Non-standard header files may not be recognized properly

[Season0518]

Observed behavior

I'm trying to migrate from clangd to ccls recently. I configured gcc-toolchain as shown in the wiki, but why doesn't it recognize gcc's header files like clangd does?

Compile_commands.json

[
{
  "directory": "/Users/season/CLionProjects/new_project/build",
  "command": "/opt/homebrew/bin/aarch64-apple-darwin24-g++-14 -DCMAKE_INTDIR=\\\"Debug\\\" -g -std=gnu++20 -arch arm64 -isysroot /Library/Developer/CommandLineTools/SDKs/MacOSX15.2.sdk -mmacosx-version-min=15.1 -o CMakeFiles/test.dir/Debug/main.cpp.o -c /Users/season/CLionProjects/new_project/main.cpp",
  "file": "/Users/season/CLionProjects/new_project/main.cpp",
  "output": "CMakeFiles/test.dir/Debug/main.cpp.o"
},
{
  "directory": "/Users/season/CLionProjects/new_project/build",
  "command": "/opt/homebrew/bin/aarch64-apple-darwin24-g++-14 -DCMAKE_INTDIR=\\\"Release\\\" -O3 -DNDEBUG -std=gnu++20 -arch arm64 -isysroot /Library/Developer/CommandLineTools/SDKs/MacOSX15.2.sdk -mmacosx-version-min=15.1 -o CMakeFiles/test.dir/Release/main.cpp.o -c /Users/season/CLionProjects/new_project/main.cpp",
  "file": "/Users/season/CLionProjects/new_project/main.cpp",
  "output": "CMakeFiles/test.dir/Release/main.cpp.o"
},
{
  "directory": "/Users/season/CLionProjects/new_project/build",
  "command": "/opt/homebrew/bin/aarch64-apple-darwin24-g++-14 -DCMAKE_INTDIR=\\\"RelWithDebInfo\\\" -O2 -g -DNDEBUG -std=gnu++20 -arch arm64 -isysroot /Library/Developer/CommandLineTools/SDKs/MacOSX15.2.sdk -mmacosx-version-min=15.1 -o CMakeFiles/test.dir/RelWithDebInfo/main.cpp.o -c /Users/season/CLionProjects/new_project/main.cpp",
  "file": "/Users/season/CLionProjects/new_project/main.cpp",
  "output": "CMakeFiles/test.dir/RelWithDebInfo/main.cpp.o"
}
]

CMakeLists

cmake_minimum_required(VERSION 3.25)
set(CMAKE_EXPORT_COMPILE_COMMANDS ON)
set(CMAKE_CXX_STANDARD 20)

project(cpp_test LANGUAGES C CXX)

add_executable(test)
target_sources(test PRIVATE main.cpp)

#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <iostream>

int main() {
  
}

ccls output:

