CMake: Add warnings as errors support for example builds

We want to ensure we don't introduce new build warnings in examples.
We have a mechanism to enable warnings as errors for qt targets like
modules or plugins, but not for targets in example projects.

To enable this, we introduce a new internal target
PlatformExampleInternal, which is linked to all targets created
by qt_add_executable and qt_add_library-like functions in projects
in the <repo>/examples subdirectory.

The detection of targets in example projects is done by checking the
QT_INTERNAL_IS_EXAMPLE_EP_BUILD variable or the
QT_INTERNAL_IS_EXAMPLE_IN_TREE_BUILD variable. These are set either by
ExternalProject_Add in the case of external example projects, or by
qt_examples_build_begin in the case of in-tree example projects.

The PlatformExampleInternal target gets the same warnings as errors
flags as the other Platform*Internal targets.

To enable gradual adoption, the new feature is opt-in rather than
opt-out, on a per-repo basis. The QT_REPO_EXAMPLES_WARNINGS_CLEAN
variable can be set to TRUE in a repo's '.cmake.conf' file.

The warnings as errors behavior is controlled as usual by configuring
qt with -DWARNINGS_ARE_ERRORS=ON.

Pick-to: 6.8
Task-number: QTBUG-108448
Change-Id: If62b47b205c111f2a1fec791e5bb6502f5b64f9c
Reviewed-by: Alexey Edelev <alexey.edelev@qt.io>
(cherry picked from commit 26f8dff7da)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>
(cherry picked from commit 0c63426e66)

cmake/QtBaseGlobalTargets.cmake

@@ -144,6 +144,7 @@ target_include_directories(GlobalConfigPrivate INTERFACE
 qt_internal_setup_public_platform_target()
 
 # defines PlatformCommonInternal PlatformModuleInternal PlatformPluginInternal PlatformToolInternal
+# PlatformExampleInternal
 include(QtInternalTargets)
 qt_internal_run_common_config_tests()
 
@@ -158,7 +159,9 @@ set(__export_targets Platform
                      PlatformModuleInternal
                      PlatformPluginInternal
                      PlatformAppInternal
-                     PlatformToolInternal)
+                     PlatformToolInternal
+                     PlatformExampleInternal
+                 )
 set(__export_name "${INSTALL_CMAKE_NAMESPACE}Targets")
 qt_install(TARGETS ${__export_targets} EXPORT "${__export_name}")
 qt_install(EXPORT ${__export_name}

cmake/QtBuildRepoExamplesHelpers.cmake

@@ -89,6 +89,9 @@ macro(qt_examples_build_begin)
         if(active_linker_flags)
             add_link_options(${active_linker_flags})
         endif()
+
+        # Marker for warnings_as_errors.
+        set(QT_INTERNAL_IS_EXAMPLE_IN_TREE_BUILD ON)
     endif()
 
     # TODO: Change this to TRUE when all examples in all repos are ported to use
@@ -467,6 +470,8 @@ function(qt_internal_add_example_external_project subdir)
         QT_ADDITIONAL_PACKAGES_PREFIX_PATH:STRING
         QT_ADDITIONAL_QML_PLUGIN_GLOB_PREFIXES:STRING
         QT_INTERNAL_SKIP_DEPLOYMENT:BOOL
+        QT_REPO_EXAMPLES_WARNINGS_CLEAN:BOOL
+        WARNINGS_ARE_ERRORS:BOOL
         CMAKE_FIND_ROOT_PATH:STRING
         CMAKE_MODULE_PATH:STRING
         BUILD_SHARED_LIBS:BOOL
@@ -608,6 +613,7 @@ function(qt_internal_add_example_external_project subdir)
         TEST_COMMAND     ""
         DEPENDS          ${deps}
         CMAKE_CACHE_ARGS ${var_defs}
+                         -DQT_INTERNAL_IS_EXAMPLE_EP_BUILD:BOOL=TRUE
                          -DCMAKE_INSTALL_PREFIX:STRING=<INSTALL_DIR>
                          -DQT_INTERNAL_SET_EXAMPLE_INSTALL_DIR_TO_DOT:BOOL=TRUE
         ${terminal_args}

cmake/QtFindPackageHelpers.cmake

@@ -466,6 +466,7 @@ function(qt_internal_is_lib_part_of_qt6_package lib out_var)
             OR lib STREQUAL "PlatformPluginInternal"
             OR lib STREQUAL "PlatformToolInternal"
             OR lib STREQUAL "PlatformCommonInternal"
+            OR lib STREQUAL "PlatformExampleInternal"
     )
         set(${out_var} "TRUE" PARENT_SCOPE)
     else()

cmake/QtFlagHandlingHelpers.cmake

@@ -313,11 +313,7 @@ endfunction()
 
 # Controls the QT_SKIP_WARNINGS_ARE_ERRORS property for the given target.
 function(qt_internal_set_skip_warnings_are_errors target value)
-    get_target_property(target_type "${target}" TYPE)
-    if(target_type STREQUAL "INTERFACE_LIBRARY")
-        return()
-    endif()
-    set_target_properties("${target}" PROPERTIES QT_SKIP_WARNINGS_ARE_ERRORS ${value})
+    _qt_internal_set_skip_warnings_are_errors("${target}" "${value}")
 endfunction()
 
