Use target_link_options to propagate object libraries

target_link_options are placed by CMake at the beginning of a linker
line. This gives us an opportunity to use the function to propagate
object libraries. This change adds one more check in the root
Config.cmake file. If CMP0099 policy is enabled, CMake enables
propagating of the linking options when linking two static libraries
using the PRIVATE linking visibility, so we can rely on the correct
linking order and expect object libraries to be propagated.

Note that on the platforms where cmake version is higher than 3.16
Qt uses CMP0099 NEW in functions like qt_add_executable. This means
that at the moment of creating an executable target the TARGET_POLICY
genex will also be NEW, so we do not take into the account the user
defined CMP0099.

If the CMP0099 policy is not available for a certain CMake version
we skip the TARGET_POLICY check and simply disable propagation of
the object libraries using target_link_options for both user and Qt
libraries. This is applicable for the CMake versions 3.16 and less.

Linking approaches have the following priorities(from higher to lower)
after this change:
  - target_link_libraries - works if link order matters not or CMake
    version greater equal 3.21.
  - target_link_options - works if CMP0099 is set to NEW by user or
    if the CMake version is greater than or equal to 3.17 and an
    executable is created using Qt functions.
  - object library finalizer - works if CMake version is greater equal
    3.19 or qt6_finalize_target is called explicitly.
  - target_sources - is used when all the other approaches could not
    be used.

Amends a1fd4f51ada82854f35654158a334454e760a9f7
Amends 3329212815777e33dfb4697b748d10927d73f44c

Pick-to: 6.2
Change-Id: I14f88caeb04e357191c840abeab89b03e210b796
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>

1 files changed, 19 insertions, 0 deletions

diff --git a/cmake/QtBuildInternals/QtBuildInternalsConfig.cmake b/cmake/QtBuildInternals/QtBuildInternalsConfig.cmake
index 2547282dca..8fa0e69c3e 100644
--- a/cmake/QtBuildInternals/QtBuildInternalsConfig.cmake
+++ b/cmake/QtBuildInternals/QtBuildInternalsConfig.cmake
@@ -977,8 +977,27 @@ function(qt_internal_static_link_order_test)
     )
 endfunction()
 
+function(qt_internal_check_cmp0099_available)
+    # Don't care about CMP0099 in CMake versions greater than or equal to 3.21
+    if(CMAKE_VERSION VERSION_GREATER_EQUAL 3.21)
+        return()
+    endif()
+
+    __qt_internal_check_cmp0099_available(result)
+    if(result)
+        set(summary_message "yes")
+    else()
+        set(summary_message "no")
+    endif()
+    qt_configure_add_summary_entry(TYPE "message"
+        ARGS "CMake policy CMP0099 is supported"
+        MESSAGE "${summary_message}"
+    )
+endfunction()
+
 function(qt_internal_run_common_config_tests)
     qt_configure_add_summary_section(NAME "Common build options")
     qt_internal_static_link_order_test()
+    qt_internal_check_cmp0099_available()
     qt_configure_end_summary_section()
 endfunction()