Unsolved Strange MOC related error with Q_DECLARE_METATYPE()

[oblivioncth]

I'm porting an application from Qt5 to Qt6 and ran into a bizarre issue.

Minimal Example
[CMakeLists.txt]

cmake_minimum_required(VERSION 3.19)

project(MetaTypeIssue VERSION 0.1 LANGUAGES CXX)

set(CMAKE_CXX_STANDARD 17)
set(CMAKE_CXX_STANDARD_REQUIRED ON)

set(CMAKE_AUTOMOC ON)

find_package(QT NAMES Qt6 Qt5 REQUIRED COMPONENTS Widgets)
find_package(Qt${QT_VERSION_MAJOR} REQUIRED COMPONENTS Widgets)

set(PROJECT_SOURCES
        main.cpp
        m.h
        foo.h
)
if(${QT_VERSION_MAJOR} GREATER_EQUAL 6)
    qt_add_executable(${PROJECT_NAME} ${PROJECT_SOURCES})
else()
    add_executable(${PROJECT_NAME} ${PROJECT_SOURCES})
endif()

target_link_libraries(MetaTypeIssue PRIVATE Qt${QT_VERSION_MAJOR}::Core)

[foo.h]

#ifndef FOO_H
#define FOO_H

#include <QObject>

class Foo : public QObject
{
    Q_OBJECT

public:
    Foo() {}

signals:

    void sig(std::shared_ptr<int> var);
};

#endif // FOO_H

[m.h]

#ifndef BAR_H
#define BAR_H

#include <QObject>

class Bar : public QObject
{
    Q_OBJECT

public:
    Bar() {}
};
Q_DECLARE_METATYPE(std::shared_ptr<int>);

#endif // BAR_H

[main.cpp]

#include <QCoreApplication>

int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);
    return a.exec();
}

Trying to compile this setup with Qt 6 (6.2.4) results in:

m.h:13: error: C2908: explicit specialization; 'QMetaTypeId<T>' has already been instantiated
m.h:13: error: C2766: explicit specialization; 'QMetaTypeId<T>' has already been defined
qmetatype.h:1132: see previous definition of 'QMetaTypeId<std::shared_ptr<int> >'

Some changes that "prevent" the errors:

• Change the parameter type of Foo::sig() to be different than that in Q_DECLARE_METATYPE() (obviously breaks functionality)
• Disable auto-MOC (obviously causes linker errors)
• Rename m.h to any name that doesn't start with an 'm'
• Compile with Qt 5 (tested with 5.12.3)

I was hoping to see if anyone could reproduce this or note if/why this outright shouldn't work if that really is the case for some reason.

Here is the above MRE for convenience: https://www.mediafire.com/file/lm35jkzipj0hmwy/MetaTypeIssue.zip/file

The original application was able to use shared_ptr<int> as a signal argument just fine, though it was only through a queued connection briefly before threading changes were made, but I'm pretty sure it worked the same when queued.

EDIT: Corrected accidental usage of unique_ptr in example code instead of shared_ptr (what it actually was/is).

[MB12]

Hello, were you able to resolve this error?
I am running into this issue when upgrading from QT5 to QT6. Any ideas on troubleshooting this would be appreciated.

[MB12]

@oblivioncth
This is to confirm that I am able to reproduce the issue on QT 6.4.2 on MacOS.
The project I am trying to port is much bigger and the filename that results in this error does not start with 'm'.

[ 25%] Automatic MOC for target MetaTypeIssue
[ 25%] Built target MetaTypeIssue_autogen
[ 50%] Building CXX object CMakeFiles/MetaTypeIssue.dir/MetaTypeIssue_autogen/mocs_compilation.cpp.o
In file included from /var/tmp/meta_issue/build/MetaTypeIssue_autogen/mocs_compilation.cpp:3:
In file included from /var/tmp/meta_issue/build/MetaTypeIssue_autogen/EWIEGA46WW/moc_m.cpp:10:
/var/tmp/meta_issue/build/MetaTypeIssue_autogen/EWIEGA46WW/../../../m.h:13:1: error: explicit specialization of 'QMetaTypeId<std::shared_ptr<int>>' after instantiation
Q_DECLARE_METATYPE(std::shared_ptr<int>);
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/opt/homebrew/include/QtCore/qmetatype.h:1399:34: note: expanded from macro 'Q_DECLARE_METATYPE'
#define Q_DECLARE_METATYPE(TYPE) Q_DECLARE_METATYPE_IMPL(TYPE)
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/opt/homebrew/include/QtCore/qmetatype.h:1403:12: note: expanded from macro 'Q_DECLARE_METATYPE_IMPL'
struct QMetaTypeId< TYPE >
^~~~~~~~~~~~~~~~~~~
/opt/homebrew/include/QtCore/qmetatype.h:1122:22: note: implicit instantiation first required here
enum { Defined = QMetaTypeId<T>::Defined, IsBuiltIn=false };
^
1 error generated.

[MB12]

I have created a bug report regarding this. I used the minimal example that you have created in the bug report. Thanks for sharing your minimal example.

https://bugreports.qt.io/browse/QTBUG-111968