Module not found in clangd on Windows (Mingw-w64 UCRT)

[zHaOdANiuu]

Environment

• OS: Microsoft Windows [Version 10.0.19045.3803]
• Compiler: MinGW-w64 (UCRT64) – g++ 16.1.0 (from msys2 2026-03-22 bundle)
• Build system: CMake 4.3.1
• IDE / LSP client: VSCode + clangd plugin

Problem Summary

import std; is not recognized by clangd, while other modules work correctly

Workaround: copying bits/std.cc from MinGW include directory to the current project directory makes std module visible to clangd.

• Other defined modules work fine with clangd.
• The problem is specific to the std module from the standard library.

dir

-rw-r--r-- 1 you None   94 May  5 20:09 .clangd
drwxr-xr-x 1 you None    0 May  5 20:33 build
-rw-r--r-- 1 you None  330 May  5 17:40 Cmakelists.txt
-rw-r--r-- 1 you None   58 May  5 20:41 main.cpp

main.cpp

import std;
int main() { std::println("hello world"); }

CMakeLists.txt

cmake_minimum_required(VERSION 4.3.1)
set(CMAKE_EXPERIMENTAL_CXX_IMPORT_STD "451f2fe2-a8a2-47c3-bc32-94786d8fc91b")
set(CMAKE_CXX_STANDARD 23)
set(CMAKE_CXX_MODULE_STD ON)
set(CMAKE_EXPORT_COMPILE_COMMANDS ON)
project(test LANGUAGES CXX)
add_executable(test main.cpp)
target_link_libraries(test PRIVATE stdc++exp)

compile_commands.json

[
{
  "directory": "D:/home/proj/tests/modules/build",
  "command": "D:\\bin\\scoop\\apps\\msys2\\2026-03-22\\ucrt64\\bin\\g++.exe -std=gnu++23 -fmodules-ts -fmodule-mapper=CMakeFiles\\test.dir\\main.cpp.obj.modmap -MD -fdeps-format=p1689r5 -x c++ -o CMakeFiles\\test.dir\\main.cpp.obj -c D:\\home\\proj\\tests\\modules\\main.cpp",
  "file": "D:/home/proj/tests/modules/main.cpp",
  "output": "D:/home/proj/tests/modules/build/CMakeFiles/test.dir/main.cpp.obj"
},
{
  "directory": "D:/home/proj/tests/modules/build",
  "command": "D:\\bin\\scoop\\apps\\msys2\\2026-03-22\\ucrt64\\bin\\g++.exe -std=gnu++23 -fmodules-ts -fmodule-mapper=CMakeFiles\\__cmake_cxx_std_23.dir\\D_\\bin\\scoop\\apps\\msys2\\2026-03-22\\ucrt64\\include\\c++\\16.1.0\\bits\\std.cc.obj.modmap -MD -fdeps-format=p1689r5 -x c++ -o CMakeFiles\\__cmake_cxx_std_23.dir\\D_\\bin\\scoop\\apps\\msys2\\2026-03-22\\ucrt64\\include\\c++\\16.1.0\\bits\\std.cc.obj -c D:\\bin\\scoop\\apps\\msys2\\2026-03-22\\ucrt64\\include\\c++\\16.1.0\\bits\\std.cc",
  "file": "D:/bin/scoop/apps/msys2/2026-03-22/ucrt64/include/c++/16.1.0/bits/std.cc",
  "output": "D:/home/proj/tests/modules/build/CMakeFiles/__cmake_cxx_std_23.dir/D_/bin/scoop/apps/msys2/2026-03-22/ucrt64/include/c++/16.1.0/bits/std.cc.obj"
},
{
  "directory": "D:/home/proj/tests/modules/build",
  "command": "D:\\bin\\scoop\\apps\\msys2\\2026-03-22\\ucrt64\\bin\\g++.exe -std=gnu++23 -fmodules-ts -fmodule-mapper=CMakeFiles\\__cmake_cxx_std_23.dir\\D_\\bin\\scoop\\apps\\msys2\\2026-03-22\\ucrt64\\include\\c++\\16.1.0\\bits\\std.compat.cc.obj.modmap -MD -fdeps-format=p1689r5 -x c++ -o CMakeFiles\\__cmake_cxx_std_23.dir\\D_\\bin\\scoop\\apps\\msys2\\2026-03-22\\ucrt64\\include\\c++\\16.1.0\\bits\\std.compat.cc.obj -c D:\\bin\\scoop\\apps\\msys2\\2026-03-22\\ucrt64\\include\\c++\\16.1.0\\bits\\std.compat.cc",
  "file": "D:/bin/scoop/apps/msys2/2026-03-22/ucrt64/include/c++/16.1.0/bits/std.compat.cc",
  "output": "D:/home/proj/tests/modules/build/CMakeFiles/__cmake_cxx_std_23.dir/D_/bin/scoop/apps/msys2/2026-03-22/ucrt64/include/c++/16.1.0/bits/std.compat.cc.obj"
}
]

