CMake: Bump min required CMake version for static Qt builds to 3.20

Building a static library Qt configuration will now require a
minimum CMake version of 3.20.

Qt builders and packagers can still opt out of the mentioned minimum
required version by configuring Qt with QT_FORCE_MIN_CMAKE_VERSION.
Such a Qt configuration is /NOT SUPPORTED/.

To facilitate these changes, the minimum version check has been moved
to happen after the BUILD_SHARED_LIBS option is computed by either
QtAutoDetect.cmake or set by a user provided cmake toolchain file.

Introduce a new QT_MIN_SUPPORTED_CMAKE_VERSION_FOR_STATIC_QT variable
in .cmake.conf to mark the minimum version for a static Qt build.

Pick-to: 6.2
Task-number: QTBUG-95018
Change-Id: Idc1875729f26a7c635b6bd26ac0c1be973917c13
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>
Reviewed-by: Alexey Edelev <alexey.edelev@qt.io>
Reviewed-by: Craig Scott <craig.scott@qt.io>

3 files changed, 45 insertions, 25 deletions

diff --git a/.cmake.conf b/.cmake.conf
index 848faeb55d..ff95a9c186 100644
--- a/.cmake.conf
+++ b/.cmake.conf
@@ -3,6 +3,7 @@ set(QT_REPO_MODULE_PRERELEASE_VERSION_SEGMENT "alpha1")
 
 # Minimum requirement for building Qt
 set(QT_MIN_SUPPORTED_CMAKE_VERSION "3.16")
+set(QT_MIN_SUPPORTED_CMAKE_VERSION_FOR_STATIC_QT "3.20")
 
 # 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
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 35ee14102e..c3dcc2f731 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -28,10 +28,6 @@ endif()
 unset(build_dir_absolute)
 unset(build_dir_realpath)
 
-# Early check to reduce chance of warning being lost in the output
-include("${CMAKE_CURRENT_SOURCE_DIR}/cmake/QtCMakeVersionHelpers.cmake")
-qt_internal_check_for_suitable_cmake_version()
-
 # Run auto detection routines, but not when doing standalone tests. In that case, the detection
 # results are taken from either QtBuildInternals or the qt.toolchain.cmake file. Also, inhibit
 # auto-detection in a top-level build, because the top-level project file already includes it.
@@ -39,6 +35,7 @@ if(NOT QT_BUILD_STANDALONE_TESTS AND NOT QT_SUPERBUILD)
     include(${CMAKE_CURRENT_SOURCE_DIR}/cmake/QtAutoDetect.cmake)
 endif()
 
+# This call will load any provided cmake toolchain file.
 project(QtBase
     VERSION "${QT_REPO_MODULE_VERSION}"
     DESCRIPTION "Qt Base Libraries"
@@ -46,6 +43,22 @@ project(QtBase
     LANGUAGES CXX C ASM
 )
 
+# Should this Qt be static or dynamically linked?
+option(BUILD_SHARED_LIBS "Build Qt statically or dynamically" ON)
+set(QT_BUILD_SHARED_LIBS ${BUILD_SHARED_LIBS})
+
+# This variable is also set in Qt6CoreConfigExtras.cmake, but it's not loaded when building
+# qtbase. Set it here so qt_add_plugin can compute the proper plugin flavor.
+set(QT6_IS_SHARED_LIBS_BUILD ${BUILD_SHARED_LIBS})
+
+# BUILD_SHARED_LIBS influences the minimum required CMake version. The value is set either by:
+#   a cache variable provided on the configure command line
+#   or set by QtAutoDetect.cmake depending on the platform
+#   or specified via a toolchain file that is loaded by the project() call
+#   or set by the option() call above
+include("${CMAKE_CURRENT_SOURCE_DIR}/cmake/QtCMakeVersionHelpers.cmake")
+qt_internal_check_for_suitable_cmake_version()
+
 if(NOT QT_BUILD_STANDALONE_TESTS)
     ## Add some paths to check for cmake modules:
     list(PREPEND CMAKE_MODULE_PATH
@@ -77,14 +90,6 @@ endif()
 qt_build_repo_begin()
 
