target_link_libraries: Allow use with targets in other directories

Previously the command did not allow naming targets on the LHS that
were not created in the calling directory.  Lift this restriction to
enable more flexible use by projects.

Targets named on the RHS will need to be looked up during generation in
the scope of the call site rather than the scope of the LHS target.
Introduce an internal syntax in `[INTERFACE_]LINK_LIBRARIES` properties
to specify target names that need to be looked up in a directory other
than that containing the target on which the property is set.  Add
minimal documentation of the syntax to help users that encounter it.

Unfortunately CMake previously did allow such calls in the case that
only `INTERFACE` libraries are specified, but those libraries would be
looked up in the target's directory rather than the caller's.  Add
policy `CMP0079` to enable the new behavior with new lookup scope in a
compatible way.

Fixes: #17943

3 files changed, 14 insertions, 0 deletions

diff --git a/Help/prop_tgt/INTERFACE_LINK_LIBRARIES.rst b/Help/prop_tgt/INTERFACE_LINK_LIBRARIES.rst
index 832d12b..bf7f72f 100644
--- a/Help/prop_tgt/INTERFACE_LINK_LIBRARIES.rst
+++ b/Help/prop_tgt/INTERFACE_LINK_LIBRARIES.rst
@@ -17,6 +17,8 @@ with the syntax ``$<...>``.  See the :manual:`cmake-generator-expressions(7)`
 manual for available expressions.  See the :manual:`cmake-buildsystem(7)`
 manual for more on defining buildsystem properties.
 
+.. include:: LINK_LIBRARIES_INDIRECTION.txt
+
 Creating Relocatable Packages
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
diff --git a/Help/prop_tgt/LINK_LIBRARIES.rst b/Help/prop_tgt/LINK_LIBRARIES.rst
index aa4b9f5..d88e798 100644
--- a/Help/prop_tgt/LINK_LIBRARIES.rst
+++ b/Help/prop_tgt/LINK_LIBRARIES.rst
@@ -15,3 +15,5 @@ Contents of ``LINK_LIBRARIES`` may use "generator expressions" with the
 syntax ``$<...>``.  See the :manual:`cmake-generator-expressions(7)` manual
 for available expressions.  See the :manual:`cmake-buildsystem(7)` manual
 for more on defining buildsystem properties.
+
+.. include:: LINK_LIBRARIES_INDIRECTION.txt
diff --git a/Help/prop_tgt/LINK_LIBRARIES_INDIRECTION.txt b/Help/prop_tgt/LINK_LIBRARIES_INDIRECTION.txt
new file mode 100644
index 0000000..1fdb6ad
--- /dev/null
+++ b/Help/prop_tgt/LINK_LIBRARIES_INDIRECTION.txt
@@ -0,0 +1,10 @@
+.. note::
+  A call to :command:`target_link_libraries(<target> ...)` may update this
+  property on ``<target>``.  If ``<target>`` was not created in the same
+  directory as the call then :command:`target_link_libraries` will add a
+  suffix of the form ``::@<directory-id>`` to each entry, where the
+  ``::@`` is a separator and the ``<directory-id>`` is unspecified.
+  This tells the generators that the named libraries must be looked up in
+  the scope of the caller rather than in the scope in which the
+  ``<target>`` was created.  Valid directory ids are stripped on export
+  by the :command:`install(EXPORT)` and :command:`export` commands.