The reproduction step doesn't fit into an issue description. I've created a tiny project to showcase the bug: https://github.com/SidneyCogdill/clangd_modules_bug_showcase

git clone https://github.com/SidneyCogdill/clangd_modules_bug_showcase.git

Audit the code before opening it in your editor because it's good security practice, it's a tiny project and shouldn't be hard.

Basically:

• module unique_fd requires unique_wrapper as an dependency:

// unique_fd.cpp
module;

#include <print>

export module unique_fd;

import unique_wrapper;

export struct UniqueFd { ... };

// unique_wrapper.cpp
module;

#include <functional>
#include <stdexcept>
#include <type_traits>

export module unique_wrapper;

export template <typename Handle> requires(...)
struct UniqueWrapper { ... };

• When unique_fd is used in my_app, unique_wrapper has to be explicitly imported before unique_fd, otherwise clangd will fail to compile unique_fd:

module;

#include <print>

export module my_app;

import unique_wrapper; // <- This is required to make clangd successfully compile `unique_fd`.
import unique_fd;

clang can compile the program successfully, but clangd will complain "Module XXX not found`. more importantly it will point at whatever the first import statement is at rather than the problematic module. One can only obtain useful information from the log output.

clangd version:

~> clangd-21 --version
Ubuntu clangd version 21.0.0 (++20250501084208+9b1051281e43-1~exp1~20250501084221.26)
Features: linux+grpc
Platform: x86_64-pc-linux-gnu

arguments:

clangd \
    --compile-commands-dir=<relative path to build folder> \
    --experimental-modules-support \
    --header-insertion=never \
    --background-index \
    -j=6 \
    --pch-storage=disk