7 files changed, 124 insertions, 2 deletions
diff --git a/.cmake.conf b/.cmake.conf
index 7e3a04a0df1..5d7ef22e8ea 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
diff --git a/cmake/QtBaseGlobalTargets.cmake b/cmake/QtBaseGlobalTargets.cmake
index dd7ceecb357..1b7f8d8aa2b 100644
--- a/cmake/QtBaseGlobalTargets.cmake
+++ b/cmake/QtBaseGlobalTargets.cmake
@@ -153,6 +153,13 @@ qt_internal_export_modern_cmake_config_targets_file(TARGETS ${__export_targets}
 qt_internal_get_supported_min_cmake_version_for_using_qt(supported_min_version_for_using_qt)
 qt_internal_get_computed_min_cmake_version_for_using_qt(computed_min_version_for_using_qt)
 
+# Save the minimum required CMake version to use Qt that should appear in a project's
+# cmake_minimum_required() call.
+qt_internal_get_supported_min_cmake_version_for_using_qt_in_cmake_min_required(
+    supported_min_version_for_using_qt_in_cmake_min_required)
+qt_internal_get_computed_min_cmake_version_for_using_qt_in_cmake_min_required(
+    computed_min_version_for_using_qt_in_cmake_min_required)
+
 # Get the lower and upper policy range to embed into the Qt6 config file.
 qt_internal_get_min_new_policy_cmake_version(min_new_policy_version)
 qt_internal_get_max_new_policy_cmake_version(max_new_policy_version)
diff --git a/cmake/QtCMakeVersionHelpers.cmake b/cmake/QtCMakeVersionHelpers.cmake
index a42e1a523f1..52f65ebc125 100644
--- a/cmake/QtCMakeVersionHelpers.cmake
+++ b/cmake/QtCMakeVersionHelpers.cmake
@@ -36,6 +36,25 @@ function(qt_internal_get_supported_min_cmake_version_for_using_qt out_var)
     set(${out_var} "${supported_version}" PARENT_SCOPE)
 endfunction()
 
+# Returns the minimum CMake version that needs to be specified in the cmake_minimum_required() call
+# of a Qt user project as originally advertised by Qt.
+function(qt_internal_get_supported_min_cmake_version_for_using_qt_in_cmake_min_required 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()
+
+    if(BUILD_SHARED_LIBS)
+        set(supported_version
+            "${QT_SUPPORTED_MIN_CMAKE_VERSION_FOR_USING_QT_SHARED_IN_CMAKE_MIN_REQUIRED}")
+    else()
+        set(supported_version
+            "${QT_SUPPORTED_MIN_CMAKE_VERSION_FOR_USING_QT_STATIC_IN_CMAKE_MIN_REQUIRED}")
+    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_min_cmake_version_for_building_qt out_var)
     # An explicit override for those that take it upon themselves to fix the build system
@@ -74,6 +93,23 @@ function(qt_internal_get_computed_min_cmake_version_for_using_qt out_var)
     set(${out_var} "${computed_min_version}" PARENT_SCOPE)
 endfunction()
 
+# Returns the computed minimum CMake version that needs to be specified in the
+# cmake_minimum_required() call of a Qt user project.
+function(qt_internal_get_computed_min_cmake_version_for_using_qt_in_cmake_min_required out_var)
+    # Allow overriding the version for user projects, without forcing
+    # each project developer to have to override it manually.
+    if(QT_FORCE_MIN_CMAKE_VERSION_FOR_USING_QT_IN_CMAKE_MIN_REQUIRED)
+        set(computed_min_version "${QT_FORCE_MIN_CMAKE_VERSION_FOR_USING_QT_IN_CMAKE_MIN_REQUIRED}")
+
+    # No override was given, thus initialize with the default minimum.
+    else()
+        qt_internal_get_supported_min_cmake_version_for_using_qt_in_cmake_min_required(
+            min_supported_version)
+        set(computed_min_version "${min_supported_version}")
+    endif()
+    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
diff --git a/cmake/QtConfig.cmake.in b/cmake/QtConfig.cmake.in
index 75e15226ddf..8b455b0bb64 100644
--- a/cmake/QtConfig.cmake.in
+++ b/cmake/QtConfig.cmake.in
@@ -1,5 +1,9 @@
 @PACKAGE_INIT@
 