.clangd

CompileFlags:
  Remove:
    - -fmodules-ts
    - -fdeps-format=*
    - -fmodule-mapper=*

log

I[21:08:58.823] clangd version 22.1.4 (https://github.com/msys2/MINGW-packages e97dcec28087f0155bb8cb6808ee2ab95f551f1b)
I[21:08:58.823] Features: windows
I[21:08:58.823] PID: 9272
I[21:08:58.823] Working directory: d:/home/proj/tests/modules
I[21:08:58.823] argv[0]: D:/bin/scoop/apps/msys2/current/ucrt64/bin/clangd.exe
I[21:08:58.823] argv[1]: --clang-tidy
I[21:08:58.823] argv[2]: --header-insertion=never
I[21:08:58.823] argv[3]: --background-index
I[21:08:58.823] argv[4]: --limit-results=20
I[21:08:58.823] argv[5]: --experimental-modules-support
I[21:08:58.823] argv[6]: --query-driver=D:/bin/scoop/apps/msys2/2026-03-22/ucrt64/bin/g*.exe
I[21:08:58.826] Starting LSP over stdin/stdout
I[21:08:58.850] <-- initialize(0)
I[21:08:58.854] --> reply:initialize(0) 3 ms
I[21:08:58.865] <-- initialized
I[21:08:58.879] <-- textDocument/didOpen
I[21:08:58.882] <-- textDocument/documentLink(1)
I[21:08:58.884] Loading config file at d:/home/proj/tests/modules/.clangd
I[21:08:58.887] --> textDocument/publishDiagnostics
I[21:08:58.888] Loaded compilation database from d:/home/proj/tests/modules/build/compile_commands.json
I[21:08:58.892] Loading config file at D:/home/proj/tests/modules/.clangd
I[21:08:58.893] --> textDocument/publishDiagnostics
I[21:08:58.897] --> window/workDoneProgress/create(0)
I[21:08:58.897] Enqueueing 1 commands for indexing
I[21:08:58.899] <-- reply(0)
I[21:08:58.899] --> $/progress
I[21:08:58.899] --> $/progress
I[21:08:58.904] --> $/progress
I[21:08:58.904] --> $/progress
I[21:08:58.975] System includes extractor: successfully executed D:\bin\scoop\apps\msys2\2026-03-22\ucrt64\bin\g++.exe
	got includes: "D:/bin/scoop/apps/msys2/2026-03-22/ucrt64/bin/../lib/gcc/x86_64-w64-mingw32/16.1.0/../../../../include/c++/16.1.0, D:/bin/scoop/apps/msys2/2026-03-22/ucrt64/bin/../lib/gcc/x86_64-w64-mingw32/16.1.0/../../../../include/c++/16.1.0/x86_64-w64-mingw32, D:/bin/scoop/apps/msys2/2026-03-22/ucrt64/bin/../lib/gcc/x86_64-w64-mingw32/16.1.0/../../../../include/c++/16.1.0/backward, D:/bin/scoop/apps/msys2/2026-03-22/ucrt64/bin/../lib/gcc/x86_64-w64-mingw32/16.1.0/../../../../include, D:/bin/scoop/apps/msys2/2026-03-22/ucrt64/bin/../lib/gcc/x86_64-w64-mingw32/16.1.0/include-fixed"
	got target: "x86_64-w64-mingw32"
I[21:08:58.976] ASTWorker building file d:/home/proj/tests/modules/main.cpp version 1 with command 
[D:/home/proj/tests/modules/build]
D:/bin/scoop/apps/msys2/2026-03-22/ucrt64/bin/g++.exe --driver-mode=g++ -std=gnu++23 -MD -x c++ -o "CMakeFiles\\test.dir\\main.cpp.obj" -c -isystem D:/bin/scoop/apps/msys2/2026-03-22/ucrt64/bin/../lib/gcc/x86_64-w64-mingw32/16.1.0/../../../../include/c++/16.1.0 -isystem D:/bin/scoop/apps/msys2/2026-03-22/ucrt64/bin/../lib/gcc/x86_64-w64-mingw32/16.1.0/../../../../include/c++/16.1.0/x86_64-w64-mingw32 -isystem D:/bin/scoop/apps/msys2/2026-03-22/ucrt64/bin/../lib/gcc/x86_64-w64-mingw32/16.1.0/../../../../include/c++/16.1.0/backward -isystem D:/bin/scoop/apps/msys2/2026-03-22/ucrt64/bin/../lib/gcc/x86_64-w64-mingw32/16.1.0/../../../../include -isystem D:/bin/scoop/apps/msys2/2026-03-22/ucrt64/bin/../lib/gcc/x86_64-w64-mingw32/16.1.0/include-fixed --target=x86_64-w64-mingw32 -resource-dir=D:/bin/scoop/apps/msys2/2026-03-22/ucrt64/lib/clang/22 -- d:/home/proj/tests/modules/main.cpp
I[21:08:58.979] --> textDocument/clangd.fileStatus
I[21:08:59.007] Built preamble of size 256224 for file d:/home/proj/tests/modules/main.cpp version 1 in 0.03 seconds
I[21:08:59.352] Failed to find compilation database for D:/bin/scoop/apps/msys2/2026-03-22/ucrt64/include/c++/16.1.0/bits/std.cc
I[21:08:59.846] Failed to find compilation database for D:/bin/scoop/apps/msys2/2026-03-22/ucrt64/include/c++/16.1.0/bits/std.cc
E[21:08:59.846] Failed to build module std; due to No compile command for D:/bin/scoop/apps/msys2/2026-03-22/ucrt64/include/c++/16.1.0/bits/std.cc
I[21:08:59.849] Built prerequisite modules for file d:/home/proj/tests/modules/main.cpp in 0.84 seconds
I[21:08:59.851] --> workspace/semanticTokens/refresh(1)
I[21:08:59.853] Indexing c++23 standard library in the context of d:/home/proj/tests/modules/main.cpp
I[21:08:59.869] --> textDocument/publishDiagnostics
I[21:08:59.869] --> textDocument/inactiveRegions
I[21:08:59.869] --> reply:textDocument/documentLink(1) 987 ms
I[21:08:59.870] --> textDocument/clangd.fileStatus
I[21:09:00.195] <-- $/cancelRequest
I[21:09:00.205] <-- textDocument/documentLink(2)
I[21:09:00.205] --> reply:textDocument/documentLink(2) 0 ms
I[21:09:00.205] --> textDocument/clangd.fileStatus
I[21:09:00.208] <-- textDocument/foldingRange(3)
I[21:09:00.208] --> reply:textDocument/foldingRange(3) 0 ms
I[21:09:00.212] <-- textDocument/semanticTokens/full(4)
I[21:09:00.212] --> reply:textDocument/semanticTokens/full(4) 0 ms
I[21:09:00.212] --> textDocument/clangd.fileStatus
I[21:09:00.229] <-- $/setTrace
I[21:09:00.229] unhandled notification $/setTrace
I[21:09:00.231] <-- $/cancelRequest
I[21:09:00.232] <-- textDocument/documentSymbol(5)
I[21:09:00.232] --> reply:textDocument/documentSymbol(5) 0 ms
I[21:09:00.232] --> textDocument/clangd.fileStatus
I[21:09:00.233] <-- textDocument/documentLink(6)
I[21:09:00.233] --> reply:textDocument/documentLink(6) 0 ms
I[21:09:00.233] --> textDocument/clangd.fileStatus
I[21:09:00.234] <-- reply(1)
I[21:09:00.235] <-- $/cancelRequest
I[21:09:00.235] <-- textDocument/documentLink(7)
I[21:09:00.235] --> reply:textDocument/documentLink(7) 0 ms
I[21:09:00.235] --> textDocument/clangd.fileStatus
I[21:09:01.297] <-- textDocument/semanticTokens/full/delta(8)
I[21:09:01.297] --> reply:textDocument/semanticTokens/full/delta(8) 0 ms
I[21:09:01.297] --> textDocument/clangd.fileStatus
I[21:09:03.400] Indexed c++23 standard library: 17455 symbols, 10867 filtered

