CMake: Enable NEW policies by CMake version with a global default

When a CMake release introduces a new policy that affects most Qt
modules, it may be appropriate to make each module aware of that newer
CMake version and use the NEW policy without raising the minimum CMake
version requirement. To reduce the churn associated with making that
change across all Qt modules individually, this change allows it to be
updated in a central place (qtbase), but in a way that allows a Qt
module to override it in its own .cmake.conf file if required (e.g. to
address the issues identified by policy warnings at a later time). The
policies are modified at the start of the call to
qt_build_repo_begin().

For commands defined by the qtbase module, qtbase needs to be in
control of the policy settings at the point where those commands are
defined. The above mechanism should not affect the policy settings for
these commands, so the various *Config.cmake.in files must not specify
policy ranges in a way that a Qt module's .cmake.conf file could
influence.

Starting with CMake 3.12, policies can be specified as a version range
with the cmake_minimum_required() and cmake_policy() commands. All
policies introduced in CMake versions up to the upper limit of that
range will be set to NEW. The actual version of CMake being used only
has to be at least the lower limit of the specified version range.
This change uses cmake_minimum_required() rather than cmake_policy()
due to the latter not halting further processing upon failure.
See the following:

    https://gitlab.kitware.com/cmake/cmake/-/issues/21557

Task-number: QTBUG-88700
Pick-to: 6.0
Change-Id: I0a1f2611dd629f847a18186394f500d7f52753bc
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>

1 files changed, 10 insertions, 5 deletions

diff --git a/cmake/QtPostProcessHelpers.cmake b/cmake/QtPostProcessHelpers.cmake
index 09a5b96479..37213f13a5 100644
--- a/cmake/QtPostProcessHelpers.cmake
+++ b/cmake/QtPostProcessHelpers.cmake
@@ -549,14 +549,19 @@ endif()\n")
                 "set(QT_QPA_DEFAULT_PLATFORM \"${QT_QPA_DEFAULT_PLATFORM}\" CACHE STRING \"\")\n")
         endif()
 
-        # Save the supported and computed minimum CMake versions to ensure the same minimum is
-        # checked for when building other child repos (qtsvg, etc).
+        # Save minimum and policy-related CMake versions to ensure the same minimum is
+        # checked for when building other child repos (qtsvg, etc) and the policy settings
+        # will be consistent unless the child repos explicitly override them.
         qt_internal_get_qt_supported_minimum_cmake_version(min_supported_version)
         qt_internal_get_computed_minimum_cmake_version(computed_min_version)
+        qt_internal_get_min_new_policy_cmake_version(lower_policy_version)
+        qt_internal_get_max_new_policy_cmake_version(upper_policy_version)
         string(APPEND QT_EXTRA_BUILD_INTERNALS_VARS
-            "set(QT_MIN_SUPPORTED_CMAKE_VERSION \"${min_supported_version}\" CACHE STRING \"Minimum supported CMake version required to build Qt\")\n")
-        string(APPEND QT_EXTRA_BUILD_INTERNALS_VARS
-            "set(QT_COMPUTED_MIN_CMAKE_VERSION \"${computed_min_version}\" CACHE STRING \"Computed minimum CMake version required to build Qt\")\n")
+            "set(QT_MIN_SUPPORTED_CMAKE_VERSION \"${min_supported_version}\" CACHE STRING \"Minimum supported CMake version required to build Qt\")\n"
+            "set(QT_COMPUTED_MIN_CMAKE_VERSION \"${computed_min_version}\" CACHE STRING \"Computed minimum CMake version required to build Qt\")\n"
+            "set(QT_MIN_NEW_POLICY_CMAKE_VERSION \"${lower_policy_version}\" CACHE STRING \"Oldest CMake version for which NEW policies should be enabled\")\n"
+            "set(QT_MAX_NEW_POLICY_CMAKE_VERSION \"${upper_policy_version}\" CACHE STRING \"Latest CMake version for which NEW policies should be enabled\")\n"
+        )
 
         # Rpath related things that need to be re-used when building other repos.
         string(APPEND QT_EXTRA_BUILD_INTERNALS_VARS