+# Used by __qt_internal_warn_if_project_min_cmake_version_is_not_met
+if(NOT _qt_project_last_cmake_minimum_required_version)
+    set(_qt_project_last_cmake_minimum_required_version "${CMAKE_MINIMUM_REQUIRED_VERSION}")
+endif()
 cmake_minimum_required(VERSION @min_new_policy_version@...@max_new_policy_version@)
 
 include("${CMAKE_CURRENT_LIST_DIR}/@INSTALL_CMAKE_NAMESPACE@ConfigExtras.cmake")
diff --git a/cmake/QtConfigExtras.cmake.in b/cmake/QtConfigExtras.cmake.in
index bde3460cdcb..627d3265693 100644
--- a/cmake/QtConfigExtras.cmake.in
+++ b/cmake/QtConfigExtras.cmake.in
@@ -1,2 +1,7 @@
 set(QT_SUPPORTED_MIN_CMAKE_VERSION_FOR_USING_QT "@supported_min_version_for_using_qt@")
 set(QT_COMPUTED_MIN_CMAKE_VERSION_FOR_USING_QT "@computed_min_version_for_using_qt@")
+
+set(QT_SUPPORTED_MIN_CMAKE_VERSION_FOR_USING_QT_IN_CMAKE_MIN_REQUIRED
+    "@supported_min_version_for_using_qt_in_cmake_min_required@")
+set(QT_COMPUTED_MIN_CMAKE_VERSION_FOR_USING_QT_IN_CMAKE_MIN_REQUIRED
+    "@computed_min_version_for_using_qt_in_cmake_min_required@")
diff --git a/cmake/QtModuleConfig.cmake.in b/cmake/QtModuleConfig.cmake.in
index 76fde2dc1fc..66e90fc6b18 100644
--- a/cmake/QtModuleConfig.cmake.in
+++ b/cmake/QtModuleConfig.cmake.in
@@ -1,5 +1,9 @@
 @PACKAGE_INIT@
 
+# Used by __qt_internal_warn_if_project_min_cmake_version_is_not_met
+if(NOT _qt_project_last_cmake_minimum_required_version)
+    set(_qt_project_last_cmake_minimum_required_version "${CMAKE_MINIMUM_REQUIRED_VERSION}")
+endif()
 cmake_minimum_required(VERSION @min_new_policy_version@...@max_new_policy_version@)
 
 include(CMakeFindDependencyMacro)
diff --git a/cmake/QtPublicCMakeVersionHelpers.cmake b/cmake/QtPublicCMakeVersionHelpers.cmake
index 2b3c697f4c4..638c9edc2d0 100644
--- a/cmake/QtPublicCMakeVersionHelpers.cmake
+++ b/cmake/QtPublicCMakeVersionHelpers.cmake
@@ -4,6 +4,12 @@ function(__qt_internal_get_supported_min_cmake_version_for_using_qt out_var)
     set(${out_var} "${supported_version}" PARENT_SCOPE)
 endfunction()
 
+function(__qt_internal_get_supported_min_cmake_version_for_using_qt_in_cmake_min_required out_var)
+    # This is recorded in Qt6ConfigExtras.cmake
+    set(supported_version "${QT_SUPPORTED_MIN_CMAKE_VERSION_FOR_USING_QT_IN_CMAKE_MIN_REQUIRED}")
+    set(${out_var} "${supported_version}" PARENT_SCOPE)
+endfunction()
+
 function(__qt_internal_get_computed_min_cmake_version_for_using_qt out_var)
     # Allow override when configuring user project.
     if(QT_FORCE_MIN_CMAKE_VERSION_FOR_USING_QT)
@@ -20,6 +26,23 @@ function(__qt_internal_get_computed_min_cmake_version_for_using_qt out_var)
     set(${out_var} "${computed_min_version}" PARENT_SCOPE)
 endfunction()
 