 if(NOT QT_BUILD_STANDALONE_TESTS)
-    ## Should this Qt be static or dynamically linked?
-    option(BUILD_SHARED_LIBS "Build Qt statically or dynamically" ON)
-    set(QT_BUILD_SHARED_LIBS ${BUILD_SHARED_LIBS})
-
-    # This variable is also set in Qt6CoreConfigExtras.cmake, but it's not loaded when building
-    # qtbase. Set it here so qt_add_plugin can compute the proper plugin flavor.
-    set(QT6_IS_SHARED_LIBS_BUILD ${BUILD_SHARED_LIBS})
-
     ## Should this Qt be built with Werror?
     option(WARNINGS_ARE_ERRORS "Build Qt with warnings as errors" ${FEATURE_developer_build})
 
diff --git a/cmake/QtCMakeVersionHelpers.cmake b/cmake/QtCMakeVersionHelpers.cmake
index 4d4113bb43..31eb78b667 100644
--- a/cmake/QtCMakeVersionHelpers.cmake
+++ b/cmake/QtCMakeVersionHelpers.cmake
@@ -1,29 +1,40 @@
 # Returns the minimum supported CMake version required to build Qt as originally advertised by Qt.
 function(qt_internal_get_qt_supported_minimum_cmake_version out_var)
+    if(NOT DEFINED BUILD_SHARED_LIBS)
+        message(FATAL_ERROR "BUILD_SHARED_LIBS is needed to decide the minimum CMake version. "
+            "It should have been set by this point.")
+    endif()
+
     # QT_MIN_SUPPORTED_CMAKE_VERSION is set either in .cmake.conf or in QtBuildInternalsExtras.cmake
-    # when building a child repo.
+    # when building a repo.
     set(supported_version "${QT_MIN_SUPPORTED_CMAKE_VERSION}")
+
+    if(NOT BUILD_SHARED_LIBS)
+        set(supported_version "${QT_MIN_SUPPORTED_CMAKE_VERSION_FOR_STATIC_QT}")
+    endif()
     set(${out_var} "${supported_version}" PARENT_SCOPE)
 endfunction()
 
 # Returns the computed minimum supported CMake version required to build Qt.
 function(qt_internal_get_computed_minimum_cmake_version out_var)
-    qt_internal_get_qt_supported_minimum_cmake_version(min_supported_version)
-    set(computed_min_version "${min_supported_version}")
-
-    # Set in QtBuildInternalsExtras.cmake.
-    if(QT_COMPUTED_MIN_CMAKE_VERSION)
-        set(computed_min_version "${QT_COMPUTED_MIN_CMAKE_VERSION}")
-    endif()
-
     # An explicit override for those that take it upon themselves to fix the build system
     # when using a CMake version lower than the one officially supported.
     # Also useful for build testing locally with different minimum versions to observe different
     # policy behaviors.
     if(QT_FORCE_MIN_CMAKE_VERSION)
         set(computed_min_version "${QT_FORCE_MIN_CMAKE_VERSION}")
-    endif()
 
+    # Set in QtBuildInternalsExtras.cmake, which means it was already computed as part of qtbase
+    # configuration.
+    elseif(QT_COMPUTED_MIN_CMAKE_VERSION)
+        set(computed_min_version "${QT_COMPUTED_MIN_CMAKE_VERSION}")
+
+    # No override was given and the version was not computed before, thus initialize with the
+    # default minimum.
+    else()
+        qt_internal_get_qt_supported_minimum_cmake_version(min_supported_version)
+        set(computed_min_version "${min_supported_version}")
+    endif()
     set(${out_var} "${computed_min_version}" PARENT_SCOPE)
 endfunction()
 
@@ -64,7 +75,7 @@ function(qt_internal_check_for_suitable_cmake_version)
     qt_internal_warn_about_unsuitable_cmake_versions()
 endfunction()
 
-# Function to be used in child repos like qtsvg to require a minimum CMake version.
+# Function to be used in downstream repos like qtsvg to require a minimum CMake version.
 #
 # 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.
@@ -73,8 +84,11 @@ function(qt_internal_require_suitable_cmake_version)
     qt_internal_get_computed_minimum_cmake_version(computed_min_version)
 
     if(CMAKE_VERSION VERSION_LESS computed_min_version)
-        message(FATAL_ERROR "CMake ${computed_min_version} or higher is required. "
-                            "You are running version ${CMAKE_VERSION}")
+        message(FATAL_ERROR
+            "CMake ${computed_min_version} or higher is required. "
+            "You are running version ${CMAKE_VERSION} "
+            "\nQt requires newer CMake features to build correctly. You can lower the minimum "
+            "required version by passing a QT_FORCE_MIN_CMAKE_VERSION cache variable when configuring Qt.")
     endif()
 endfunction()