CMake: Warn if cmake_minimum_required has an unsupported low version

Qt6Config.cmake calls cmake_minimum_required to ensure a recent enough
CMake version is used in projects.

That call does not set policies in the calling subdirectory scope,
because find_package introduces a new policy scope.

If a project using Qt has a 'cmake_minimum_required(VERSION 3.1)'
call and is configured with a recent CMake, many policies will
still be set to OLD.

One such policy is CMP0071 (Run AUTOMOC on GENERATED files). The
policy value is queried at generation time rather than at target
definition time, which means we can't influence the policy value
(e.g. inside the implementation of qt_add_executable for example)

The inability to influence the policy value for targets created by our
own CMake functions is unfortunate and can lead to issues (in the case
of the above policy to compilation / linker issues).

Record the version of the last cmake_minimum_required call before
the Qt packages are found and error out if the version is lower than
the minimum supported one.

A project can reduce the error into a warning by specifying a
-DQT_FORCE_MIN_CMAKE_VERSION_FOR_USING_QT_IN_CMAKE_MIN_REQUIRED=3.xyz
option when configuring the project.
If the option is used and build issues arise, no official support is
given.

All the CMake example projects shipped with Qt specify a minimum
version of 3.16 already (which is the minimum for shared Qt builds),
so it shouldn't be an issue to require that in other user projects as
well.

Implementation wise, we follow the existing pattern to record
what the minimum and computed versions for static and shared Qt
builds are at qtbase configure time.
These are then checked before the Qt6 or QtFoo packages are
find_package'd.

Amends 6518bcc167d47e1c27d082c21551b9a838b04e5d

Pick-to: 6.2
Task-number: QTBUG-95018
Task-number: QTBUG-95832
Change-Id: I1a1d06d82f566c92192a699045127943604c8353
Reviewed-by: Craig Scott <craig.scott@qt.io>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>

1 files changed, 10 insertions, 2 deletions

diff --git a/.cmake.conf b/.cmake.conf
index 7e3a04a0df..5d7ef22e8e 100644
--- a/.cmake.conf
+++ b/.cmake.conf
@@ -1,16 +1,24 @@
 set(QT_REPO_MODULE_VERSION "6.3.0")
 set(QT_REPO_MODULE_PRERELEASE_VERSION_SEGMENT "alpha1")
 
-# Minimum requirement for building Qt
+# Minimum required CMake version for building Qt.
 set(QT_SUPPORTED_MIN_CMAKE_VERSION_FOR_BUILDING_QT_SHARED "3.16")
 set(QT_SUPPORTED_MIN_CMAKE_VERSION_FOR_BUILDING_QT_STATIC "3.21")
 
-# Minimum requirement for consuming Qt in a user project.
+# Minimum required CMake version for using Qt in a user project.
 # This might be different in the future, e.g. be lower than the requirement for
 # building Qt.
 set(QT_SUPPORTED_MIN_CMAKE_VERSION_FOR_USING_QT_SHARED "3.16")
 set(QT_SUPPORTED_MIN_CMAKE_VERSION_FOR_USING_QT_STATIC "3.21")
 
+# Minimum CMake version that needs to appear in the cmake_minimum_required() call of a Qt user
+# project, so that all policies defined up to the specified CMake version are using NEW behavior.
+# With a value of 3.16, all policies CMP0000-CMP0097 will use NEW behavior.
+#
+# For now, both shared and static Qt builds share the same minimum.
+set(QT_SUPPORTED_MIN_CMAKE_VERSION_FOR_USING_QT_SHARED_IN_CMAKE_MIN_REQUIRED "3.16")
+set(QT_SUPPORTED_MIN_CMAKE_VERSION_FOR_USING_QT_STATIC_IN_CMAKE_MIN_REQUIRED "3.16")
+
 # Policy settings for commands defined by qtbase. These will also be injected
 # into the top level policy scope of each Qt module when building Qt so that
 # modules have the same policy settings as qtbase by default. They can be