CMake: Add Qt6 forward compatible CMake API and targets

Create Qt:: versionless targets for libraries and tools. So
Qt::Core will link to Qt5::Core.

Add additional feature properties to targets, with the same name
they have in Qt6: QT_ENABLED_PUBLIC_FEATURES,
QT_DISABLED_PUBLIC_FEATURES, QT_ENABLED_PRIVATE_FEATURES,
QT_DISABLED_PRIVATE_FEATURES, to be forward-compatible with Qt6.
Prefix properties with INTERFACE_ for interface libraries.

Create functions with no major version in their prefix, so qt_foo
instead of qt5_foo.
The non-versioned functions will call the versioned functions,
depending on the value of QT_DEFAULT_MAJOR_VERSION, which can be
set by an application developer before finding the Qt package.

Set QT_DEFAULT_MAJOR_VERSION to 5 if the value has not been defined
in the current scope.

Application developers can set QT_NO_CREATE_VERSIONLESS_FUNCTIONS to
TRUE before calling find_package(Qt5) to suppress creation of the
non-versioned functions.

Application developers can set QT_NO_CREATE_VERSIONLESS_TARGETS to
TRUE before calling find_package(Qt5) to suppress creation of the
non-versioned targets.

Setting these can be useful when both find_package(Qt5) and
find_package(Qt6) are in the same project.
If none of these are set by the user, then the first find_package(Qt5)
will create versionless targets with the major version being "5",
which means the second find_package(Qt6) will not create versionless
targets.

Handle versionless plugin names in qt_import_plugins, so both
Qt::QCocoaIntegrationPlugin and Qt5/6::QCocoaIntegrationPlugin
are recognized by the function.

Allow specifying multiple types in EXCLUDE_BY_TYPE in
qt_import_plugins, to be consitent with the Qt 6 version.

Make sure to set the QT_PLUGIN_CLASS_NAME property to compatible with
Qt 6.

Task-number: QTBUG-74137
Task-number: QTBUG-80477
Change-Id: Ib89d090ea6f7794d7debd64f03f29da963a17ca7
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>

2 files changed, 23 insertions, 0 deletions

diff --git a/src/widgets/Qt5WidgetsConfigExtras.cmake.in b/src/widgets/Qt5WidgetsConfigExtras.cmake.in
index 99d87e2e46..236d1d4159 100644
--- a/src/widgets/Qt5WidgetsConfigExtras.cmake.in
+++ b/src/widgets/Qt5WidgetsConfigExtras.cmake.in
@@ -17,3 +17,14 @@ endif()
 include(\"${CMAKE_CURRENT_LIST_DIR}/Qt5Widgets_AccessibleFactory.cmake\" OPTIONAL)
 
 set(Qt5Widgets_UIC_EXECUTABLE Qt5::uic)
+
+# Create versionless tool targets.
+foreach(__qt_tool uic)
+    if(NOT \"${QT_NO_CREATE_VERSIONLESS_TARGETS}\" AND NOT TARGET Qt::${__qt_tool}
+       AND TARGET Qt5::${__qt_tool})
+        add_executable(Qt::${__qt_tool} IMPORTED)
+        get_target_property(__qt_imported_location Qt5::${__qt_tool} IMPORTED_LOCATION)
+        set_target_properties(Qt::${__qt_tool}
+                              PROPERTIES IMPORTED_LOCATION \"${__qt_imported_location}\")
+    endif()
+endforeach()
diff --git a/src/widgets/Qt5WidgetsMacros.cmake b/src/widgets/Qt5WidgetsMacros.cmake
index 21e73d4f0c..35d2a2dc54 100644
--- a/src/widgets/Qt5WidgetsMacros.cmake
+++ b/src/widgets/Qt5WidgetsMacros.cmake
@@ -66,3 +66,15 @@ function(qt5_wrap_ui outfiles )
     endforeach()
     set(${outfiles} ${${outfiles}} PARENT_SCOPE)
 endfunction()
+
+# This will override the CMake upstream command, because that one is for Qt 3.
+if(NOT QT_NO_CREATE_VERSIONLESS_FUNCTIONS)
+    function(qt_wrap_ui outfiles)
+        if(QT_DEFAULT_MAJOR_VERSION EQUAL 5)
+            qt5_wrap_ui("${outfiles}" ${ARGN})
+        elseif(QT_DEFAULT_MAJOR_VERSION EQUAL 6)
+            qt6_wrap_ui("${outfiles}" ${ARGN})
+        endif()
+        set("${outfiles}" "${${outfiles}}" PARENT_SCOPE)
+    endfunction()
+endif()