Server CWD is /Users/season/CLionProjects/new_project
21:20:44                initialize.cc:330 I initialize in directory /Users/season/CLionProjects/new_project with uri file:///Users/season/CLionProjects/new_project
21:20:44                initialize.cc:353 I initializationOptions: {"compilationDatabaseCommand":"","compilationDatabaseDirectory":"","cache":{"directory":".ccls-cache","format":"binary","hierarchicalPath":false,"retainInMemory":2},"capabilities":{"documentOnTypeFormattingProvider":{"firstTriggerCharacter":"}","moreTriggerCharacter":[]},"foldingRangeProvider":true,"workspace":{"workspaceFolders":{"supported":true,"changeNotifications":true}}},"clang":{"excludeArgs":[],"extraArgs":["/opt/homebrew/bin/aarch64-apple-darwin24-g++-14"],"pathMappings":[],"resourceDir":""},"client":{"diagnosticsRelatedInformation":true,"hierarchicalDocumentSymbolSupport":true,"linkSupport":true,"snippetSupport":false},"codeLens":{"localVariables":false},"completion":{"caseSensitivity":2,"detailedLabel":false,"dropOldRequests":true,"duplicateOptional":false,"filterAndSort":true,"include":{"blacklist":[],"maxPathSize":37,"suffixWhitelist":[".h",".hpp",".hh",".inc",".h",".hpp",".hh"],"whitelist":[]},"maxNum":100,"placeholder":true},"diagnostics":{"blacklist":[],"onChange":1000,"onOpen":0,"onSave":0,"spellChecking":true,"whitelist":[]},"highlight":{"largeFileSize":2097152,"rainbow":0,"blacklist":[],"whitelist":[]},"index":{"blacklist":[],"comments":2,"initialNoLinkage":false,"initialBlacklist":[],"initialWhitelist":[],"maxInitializerLines":15,"multiVersion":0,"multiVersionBlacklist":[],"multiVersionWhitelist":[],"name":{"suppressUnwrittenScope":false},"onChange":false,"parametersInDeclarations":true,"threads":0,"trackDependency":2,"whitelist":[]},"request":{"timeout":5000},"session":{"maxNum":10},"workspaceSymbol":{"caseSensitivity":1,"maxNum":1000,"sort":true},"xref":{"maxNum":2000}}
21:20:44                initialize.cc:386 I use -resource-dir=/opt/homebrew/opt/llvm/lib/clang/19
21:20:44                initialize.cc:418 I workspace folder: /Users/season/CLionProjects/new_project/
21:20:44                initialize.cc:443 I start 12 indexers
21:20:44                initialize.cc:447 I dispatch initial index requests
21:20:44                  pipeline.cc:510 I loaded project. Refresh semantic highlight for all working file.
21:20:44              sema_manager.cc:776 I create session for /Users/season/CLionProjects/new_project/main.cpp
21:20:45                  pipeline.cc:386 I parse /Users/season/CLionProjects/new_project/main.cpp error:1 'iostream' file not found
21:20:45                  pipeline.cc:397 I store index for /opt/homebrew/Cellar/llvm/19.1.4/lib/clang/19/include/__stddef_null.h (delta: 1)
21:20:45                  pipeline.cc:397 I store index for /opt/homebrew/Cellar/llvm/19.1.4/lib/clang/19/include/__stddef_wchar_t.h (delta: 0)
21:20:45                  pipeline.cc:397 I store index for /opt/homebrew/Cellar/llvm/19.1.4/lib/clang/19/include/__stddef_header_macro.h (delta: 0)
21:20:45                  pipeline.cc:397 I store index for /opt/homebrew/Cellar/llvm/19.1.4/lib/clang/19/include/__stddef_nullptr_t.h (delta: 0)
21:20:45                  pipeline.cc:397 I store index for /opt/homebrew/Cellar/llvm/19.1.4/lib/clang/19/include/__stddef_ptrdiff_t.h (delta: 0)
21:20:45                  pipeline.cc:397 I store index for /opt/homebrew/Cellar/llvm/19.1.4/lib/clang/19/include/stddef.h (delta: 0)
21:20:45                  pipeline.cc:397 I store index for /opt/homebrew/Cellar/llvm/19.1.4/lib/clang/19/include/__stddef_offsetof.h (delta: 0)
21:20:45                  pipeline.cc:397 I store index for /opt/homebrew/Cellar/llvm/19.1.4/lib/clang/19/include/__stddef_size_t.h (delta: 0)
21:20:45                  pipeline.cc:397 I store index for /opt/homebrew/Cellar/llvm/19.1.4/lib/clang/19/include/__stddef_max_align_t.h (delta: 0)
21:20:45                  pipeline.cc:397 I store index for /Users/season/CLionProjects/new_project/main.cpp (delta: 0)
21:20:55                  pipeline.cc:386 I parse /Users/season/CLionProjects/new_project/main.cpp error:1 'bits/stdc++.h' file not found
21:20:55                  pipeline.cc:397 I store index for /Users/season/CLionProjects/new_project/main.cpp (delta: 1)
21:20:59                  pipeline.cc:386 I parse /Users/season/CLionProjects/new_project/main.cpp error:1 'bits/stdc++.h' file not found
21:20:59                  pipeline.cc:397 I store index for /Users/season/CLionProjects/new_project/main.cpp (delta: 1)
21:21:08                  pipeline.cc:386 I parse /Users/season/CLionProjects/new_project/main.cpp error:1 'bits/stdc++.h' file not found
21:21:08                  pipeline.cc:397 I store index for /Users/season/CLionProjects/new_project/main.cpp (delta: 1)

Expected behavior

The header file is recognized normally

Screenshot from clangd

Steps to reproduce

1. Create a empty folder
2. Create CMakeLists and main.cpp
3. Configure Project

System information

• ccls version (git describe --tags):
  Homebrew ccls version
  clang version 19.1.4
• OS: Macos 15.1.1 (24B91)
• Editor: VSCode
• vscode-ccls version: v0.1.29