From c3eaa283c3d8a677727d6851b62a211a9e20cd7a Mon Sep 17 00:00:00 2001
From: Alexandru Croitor <alexandru.croitor@qt.io>
Date: Thu, 24 Apr 2025 18:14:25 +0200
Subject: [PATCH] CMake: Skip walking empty targets in
 __qt_internal_walk_libs()

If one of the interface link libraries of a target is
  "$<LINK_ONLY:$<BUILD_LOCAL_INTERFACE:Foo>>"
this will be exported by cmake as
  "$<LINK_ONLY:>"
in the exported INTERFACE_LINK_LIBRARIES property.

When walking the dependent libraries of a target using
__qt_internal_walk_libs(), this value will become empty after
unwrapping the LINK_ONLY genex.

In such a case, we should skip further processing of the library.

Otherwise in some weird unknown conditions, CMake might consider the
empty name to be a valid target, and cause errors when trying to read
properties from it.

Amends ad7b94e163ac5c3959a7e38d7f48536be288a187

Fixes: QTBUG-136039
Change-Id: I143dec697e10530379486462697cd299940ee463
Reviewed-by: Alexey Edelev <alexey.edelev@qt.io>
(cherry picked from commit 9bc8ce6608eff2bd07b0044be80db4e9b6d8f5cd)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>
---
 cmake/QtPublicWalkLibsHelpers.cmake | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/cmake/QtPublicWalkLibsHelpers.cmake b/cmake/QtPublicWalkLibsHelpers.cmake
index 7aa26c1af00..d0826e08a27 100644
--- a/cmake/QtPublicWalkLibsHelpers.cmake
+++ b/cmake/QtPublicWalkLibsHelpers.cmake
@@ -186,9 +186,19 @@ function(__qt_internal_walk_libs
             # Unwrap targets like $<LINK_ONLY:$<BUILD_INTERFACE:Qt6::CorePrivate>>
             while(lib_target
                     MATCHES "^\\$<(LINK_ONLY|BUILD_INTERFACE|BUILD_LOCAL_INTERFACE):(.*)>$")
-                set(lib_target ${CMAKE_MATCH_2})
+                set(lib_target "${CMAKE_MATCH_2}")
             endwhile()
 
+            # If one of the values is "$<LINK_ONLY:$<BUILD_LOCAL_INTERFACE:Foo>>", this will be
+            # exported by cmake as "$<LINK_ONLY:>", which will become an empty value after the
+            # unwrapping above.
+            # In that case, skip the processing. Otherwise in some weird unknown conditions,
+            # CMake might consider the empty name to be a valid target, and cause errors further
+            # down.
+            if("${lib_target}" STREQUAL "")
+                continue()
+            endif()
+
             # Skip CMAKE_DIRECTORY_ID_SEP. If a target_link_libraries is applied to a target
             # that was defined in a different scope, CMake appends and prepends a special directory
             # id separator. Filter those out.