entt fails to compile in C++20 when used inside a module interface file

[KiviTK]

I tried to wrap some entt functionality for a project in C++20 modules, however when entt::registry is used in a module interface the compilation fails in this way:

[1/16] Scanning F:\Pawprint\source\ecs\world.cxxm for CXX dependencies
world.cxxm
[2/16] Generating CXX dyndep file CMakeFiles\Pawprint.dir\CXX.dd
[3/6] Building CXX object CMakeFiles\Pawprint.dir\source\ecs\world.cxxm.obj
FAILED: CMakeFiles/Pawprint.dir/source/ecs/world.cxxm.obj 
C:\PROGRA~1\MICROS~2\2022\COMMUN~1\VC\Tools\MSVC\1440~1.338\bin\Hostx64\x64\cl.exe  /nologo /TP -DGRAPHICS_API_OPENGL_33 -DPLATFORM_DESKTOP -DSPDLOG_COMPILED_LIB -DTRACY_ENABLE -IF:\Pawprint\external -IF:\Pawprint\external\imgui -IF:\Pawprint\external\rlimgui -IF:\Pawprint\external\json\single_include\nlohmann -IF:\Pawprint\cmake-build-debug\_deps\raylib-src\src -IF:\Pawprint\cmake-build-debug\_deps\raylib-src\src\external\glfw\include -IF:\Pawprint\external\spdlog\include -external:IF:\Pawprint\external\tracy\public -external:W0 /DWIN32 /D_WINDOWS /EHsc /Ob0 /Od /RTC1 -std:c++latest -MDd -Zi /utf-8 /showIncludes @CMakeFiles\Pawprint.dir\source\ecs\world.cxxm.obj.modmap /FoCMakeFiles\Pawprint.dir\source\ecs\world.cxxm.obj /FdCMakeFiles\Pawprint.dir\Pawprint.pdb /FS -c F:\Pawprint\source\ecs\world.cxxm
world.cxxm
F:\Pawprint\external\entt.h(1435): error C2760: syntax error: '&&' was unexpected here; expected 'expression'
F:\Pawprint\external\entt.h(1435): note: the template instantiation context (the oldest one first) is
F:\Pawprint\source\ecs\world.ixx(9): note: see reference to class template instantiation 'entt::basic_registry<entt::entity,std::allocator<entt::entity>>' being compiled
F:\Pawprint\external\entt.h(38215): note: see reference to class template instantiation 'entt::dense_map<entt::id_type,std::shared_ptr<entt::basic_sparse_set<Entity,Allocator>>,entt::identity,std::equal_to<entt::id_type>,std::allocator<std::pair<const entt::id_type,std::shared_ptr<entt::basic_sparse_set<Entity,Allocator>>>>>' being compiled
        with
        [
            Entity=entt::entity,
            Allocator=std::allocator<entt::entity>
        ]
F:\Pawprint\external\entt.h(4053): note: see reference to class template instantiation 'entt::compressed_pair<std::vector<size_t,std::allocator<size_t>>,entt::identity>' being compiled
F:\Pawprint\external\entt.h(1435): error C2057: expected constant expression
F:\Pawprint\external\entt.h(1435): error C2059: syntax error: '&&'
F:\Pawprint\external\entt.h(3453): error C2760: syntax error: '&&' was unexpected here; expected 'expression'
F:\Pawprint\external\entt.h(3453): error C2057: expected constant expression
F:\Pawprint\external\entt.h(3453): error C2059: syntax error: '&&'
[4/6] Building CXX object CMakeFiles\PawprintExample.dir\example\main.cpp.obj
FAILED: CMakeFiles/PawprintExample.dir/example/main.cpp.obj 
C:\PROGRA~1\MICROS~2\2022\COMMUN~1\VC\Tools\MSVC\1440~1.338\bin\Hostx64\x64\cl.exe  /nologo /TP -DGRAPHICS_API_OPENGL_33 -DPLATFORM_DESKTOP -DSPDLOG_COMPILED_LIB -DTRACY_ENABLE -IF:\Pawprint\external -IF:\Pawprint\external\imgui -IF:\Pawprint\external\rlimgui -IF:\Pawprint\external\json\single_include\nlohmann -IF:\Pawprint -IF:\Pawprint\cmake-build-debug\_deps\raylib-src\src -IF:\Pawprint\cmake-build-debug\_deps\raylib-src\src\external\glfw\include -IF:\Pawprint\external\spdlog\include -external:IF:\Pawprint\external\tracy\public -external:W0 /DWIN32 /D_WINDOWS /EHsc /Ob0 /Od /RTC1 -std:c++latest -MDd -Zi /utf-8 /showIncludes @CMakeFiles\PawprintExample.dir\example\main.cpp.obj.modmap /FoCMakeFiles\PawprintExample.dir\example\main.cpp.obj /FdCMakeFiles\PawprintExample.dir\ /FS -c F:\Pawprint\example\main.cpp
F:\Pawprint\external\entt.h(1435): error C2760: syntax error: '&&' was unexpected here; expected 'expression'
F:\Pawprint\external\entt.h(1435): note: the template instantiation context (the oldest one first) is
F:\Pawprint\source\ecs\world.ixx(9): note: see reference to class template instantiation 'entt::basic_registry<entt::entity,std::allocator<entt::entity>>' being compiled
F:\Pawprint\external\entt.h(38215): note: see reference to class template instantiation 'entt::dense_map<entt::id_type,std::shared_ptr<entt::basic_sparse_set<Entity,Allocator>>,entt::identity,std::equal_to<entt::id_type>,std::allocator<std::pair<const entt::id_type,std::shared_ptr<entt::basic_sparse_set<Entity,Allocator>>>>>' being compiled
        with
        [
            Entity=entt::entity,
            Allocator=std::allocator<entt::entity>
        ]
F:\Pawprint\external\entt.h(4053): note: see reference to class template instantiation 'entt::compressed_pair<std::vector<size_t,std::allocator<size_t>>,entt::identity>' being compiled
F:\Pawprint\external\entt.h(1435): error C2057: expected constant expression
F:\Pawprint\external\entt.h(1435): error C2059: syntax error: '&&'
F:\Pawprint\external\entt.h(3453): error C2760: syntax error: '&&' was unexpected here; expected 'expression'
F:\Pawprint\external\entt.h(3453): error C2057: expected constant expression
F:\Pawprint\external\entt.h(3453): error C2059: syntax error: '&&'
ninja: build stopped: subcommand failed.

