[C++-Qt] qHash: invalid initialization of reference in autogenerated code

Description

When generating code using the cpp-qt-client for Qt5 in version 6 or 7, compiling will lead to errors of type

[ 11%] Building CXX object CMakeFiles/QtOpenAPIClient.dir/OAIOrder.cpp.o
In file included from /cpp/client/OAIOrder_dimensions.h:25:0,
                 from /cpp/client/OAIOrder.h:23,
                 from /cpp/client/OAIOrder.cpp:12:
/cpp/client/OAIObject.h: In function 'uint OpenAPI::qHash(const OpenAPI::OAIObject&, uint)':
/cpp/client/OAIObject.h:72:34: error: invalid initialization of reference of type 'const OpenAPI::OAIObject&' from expression of type 'QJsonObject'
     return qHash(obj.asJsonObject(), seed);
                  ~~~~~~~~~~~~~~~~^~
/cpp/client/OAIObject.h:71:1: note: in passing argument 1 of 'uint OpenAPI::qHash(const OpenAPI::OAIObject&, uint)'
 qHash(const OAIObject& obj, uint seed = 0) noexcept{
 ^~~~~
make[2]: *** [CMakeFiles/QtOpenAPIClient.dir/build.make:76: CMakeFiles/QtOpenAPIClient.dir/OAIOrder.cpp.o] Error 1
make[1]: *** [CMakeFiles/Makefile2:83: CMakeFiles/QtOpenAPIClient.dir/all] Error 2

openapi-generator version

I have experienced this with the openapi-generator-cli available in the openapitools/openapi-generator-cli:v6.6.0 and v7.2.0 images.
With v5.4.0 I got other errors preventing me from compiling.
I can confirm that v4.3.1 works with my configuration and setup.

I compile on openSUSE Leap 15.5 using CMake, and I am pretty sure I have all the required packages available.

OpenAPI declaration file content or url

I follow the steps described in the Qt example found at
https://www.qt.io/resources/videos/qt-and-openapi-swagger-a-tutorial-dev-des-2021, or
https://blog.basyskom.com/2021/qml-qt-quick-and-swagger-openapi-tutorial.

The specification file is:
https://github.com/basysKom/swagger_qt_demo/blob/182e70418779e07998d48a99ff7247f23ae7d183/swagger.yaml

Command line used for generation

To generate, I use

docker run --rm -v ${PWD}:/local/ openapitools/openapi-generator-cli:v7.2.0 generate -i /local/swagger.yaml -g cpp-qt-client -o /local/generated/cpp/

Steps to reproduce

• Download the file from the demo (see above)
• Generate the Qt code
• Build a docker image based on openSUSE Leap 15.5, with make, cmake, and all the required Qt devel packages (most likely, the error also occurs with a different setup)
• run the image with the generated code mounted

  docker run -ti -v ${PWD}/generated/cpp/:/cpp/ my-docker-image:15.5 /usr/bin/env bash
  

• create a build directory

  mkdir $HOME/apibuild/
  cd $HOME/apibuild/
  

• generate the C++ code, specify the Qt version (not sure how I am supposed to set $QT_VERSION_MAJOR else)

  cmake -DQT_VERSION_MAJOR=5 /cpp/client/
  

• try compiling

  make
  

My expectation is that this would create a static library libQtOpenAPIClient.a in C++ that I can use in my project. And it does that for version 4.3.1, but not 6 or 7.

This might have to do with Qt5 (which I use), and Qt6 differing in the return type of qHash: uint in Qt5, and size_t in Qt6.

Note that changing the autogenerated code from

inline
#if QT_VERSION < 0x060000
uint
#else
size_t
#endif
qHash(const OAIObject& obj, uint seed = 0) noexcept{
    return qHash(obj.asJsonObject(), seed);
}

to

inline
#if QT_VERSION < 0x060000
uint
#else
size_t
#endif
qHash(const OAIObject& obj, uint seed = 0) noexcept{
    return qHash(obj, seed);    // no .asJsonObject()
}

at least results in the library being compiled successfully using above settings. I cannot verify right now whether or not the resulting library behaves correctly.

Related issues/PRs

My suspicion is that this has to do with #11621, but I can't prove that.

Reproduce the failure with the linked swagger.yaml, the cpp-qt-client generator, and the CMake and make commands described in the issue. Start by inspecting generated cpp/client/OAIObject.h and its qHash implementation, then compare the generated output with the working 4.3.1 result. Done means the generated Qt5 client builds successfully into libQtOpenAPIClient.a without manual edits.