This code:

cmake_minimum_required(VERSION 3.28)
cmake_policy(VERSION 3.28)
project(X)

set(QT_DEFAULT_MAJOR_VERSION 6)
find_package(QT NAMES Qt6 Qt5 REQUIRED COMPONENTS Core Gui Widgets LinguistTools)
find_package(Qt${QT_VERSION_MAJOR} REQUIRED COMPONENTS Core Gui Widgets LinguistTools)
message(STATUS "QT Major Version: " ${QT_VERSION_MAJOR})

is supposed to select QT6 in the mixed Qt5/Qt6 environment, but it selects Qt5.

– QT Major Version: 5

The usage is suggested by the Qt official documentation:

https://doc.qt.io/qt-6/cmake-qt5-and-qt6-compatibility.html#supporting-older-qt-5-versions

Here we let find_package(<PackageName>...) try to find first Qt 6, and if that fails Qt 5, under the name QT. If either of them is found, find_package will succeed, and the CMake variable QT_VERSION_MAJOR will be defined to either 5 or 6.

To reproduce the issue, ensure you have both Qt 5 and Qt 6 development package installed on your distro.

The actual behavior is:

1. it does NOT try to find first Qt 6 and if that fails Qt 5 as described in the documentation.
2. weither QT_DEFAULT_MAJOR_VERSION is set, it will still always find and use Qt 5 instead of Qt 6

See also:

• https://gitlab.kitware.com/cmake/cmake/-/issues/25691
• https://gitlab.kitware.com/cmake/cmake/-/issues/23575
• https://github.com/ra3xdh/qucs_s/issues/592