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, 39 insertions, 3 deletions

diff --git a/cmake/QtCMakeVersionHelpers.cmake b/cmake/QtCMakeVersionHelpers.cmake
index 274af52cd1..aca1c50206 100644
--- a/cmake/QtCMakeVersionHelpers.cmake
+++ b/cmake/QtCMakeVersionHelpers.cmake
@@ -27,11 +27,36 @@ function(qt_internal_get_computed_minimum_cmake_version out_var)
     set(${out_var} "${computed_min_version}" PARENT_SCOPE)
 endfunction()
 
+# Returns the oldest CMake version for which NEW policies should be enabled.
+# It can be older than the minimum supported or computed CMake version, as it
+# is only used for policy settings. The currently running CMake must not be
+# older than this version though (doing so will result in an error).
+function(qt_internal_get_min_new_policy_cmake_version out_var)
+    # QT_MIN_NEW_POLICY_CMAKE_VERSION is set either in .cmake.conf or in
+    # QtBuildInternalsExtras.cmake when building a child repo.
+    set(lower_version "${QT_MIN_NEW_POLICY_CMAKE_VERSION}")
+    set(${out_var} "${lower_version}" PARENT_SCOPE)
+endfunction()
+
+# Returns the latest CMake version for which NEW policies should be enabled.
+# This cannot be less than the minimum CMake policy version or we will end up
+# specifying a version range with the max less than the min.
+function(qt_internal_get_max_new_policy_cmake_version out_var)
+    # QT_MAX_NEW_POLICY_CMAKE_VERSION is set either in .cmake.conf or in
+    # QtBuildInternalsExtras.cmake when building a child repo.
+    set(upper_version "${QT_MAX_NEW_POLICY_CMAKE_VERSION}")
+    qt_internal_get_min_new_policy_cmake_version(lower_version)
+    if(upper_version VERSION_LESS lower_version)
+        set(upper_version ${lower_version})
+    endif()
+    set(${out_var} "${upper_version}" PARENT_SCOPE)
+endfunction()
+
 function(qt_internal_check_for_suitable_cmake_version)
     # Implementation note.
-    # The very first cmake_required_version() call can't be placed in an include()d file.
+    # The very first cmake_minimum_required() call can't be placed in an include()d file.
     # It causes CMake to fail to configure with 'No cmake_minimum_required command is present.'
-    # The first cmake_required_version() must be called directly in the top-level CMakeLists.txt
+    # The first cmake_minimum_required() must be called directly in the top-level CMakeLists.txt
     # file.
     # That's why this function only handles output of warnings, and doesn't try to set the required
     # version.
@@ -41,7 +66,7 @@ endfunction()
 
 # Function to be used in child repos like qtsvg to require a minimum CMake version.
 #
-# Such repos don't have the required version information at cmake_required_version() time, that's
+# Such repos don't have the required version information at cmake_minimum_required() time, that's
 # why we provide this function to be called at a time when the info is available.
 function(qt_internal_require_suitable_cmake_version)
     qt_internal_check_for_suitable_cmake_version()
@@ -100,3 +125,14 @@ function(qt_internal_warn_about_unsuitable_cmake_versions)
         endif()
     endforeach()
 endfunction()
+
+# Functions don't have their own policy scope, so the policy settings modified
+# here will be those of the caller's policy scope. Note that these settings
+# will only apply to functions and macros defined after this function is called,
+# but not to any that are already defined. Ordinary CMake code not inside a
+# function or macro will be affected by these policy settings too.
+function(qt_internal_upgrade_cmake_policies)
+    qt_internal_get_computed_minimum_cmake_version(lower_version)
+    qt_internal_get_max_new_policy_cmake_version(upper_version)
+    cmake_minimum_required(VERSION ${lower_version}...${upper_version})
+endfunction()