+function(__qt_internal_get_computed_min_cmake_version_for_using_qt_in_cmake_min_required out_var)
+    # Allow override when configuring user project.
+    if(QT_FORCE_MIN_CMAKE_VERSION_FOR_USING_QT_IN_CMAKE_MIN_REQUIRED)
+        set(computed_min_version "${QT_FORCE_MIN_CMAKE_VERSION_FOR_USING_QT_IN_CMAKE_MIN_REQUIRED}")
+
+    # Set in QtConfigExtras.cmake.
+    elseif(QT_COMPUTED_MIN_CMAKE_VERSION_FOR_USING_QT_IN_CMAKE_MIN_REQUIRED)
+        set(computed_min_version
+            "${QT_COMPUTED_MIN_CMAKE_VERSION_FOR_USING_QT_IN_CMAKE_MIN_REQUIRED}")
+    else()
+        message(FATAL_ERROR
+            "Qt Developer error: Can't compute the version that should appear in cmake_minimum_required to use this Qt.")
+    endif()
+
+    set(${out_var} "${computed_min_version}" PARENT_SCOPE)
+endfunction()
+
 function(__qt_internal_warn_if_min_cmake_version_not_met)
     __qt_internal_get_supported_min_cmake_version_for_using_qt(min_supported_version)
     __qt_internal_get_computed_min_cmake_version_for_using_qt(computed_min_version)
@@ -34,6 +57,22 @@ function(__qt_internal_warn_if_min_cmake_version_not_met)
     endif()
 endfunction()
 
+function(__qt_internal_warn_if_project_min_cmake_version_is_not_met)
+    __qt_internal_get_supported_min_cmake_version_for_using_qt_in_cmake_min_required(
+        min_supported_version)
+    __qt_internal_get_computed_min_cmake_version_for_using_qt_in_cmake_min_required(
+        computed_min_version)
+
+    if(computed_min_version VERSION_LESS min_supported_version)
+        message(WARNING
+           "To use this Qt, the minimum CMake version that should appear in the project's "
+           "cmake_minimum_required() call should be: '${min_supported_version}'. "
+           "You have explicitly chosen to require a lower version: '${computed_min_version}'. "
+           "Using Qt with this version is not officially supported. Use at your own risk."
+        )
+    endif()
+endfunction()
+
 function(__qt_internal_require_suitable_cmake_version_for_using_qt)
     # Skip the public project check if we're building a Qt repo because it's too early to do
     # it at find_package(Qt6) time.
@@ -66,4 +105,23 @@ function(__qt_internal_require_suitable_cmake_version_for_using_qt)
             "project. Using Qt with this CMake version is not officially supported. "
             "Use at your own risk.")
     endif()
+
+
+    # Check that the project has a supported version specified in the last cmake_minimum_required
+    # call before the Qt6 package was found.
+    __qt_internal_warn_if_project_min_cmake_version_is_not_met()
+    __qt_internal_get_computed_min_cmake_version_for_using_qt_in_cmake_min_required(
+        computed_min_version)
+
+    if(_qt_project_last_cmake_minimum_required_version VERSION_LESS computed_min_version)
+        message(FATAL_ERROR
+            "The last cmake_minimum_required() call before the Qt package was found had the "
+            "following version specified: '${_qt_project_last_cmake_minimum_required_version}' but "
+            "it needs to be ${computed_min_version} or higher to use Qt. "
+            "You can reduce the error into a warning by passing "
+            "-DQT_FORCE_MIN_CMAKE_VERSION_FOR_USING_QT_IN_CMAKE_MIN_REQUIRED=${_qt_project_last_cmake_minimum_required_version} "
+            "when configuring the project, but you do so at your own risk (it is not an officially "
+            "supported way of building Qt projects)."
+        )
+    endif()
 endfunction()