #25979

Zig Version

0.15.2

Steps to Reproduce and Observed Behavior

Install Msys2 and install zig by

pacman -S mingw-w64-clang-x86_64-zig \
          mingw-w64-clang-x86_64-cmake \
          mingw-w64-clang-x86_64-ninja

Install VSCode and clangd extension in the marketplace.

Clone mrexodia/zig-cross to a workspace folder, in my case <workspace>/3rdparty.

Modify the *.cmd files in zig-cross/cmake by changing zig to <full-path-to-zig>, in my case C:\msys64\clang64\bin\zig.exe.

Set VSCode settings for a workspace with

{
    "clangd.path": "C:/msys64/clang64/bin/clangd.exe",
    "clangd.arguments": [
        "--query-driver=D:/Repos/zig_test/3rdparty/zig-cross/cmake/zig-cc.cmd,D:/Repos/zig_test/3rdparty/zig-cross/cmake/zig-c++.cmd",
        "--log=verbose",
        "--pretty",
        "--compile-commands-dir=${workspaceFolder}",
        "--header-insertion=never",
        "--all-scopes-completion",
        "--clang-tidy",
        "--pch-storage=memory",
        "-j=4"
    ]
}

Create a C++ source file main.cc

#include <iostream>

int main() {
  std::cout << "Hello, world" << std::endl;
  return 0;
}

Create CMakeLists.txt

cmake_minimum_required(VERSION 3.21)
project(text LANGUAGES CXX)
set(CMAKE_EXPORT_COMPILE_COMMANDS ON)
add_executable(test main.cc)

Run several commands in Msys2/UCRT64 shell

mkdir build && cd build && cmake -D CMAKE_TOOLCHAIN_FILE=3rdparty/zig-cross/x86_64-windows-gnu.cmake ..
cp -pv compile_commands.json ..

Now restart the clangd language server.

The language server doesnot work properly because of missing definitions for libc++.

After some digging, I found that there should be a __config_site.h in libc++, but it seems that zig will use -D when compiling. Clangd failed to detect these definitions, so it cannot work unless we use the verbose mode to compile and add these definitions manually in CMakeLists.txt to generate a database file which contains these definitions.

Expected Behavior

When using clang in Msys2/UCRT64, clangd works fine.