I have a very simple project where I’m trying to learn how to use modules in C++20:

> tree
.
├── foo_something.cxx
└── main.cxx

1 directory, 2 files

main.cxx:

import <iostream>;
import foo.something;

int main() {
    std::cout << "Hin";
}

foo_something.cxx:

export module foo.something;

import <format>;
import <chrono>; // This seems to cause the issue

int hi() {
    return 5;
}

And I’m building this with:

g++ -std=c++20 -fmodules-ts -x c++-system-header format -x c++-system-header chrono -x c++-system-header iostream
g++ -std=c++20 -fmodules-ts -c foo_something.cxx
g++ -std=c++20 -fmodules-ts main.cxx

On the last command though, I get this weird error:

> g++ -std=c++20 -fmodules-ts main.cxx
‘
/usr/include/c++/13/format:2862: confused by earlier errors, bailing out

If I comment out the import <chrono> in foo_something.cxx, it builds and runs fine.

My questions are:

• Is it expected than a completely unused module import can affect the compilation?
• What specifically is going wrong?

In case it’s relevant:

> g++ --version
g++ (SUSE Linux) 13.3.0
Copyright (C) 2023 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.