 # Sets the default warnings behavior according to the WARNINGS_ARE_ERRORS and

cmake/QtInternalTargets.cmake

@@ -150,6 +150,7 @@ function(qt_internal_add_global_definition definition)
     set(scope_PLUGIN PlatformPluginInternal)
     set(scope_TOOL PlatformToolInternal)
     set(scope_APP PlatformAppInternal)
+    set(scope_EXAMPLE PlatformExampleInternal)
 
     set(undef_property_name "QT_INTERNAL_UNDEF_${definition}")
 
@@ -187,6 +188,8 @@ target_link_libraries(PlatformAppInternal INTERFACE PlatformCommonInternal)
 qt_internal_add_platform_internal_target(PlatformToolInternal)
 target_link_libraries(PlatformToolInternal INTERFACE PlatformAppInternal)
 
+qt_internal_add_platform_internal_target(PlatformExampleInternal)
+
 qt_internal_add_global_definition(QT_NO_JAVA_STYLE_ITERATORS)
 qt_internal_add_global_definition(QT_NO_QASCONST)
 qt_internal_add_global_definition(QT_NO_QEXCHANGE)
@@ -200,6 +203,7 @@ qt_internal_add_global_definition(QT_NO_STD_FORMAT_SUPPORT SCOPE PLUGIN TOOL MOD
 qt_internal_set_warnings_are_errors_flags(PlatformModuleInternal INTERFACE)
 qt_internal_set_warnings_are_errors_flags(PlatformPluginInternal INTERFACE)
 qt_internal_set_warnings_are_errors_flags(PlatformAppInternal INTERFACE)
+qt_internal_set_warnings_are_errors_flags(PlatformExampleInternal INTERFACE)
 
 if(WIN32)
     # Needed for M_PI define. Same as mkspecs/features/qt_module.prf.

cmake/QtPublicTargetHelpers.cmake

@@ -480,6 +480,15 @@ function(_qt_internal_set_up_static_runtime_library target)
     endif()
 endfunction()
 
+# Controls the QT_SKIP_WARNINGS_ARE_ERRORS property for the given target.
+function(_qt_internal_set_skip_warnings_are_errors target value)
+    get_target_property(target_type "${target}" TYPE)
+    if(target_type STREQUAL "INTERFACE_LIBRARY")
+        return()
+    endif()
+    set_target_properties("${target}" PROPERTIES QT_SKIP_WARNINGS_ARE_ERRORS ${value})
+endfunction()
+
 function(_qt_internal_warn_about_example_add_subdirectory)
     # This is set by qt_build_repo_impl_examples() in QtBuildRepoHelpers.cmake, only for developer
     # builds, to catch examples that are added via add_subdirectory instead of via

src/corelib/Qt6CoreMacros.cmake

@@ -662,6 +662,31 @@ function(_qt_internal_disable_autorcc_zstd_when_not_supported target)
     endif()
 endfunction()
 
+# Link given target to PlatformExampleInternal when the target is part of an example build.
+function(_qt_internal_link_to_platform_example_internal target)
+    # The first variable is set when examples are built using ExternalProject_Add.
+    # The second is set when examples are built in-tree, the scope is in the <repo>/examples subdir.
+    if(QT_INTERNAL_IS_EXAMPLE_EP_BUILD
+        OR QT_INTERNAL_IS_EXAMPLE_IN_TREE_BUILD)
+        target_link_libraries("${target}" PRIVATE
+            ${QT_CMAKE_EXPORT_NAMESPACE}::PlatformExampleInternal)
+    endif()
+endfunction()
+
+# Set up warnings as errors for targets built in example projects.
+function(_qt_internal_setup_warnings_are_errors_for_example_target target)
+    # Only enable warnings as errors when the global variable is enabled and the repo is known
+    # to have clean examples.
+    if(QT_INTERNAL_IS_EXAMPLE_EP_BUILD
+        OR QT_INTERNAL_IS_EXAMPLE_IN_TREE_BUILD)
+        if(WARNINGS_ARE_ERRORS AND QT_REPO_EXAMPLES_WARNINGS_CLEAN)
+            _qt_internal_set_skip_warnings_are_errors("${target}" FALSE)
+        else()
+            _qt_internal_set_skip_warnings_are_errors("${target}" TRUE)
+        endif()
+    endif()
+endfunction()
+
 function(_qt_internal_create_executable target)
     if(ANDROID)
         list(REMOVE_ITEM ARGN "WIN32" "MACOSX_BUNDLE")
@@ -691,6 +716,8 @@ function(_qt_internal_create_executable target)
     endif()
 
     _qt_internal_disable_autorcc_zstd_when_not_supported("${target}")
+    _qt_internal_link_to_platform_example_internal("${target}")
+    _qt_internal_setup_warnings_are_errors_for_example_target("${target}")
     _qt_internal_set_up_static_runtime_library("${target}")
 endfunction()
 
@@ -2785,6 +2812,8 @@ function(_qt_internal_add_library target)
     cmake_policy(POP)
 
     _qt_internal_disable_autorcc_zstd_when_not_supported("${target}")
+    _qt_internal_link_to_platform_example_internal("${target}")
+    _qt_internal_setup_warnings_are_errors_for_example_target("${target}")
     _qt_internal_set_up_static_runtime_library(${target})
 
     if(NOT type_to_create STREQUAL "INTERFACE" AND NOT type_to_create STREQUAL "OBJECT")