porting guide 2.19 - describe lazy complex var eval impact on builtin filters/tests

docs/docsite/rst/porting_guides/porting_guide_core_2.19.rst

@@ -766,6 +766,24 @@ Noteworthy plugin changes
   This filter now returns ``False`` instead of ``None`` when the input is ``None``.
   The aforementioned deprecation warning is also issued in this case.
 
+* Passing nested non-scalars with embedded templates that may resolve to ``Undefined`` to Jinja2
+  filter plugins such as ``default`` and ``mandatory``, and test plugins including ``defined`` and ``undefined``
+  no longer evaluate the same, since nested non-scalars with embedded templates are only templated on use.
+  In 2.19, this assertion passes:
+
+  .. code-block:: yaml
+
+     - assert:
+         that:
+           - complex_var is defined
+           - (complex_var | default(unused)).nested is undefined
+           - complex_var.nested is undefined
+       vars:
+         complex_var:
+           nested: "{{ undefined_variable }}"
+         unused:
+           nested: default
+
 
 Porting custom scripts
 ======================