C++20 is required for the development and buiding of Qt itself (Phase II)

This is the second phase of the adoption of "C++ 20" in Qt.

The phase expands the use of "C++ 20" in Qt and mandates a compatible toolchain and code for the development of Qt itself. This also means that there are some internal APIs which might require C++20.​

In the case of embedded targets, the toolchain to build Qt itself is often the same as the one for Qt application development. Therefore it may mean that some embedded use cases require C++20 enabled toolchains unless they can get their Qt binaries from a different source.

The qualitative difference to Phase III (QTBUG-109362) is that here, we "inflict" C++20, inclusive its deprecations, only onto ourselves, not, yet, onto user projects: They need a C++20-capable C++ compiler, but their code can remain using the compiler's moral equivalent of -std=c++17.

This user story links to other issues on Qt Bug Reports which are scheduled for this phase.​

The "Fix Version" field should mark a version that contains all scoped topics, even if some of them have been provided in earlier versions.

​The tentative plan is Qt 6.9-6.11