Module fails to build with `recursive lazy load` error

[JoeBot21]

I'm having trouble getting the mp-units module to build on nixos 25.05 with the following shell.nix and packages versions:

# shell.nix
let
  pkgs = import <nixpkgs> {};
in
pkgs.mkshell {
  packages = with pkgs; [
    cmake
    gcc15
    gsl-lite
    ninja
  ];
}

• CMake 3.31.6
• GCC 15.1.0
• Ninja 1.12.1
• gsl-lite 0.43.0

The following CMakeLists.txt and main.cpp produce the error.

# CMakeLists.txt
cmake_minimum_required(version 3.31.0)
project(import-test LANGUAGES CXX)

set(CMAKE_CXX_STANDARD 23)
set(CMAKE_CXX_STANDARD_REQUIRED ON)
set(CMAKE_CXX_SCAN_FOR_MODULES ON)

include(FetchContent)
FetchContent_Declare(
  mp-units
  GIT_REPOSITORY https://github.com/mpusz/mp-units
  GIT_TAG d9dce46c21f6d915424112fd8b5e2163addc051b
  SOURCE_SUBDIR src
)
set(MP_UNITS_BUILD_CXX_MODULES ON)
set(MP_UNITS_API_STD_FORMAT ON)
FetchContent_MakeAvailable(mp-units)

add_library(import-test SHARED src/import-test/main.cpp)
target_link_libraries(import-test PRIVATE mp-units::mp-units)

// main.cpp
import mp_units;

int main() {}

Output from cmake --build

[5/10] Building CXX object _deps/mp-units-build/systems/CMakeFiles/mp-units-systems.dir/mp-units-systems.cpp.o
FAILED: _deps/mp-units-build/systems/CMakeFiles/mp-units-systems.dir/mp-units-systems.cpp.o _deps/mp-units-build/systems/CMakeFiles/mp-units-systems.dir/mp_units.systems.gcm 
/nix/store/abv8wwll56z9nxv33d5sk6ywwmnga397-gcc-wrapper-15.1.0/bin/g++ -DMP_UNITS_API_CONTRACTS=2 -DMP_UNITS_API_NATURAL_UNITS=1 -DMP_UNITS_API_NO_CRTP=1 -DMP_UNITS_API_STD_FORMAT=1 -DMP_UNITS_HOSTED=1 -I/home/joebot/git/mpy-units/build/_deps/mp-units-src/src/systems/include -I/home/joebot/git/mpy-units/build/_deps/mp-units-src/src/core/include -std=gnu++23 -MD -MT _deps/mp-units-build/systems/CMakeFiles/mp-units-systems.dir/mp-units-systems.cpp.o -MF _deps/mp-units-build/systems/CMakeFiles/mp-units-systems.dir/mp-units-systems.cpp.o.d -fmodules-ts -fmodule-mapper=_deps/mp-units-build/systems/CMakeFiles/mp-units-systems.dir/mp-units-systems.cpp.o.modmap -MD -fdeps-format=p1689r5 -x c++ -o _deps/mp-units-build/systems/CMakeFiles/mp-units-systems.dir/mp-units-systems.cpp.o -c /home/joebot/git/mpy-units/build/_deps/mp-units-src/src/systems/mp-units-systems.cpp
In file included from /home/joebot/git/mpy-units/build/_deps/mp-units-src/src/systems/include/mp-units/systems/angular/math.h:28,
                 from /home/joebot/git/mpy-units/build/_deps/mp-units-src/src/systems/include/mp-units/systems/angular.h:29,
                 from /home/joebot/git/mpy-units/build/_deps/mp-units-src/src/systems/mp-units-systems.cpp:37:
/home/joebot/git/mpy-units/build/_deps/mp-units-src/src/systems/include/mp-units/systems/angular/units.h:39:43: error: recursive lazy load
   39 | inline constexpr struct dim_angle final : base_dimension<"A"> {} dim_angle;
      |                                           ^~~~~~~~~~~~~~
/home/joebot/git/mpy-units/build/_deps/mp-units-src/src/systems/include/mp-units/systems/angular/units.h:39:43: fatal error: failed to load pendings for ‘mp_units::detail::power_v’
compilation terminated.
ninja: build stopped: subcommand failed.

Setting MP_UNITS_BUILD_CXX_MODULES OFF in CMakeLists.txt and including the mp-units headers builds successfully.

[mpusz]

According to our C++ compiler support table, C++ modules are not supported on gcc15 due to the bugs in this compiler.

[JoeBot21]

Thanks for the heads up. I had tried a basic module and that worked, but I guess mp-units being a bit more complex shows some of the bugs in the compiler.