This example error is produced by this code:

// world.ixx
module;

#include <entt.h>

export module world;

export class world {
public:
    entt::registry r;
};

// main.cpp
import world;

int main() {
    world w;
}

The code compiles perfectly fine when the exact same class is copied to a traditional C++ file, like on top of the main file as an example.

[skypjack]

It seems an issue with the module support of your compiler rather than a problem in EnTT. Am I wrong?

[KiviTK]

I am unsure, possibly.
I am using the currently latest MSVC compiler, 17.12.4 Visual Studio Community 2022.

Other libraries that are wrapped this way had no issues, this obviously does not mean that it's not a compiler issue.
Maybe if it's reproducible escalate this to a MSVC issue?

[skypjack]

Uhm, any chance that you can try the same with the clang compiler in VS? It's available out of the box already.

[KiviTK]

Right, I am just using the cl compiler and using external CMake to build the project, but I'll spin it up with clang-cl and report back soon.

[KiviTK]

Regular clang does build the project as expected. This may be an MSVC issue then. clang-cl does not build it, however.
Probably shaky module support on MSVC's end, I may file a report on there too, but I am unsure of what exactly the issue is so may not be trivial.

[skypjack]

The main problem is that I wouldn't even know how to fix it if the problem is a specific compiler on a feature that is still not widespread. I think that calling MSFT on the topic is the best move actually.
I'm open to suggestions to fix it temporarily if you know how to do that. I can't find an easy way to get around the problem yet.

[n0F4x]

I happen to #include <entt/entt.hpp> within module interface units in my project with Clang 19 (I use MSYS2) that uses entt/3.14.0 and it works fine.

[KiviTK]

Regular Clang does build it. And the header itself was not causing issues. I was able to use entt::entity for example without issues, the registry was what broke.
For the time being I am able to continue prototyping on Clang, so this is an acceptable compromise for me.
I also have no suggestions on how it could be worked around on MSVC, so feel free to just close the issue.
I will also recreate the issue and submit it to Microsoft to see if they can figure out where the compilation goes wrong.