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

1 files changed, 29 insertions, 2 deletions

diff --git a/Source/cmGeneratorTarget.cxx b/Source/cmGeneratorTarget.cxx
index 1bd98e2..efcfaf7 100644
--- a/Source/cmGeneratorTarget.cxx
+++ b/Source/cmGeneratorTarget.cxx
@@ -5648,11 +5648,38 @@ void cmGeneratorTarget::ComputeLinkImplementationLibraries(
 cmGeneratorTarget::TargetOrString cmGeneratorTarget::ResolveTargetReference(
   std::string const& name) const
 {
+  cmLocalGenerator const* lg = this->LocalGenerator;
+  std::string const* lookupName = &name;
+
+  // When target_link_libraries() is called with a LHS target that is
+  // not created in the calling directory it adds a directory id suffix
+  // that we can use to look up the calling directory.  It is that scope
+  // in which the item name is meaningful.  This case is relatively rare
+  // so we allocate a separate string only when the directory id is present.
+  std::string::size_type pos = name.find(CMAKE_DIRECTORY_ID_SEP);
+  std::string plainName;
+  if (pos != std::string::npos) {
+    // We will look up the plain name without the directory id suffix.
+    plainName = name.substr(0, pos);
+
+    // We will look up in the scope of the directory id.
+    // If we do not recognize the id then leave the original
+    // syntax in place to produce an indicative error later.
+    cmDirectoryId const dirId =
+      name.substr(pos + sizeof(CMAKE_DIRECTORY_ID_SEP) - 1);
+    if (cmLocalGenerator const* otherLG =
+          this->GlobalGenerator->FindLocalGenerator(dirId)) {
+      lg = otherLG;
+      lookupName = &plainName;
+    }
+  }
+
   TargetOrString resolved;
 
-  if (cmGeneratorTarget* tgt =
-        this->LocalGenerator->FindGeneratorTargetToUse(name)) {
+  if (cmGeneratorTarget* tgt = lg->FindGeneratorTargetToUse(*lookupName)) {
     resolved.Target = tgt;
+  } else if (lookupName == &plainName) {
+    resolved.String = std::move(plainName);
   } else {
     resolved.String = name;
   }