Possibly a path / discovery issue: clangd does not automatically understand that std is provided by bits/std.cc in this MinGW environment.

Is there a way to properly inform clangd about the location of the std module under MinGW-w64 UCRT without manually copying system headers into the project?

Thank you very much for the great work on clangd + modules support!

[HighCommander4]

How did you install clangd? In previous reports from MSYS2 users, it was found that using the clangd package from MSYS2's package manager works best in this situation.

[zHaOdANiuu]

I can provide detailed clangd logs for GCC.

[HighCommander4]

Does adding --compile-commands-dir=d:/home/proj/tests/modules/build to clangd's command like arguments (i.e. the same place --experimental-modules-support goes) help?

[zHaOdANiuu]

Thanks, that was useful — you've been a huge help. May you live forever and never die.😍

[HighCommander4]

Glad to be of help.

To explain a bit why that makes a difference: I saw the following in the log:

I[21:08:59.352] Failed to find compilation database for D:/bin/scoop/apps/msys2/2026-03-22/ucrt64/include/c++/16.1.0/bits/std.cc

Even though d:/home/proj/tests/modules/build/compile_commands.json contains a command for std.cc, clangd doesn't know to look for the command there, because clangd looks by default in the ancestor directories of the source file (so in this case, the ancestor directories of std.cc). The --compile-commands-dir=<directory> option tells clangd to instead look in <directory>/compile_commands.json for commands for all files.

This behaviour is a bit unfortunate / counter-intuitive in my opinion. A "strong workspace mode" discussed in #907 would improve on it.

[zHaOdANiuu]

thanks