CMake: Allow finding Qt CMake packages in additional locations

By default, when using the Qt6 CMake package to look for components,
the find_package() calls for the components use NO_DEFAULT_PATH to
ensure that CMake doesn't accidentally find system (distro) packages.
Instead we limit the paths to one level up from where the Qt6 package
is.

Unfortunately that doesn't quite work for finding Qt packages that
might have been installed into a different prefix than where the main
Qt prefix is.
This happens when Qt addons are built by Conan, and installed into
a separate prefix.

To allow calls like find_package(Qt6 COMPONENTS ConanAddon) to work
in a scenario as described above, introduce a new variable called
QT_ADDITIONAL_PACKAGES_PREFIX_PATH which can be used to specify
additional paths where Qt CMake packages should be found.

This is similar to previously introduced QT_EXAMPLES_CMAKE_PREFIX_PATH
variable which was meant for a similar case, but only for examples.

Additionally, allow disabling the NO_DEFAULT_PATH option by setting
the QT_DISABLE_NO_DEFAULT_PATH_IN_QT_PACKAGES cache variable to TRUE.
This would allow regular usage of CMAKE_PREFIX_PATH to work, at the
risk that system Qt CMake packages might be found.

Augments 5cd4001bf2a7f0894c6ac269860e833b02df6cde
and ffe088941378e32ea30c142cca7e63c537a41ff1.

Fixes: QTBUG-86882
Change-Id: Ia8e060cbba6d2a10c3d63d81892f2c71e4236a9a
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>

3 files changed, 35 insertions, 4 deletions

diff --git a/cmake/QtConfig.cmake.in b/cmake/QtConfig.cmake.in
index 95fc581993..f2020ec012 100644
--- a/cmake/QtConfig.cmake.in
+++ b/cmake/QtConfig.cmake.in
@@ -60,11 +60,20 @@ if(EXISTS "${CMAKE_CURRENT_LIST_DIR}/@INSTALL_CMAKE_NAMESPACE@Dependencies.cmake
     endif()
 endif()
 
+set(__qt_use_no_default_path_for_qt_packages "NO_DEFAULT_PATH")
+if(QT_DISABLE_NO_DEFAULT_PATH_IN_QT_PACKAGES)
+    set(__qt_use_no_default_path_for_qt_packages "")
+endif()
+
 foreach(module ${@INSTALL_CMAKE_NAMESPACE@_FIND_COMPONENTS})
     find_package(@INSTALL_CMAKE_NAMESPACE@${module}
         ${_@INSTALL_CMAKE_NAMESPACE@_FIND_PARTS_QUIET}
         ${_@INSTALL_CMAKE_NAMESPACE@_FIND_PARTS_REQUIRED}
-        PATHS ${_qt_cmake_dir} ${QT_EXAMPLES_CMAKE_PREFIX_PATH} NO_DEFAULT_PATH
+        PATHS
+            ${_qt_cmake_dir}
+            ${QT_ADDITIONAL_PACKAGES_PREFIX_PATH}
+            ${QT_EXAMPLES_CMAKE_PREFIX_PATH}
+        ${__qt_use_no_default_path_for_qt_packages}
     )
     if (NOT @INSTALL_CMAKE_NAMESPACE@${module}_FOUND)
         string(CONFIGURE ${_qt5_module_location_template} _expected_module_location @ONLY)
diff --git a/cmake/QtModuleDependencies.cmake.in b/cmake/QtModuleDependencies.cmake.in
index 24d132b57d..01cf9d5b3c 100644
--- a/cmake/QtModuleDependencies.cmake.in
+++ b/cmake/QtModuleDependencies.cmake.in
@@ -1,7 +1,16 @@
 # Make sure @INSTALL_CMAKE_NAMESPACE@ is found before anything else.
 set(@INSTALL_CMAKE_NAMESPACE@@target@_FOUND FALSE)
+
+set(__qt_use_no_default_path_for_qt_packages "NO_DEFAULT_PATH")
+if(QT_DISABLE_NO_DEFAULT_PATH_IN_QT_PACKAGES)
+    set(__qt_use_no_default_path_for_qt_packages "")
+endif()
 find_dependency(@INSTALL_CMAKE_NAMESPACE@ @PROJECT_VERSION@
-    PATHS "${CMAKE_CURRENT_LIST_DIR}/.." ${QT_EXAMPLES_CMAKE_PREFIX_PATH} NO_DEFAULT_PATH
+    PATHS
+        "${CMAKE_CURRENT_LIST_DIR}/.."
+        ${QT_ADDITIONAL_PACKAGES_PREFIX_PATH}
+        ${QT_EXAMPLES_CMAKE_PREFIX_PATH}
+    ${__qt_use_no_default_path_for_qt_packages}
 )
 
 # note: _third_party_deps example: "ICU\\;FALSE\\;1.0\\;i18n uc data;ZLIB\\;FALSE\\;\\;"
@@ -75,7 +84,11 @@ foreach(_target_dep ${_target_deps})
 
     if (NOT ${pkg}_FOUND)
         find_dependency(${pkg} ${version}
-            PATHS "${CMAKE_CURRENT_LIST_DIR}/.." ${QT_EXAMPLES_CMAKE_PREFIX_PATH} NO_DEFAULT_PATH
+            PATHS
+                "${CMAKE_CURRENT_LIST_DIR}/.."
+                ${QT_ADDITIONAL_PACKAGES_PREFIX_PATH}
+                ${QT_EXAMPLES_CMAKE_PREFIX_PATH}
+            ${__qt_use_no_default_path_for_qt_packages}
         )
     endif()
 endforeach()
diff --git a/cmake/QtPluginDependencies.cmake.in b/cmake/QtPluginDependencies.cmake.in
index 9d16bedeba..42365b4296 100644
--- a/cmake/QtPluginDependencies.cmake.in
+++ b/cmake/QtPluginDependencies.cmake.in
@@ -27,6 +27,11 @@ foreach(_target_dep ${_third_party_deps})
     endif()
 endforeach()
 
+set(__qt_use_no_default_path_for_qt_packages "NO_DEFAULT_PATH")
+if(QT_DISABLE_NO_DEFAULT_PATH_IN_QT_PACKAGES)
+    set(__qt_use_no_default_path_for_qt_packages "")
+endif()
+
 # note: target_deps example: "Qt6Core\;5.12.0;Qt6Gui\;5.12.0"
 set(_target_deps "@target_deps@")
 foreach(_target_dep ${_target_deps})
@@ -35,7 +40,11 @@ foreach(_target_dep ${_target_deps})
 
     if (NOT ${pkg}_FOUND)
         find_dependency(${pkg} ${version}
-            PATHS @find_dependency_paths@ ${QT_EXAMPLES_CMAKE_PREFIX_PATH} NO_DEFAULT_PATH
+            PATHS
+                @find_dependency_paths@
+                ${QT_ADDITIONAL_PACKAGES_PREFIX_PATH}
+                ${QT_EXAMPLES_CMAKE_PREFIX_PATH}
+            ${__qt_use_no_default_path_for_qt_packages}
         )
     endif()
 endforeach()