Anim: do not set slot ID type when Action is linked

When an action slot does not have an ID type, and it is assigned to some
ID, the slot is bound to that ID's type. This now no longer happens when
the Action is linked, because linked data should not be modified.

Pull Request: https://projects.blender.org/blender/blender/pulls/133670

tests/python/bl_animation_action.py

@@ -162,6 +162,59 @@ class ActionSlotAssignmentTest(unittest.TestCase):
             cube_adt.action_slot = slot
         self.assertEqual(cube_adt.action_slot, slot_cube, "The slot should not have changed")
 
+    def test_untyped_slot_assignment_local(self):
+        """Test untyped slot assignment, with a local Action."""
+
+        action = self._load_legacy_action(link=False)
+
+        # Assign the Action to a Mesh data-block. This should set the ID type of the Slot to 'MESH'.
+        mesh = bpy.data.meshes['Cube']
+        mesh.animation_data_create().action = action
+
+        slot = action.slots[0]
+        self.assertEqual('MESH', slot.target_id_type, "After assignment, the ID type should be specified.")
+        self.assertEqual("MELegacy Slot", slot.identifier)
+
+    def test_untyped_slot_assignment_linked(self):
+        """Test untyped slot assignment, with a linked Action."""
+
+        action = self._load_legacy_action(link=True)
+
+        # Assign the Action to a Mesh data-block. This should set the ID type of the Slot to 'MESH'.
+        mesh = bpy.data.meshes['Cube']
+        mesh.animation_data_create().action = action
+
+        slot = action.slots[0]
+        self.assertEqual(
+            'UNSPECIFIED',
+            slot.target_id_type,
+            "After assignment, the ID type should remain UNSPECIFIED when the Action is linked.")
+        self.assertEqual("XXLegacy Slot", slot.identifier)
+
+    @staticmethod
+    def _load_legacy_action(*, link: bool) -> bpy.types.Action:
+        # At the moment of writing, the only way to create an untyped slot is to
+        # load a legacy Action that has `id_root=0` and let the versioning code
+        # create the untyped slot.
+        blendpath = args.testdir / "legacy-action-without-idroot.blend"
+
+        # Append or link the one Action from the legacy file.
+        with bpy.data.libraries.load(str(blendpath), link=link) as (data_in, data_out):
+            data_out.actions = data_in.actions
+
+        # Using plain asserts here, because these are not part of the unit test.
+        # They're here to test that the test code itself is doing the right thing.
+        assert len(data_out.actions) == 1
+        assert isinstance(data_out.actions[0], bpy.types.Action)
+
+        # Check that the state of things is as expected.
+        action = data_out.actions[0]
+        slot = action.slots[0]
+        assert slot.target_id_type == 'UNSPECIFIED'
+        assert slot.identifier == "XXLegacy Slot"
+
+        return action
+
 
 class LimitationsTest(unittest.TestCase):
     """Test artificial limitations for the layered Action.