cmComputeLinkInformation: Track targets named by TARGET_OBJECTS sources

Since commit b6a5382217 (Ninja: depend on language module information
files directly, 2023-02-10, v3.27.0-rc1~502^2), the return value of
`cmCommonTargetGenerator::GetLinkedTargetDirectories` must account for
linked object libraries because they may provide modules (#25112).
These were added by commit b665966933 (cmComputeLinkInformation: track
OBJECT library dependencies, 2023-07-22, v3.27.1~5^2).  However, targets
named by `$<TARGET_OBJECTS:...>` sources are also needed (#25365).

The latter were added by commit 22da18b995 (Fortran: Restore support for
TARGET_OBJECTS providing modules, 2023-10-27, v3.28.0-rc4~9^2) and
commit 035302b7e3 (cmComputeLinkDepends: also copy the target from
object link items, 2023-10-27, v3.28.0-rc4~9^2~2).  However, their
approach added link entries not actually specified by projects.  It also
incorrectly re-used `cmComputeLinkDepends::AddLinkObject` for object
library targets when it is meant for their individual object files.
These problems caused additional regressions (#25417).  Revert the
implementation parts of those commits and leave behind an assertion and
comment to help avoid the mistake in the future.  Instead, track targets
named by `$<TARGET_OBJECTS:...>` sources with a dedicated member.

Issue: #25112
Issue: #25365
Fixes: #25417
Co-authored-by: Ben Boeckel <ben.boeckel@kitware.com>

5 files changed, 33 insertions, 0 deletions

diff --git a/Tests/ObjectLibrary/CMakeLists.txt b/Tests/ObjectLibrary/CMakeLists.txt
index b57761b..2bc2327 100644
--- a/Tests/ObjectLibrary/CMakeLists.txt
+++ b/Tests/ObjectLibrary/CMakeLists.txt
@@ -77,3 +77,5 @@ add_subdirectory(ExportLanguages)
 add_subdirectory(LinkObjects)
 
 add_subdirectory(Transitive)
+
+add_subdirectory(TransitiveLinkDeps)
diff --git a/Tests/ObjectLibrary/TransitiveLinkDeps/CMakeLists.txt b/Tests/ObjectLibrary/TransitiveLinkDeps/CMakeLists.txt
new file mode 100644
index 0000000..3f561fa
--- /dev/null
+++ b/Tests/ObjectLibrary/TransitiveLinkDeps/CMakeLists.txt
@@ -0,0 +1,15 @@
+add_library(implgather INTERFACE)
+
+add_library(dep STATIC dep.c)
+
+add_library(deps INTERFACE)
+target_link_libraries(deps INTERFACE dep)
+
+add_library(impl_obj OBJECT impl_obj.c)
+target_link_libraries(impl_obj PUBLIC deps)
+
+target_sources(implgather INTERFACE "$<TARGET_OBJECTS:impl_obj>")
+target_link_libraries(implgather INTERFACE impl_obj)
+
+add_executable(useimpl main.c)
+target_link_libraries(useimpl PRIVATE implgather)
diff --git a/Tests/ObjectLibrary/TransitiveLinkDeps/dep.c b/Tests/ObjectLibrary/TransitiveLinkDeps/dep.c
new file mode 100644
index 0000000..7cc62c3
--- /dev/null
+++ b/Tests/ObjectLibrary/TransitiveLinkDeps/dep.c
@@ -0,0 +1,4 @@
+int from_dep(void)
+{
+  return 0;
+}
diff --git a/Tests/ObjectLibrary/TransitiveLinkDeps/impl_obj.c b/Tests/ObjectLibrary/TransitiveLinkDeps/impl_obj.c
new file mode 100644
index 0000000..f5760b7
--- /dev/null
+++ b/Tests/ObjectLibrary/TransitiveLinkDeps/impl_obj.c
@@ -0,0 +1,6 @@
+int from_dep(void);
+
+int impl_obj(void)
+{
+  return from_dep();
+}
diff --git a/Tests/ObjectLibrary/TransitiveLinkDeps/main.c b/Tests/ObjectLibrary/TransitiveLinkDeps/main.c
new file mode 100644
index 0000000..5661f57
--- /dev/null
+++ b/Tests/ObjectLibrary/TransitiveLinkDeps/main.c
@@ -0,0 +1,6 @@
+int impl_obj(void);
+
+int main(int argc, char* argv[])
+{
+  return impl_obj();
+}