now passes the test of skipping list when dict attribute is undefined, added deprecation warning as this seems like bad behaviour

lib/ansible/executor/task_executor.py

@@ -26,7 +26,7 @@ import sys
 import time
 
 from ansible import constants as C
-from ansible.errors import AnsibleError, AnsibleParserError
+from ansible.errors import AnsibleError, AnsibleParserError, AnsibleUndefinedVariable
 from ansible.playbook.conditional import Conditional
 from ansible.playbook.task import Task
 from ansible.plugins import connection_loader, action_loader
@@ -154,7 +154,14 @@ class TaskExecutor:
         if self._task.loop:
             if self._task.loop in self._shared_loader_obj.lookup_loader:
                 #TODO: remove convert_bare true and deprecate this in with_ 
-                loop_terms = listify_lookup_plugin_terms(terms=self._task.loop_args, templar=templar, loader=self._loader, fail_on_undefined=True, convert_bare=True)
+                try:
+                    loop_terms = listify_lookup_plugin_terms(terms=self._task.loop_args, templar=templar, loader=self._loader, fail_on_undefined=True, convert_bare=True)
+                except AnsibleUndefinedVariable as e:
+                    if 'has no attribute' in str(e):
+                        loop_terms = []
+                        self._display.deprecated("Skipping task due to undefined attribute, in the future this will be a fatal error.")
+                    else:
+                        raise
                 items = self._shared_loader_obj.lookup_loader.get(self._task.loop, loader=self._loader, templar=templar).run(terms=loop_terms, variables=vars_copy)
             else:
                 raise AnsibleError("Unexpected failure in finding the lookup named '%s' in the available lookup plugins" % self._task.loop)

lib/ansible/template/__init__.py

@@ -256,8 +256,8 @@ class Templar:
             # safely catch run failures per #5059
             try:
                 ran = instance.run(loop_terms, variables=self._available_variables, **kwargs)
-            except (AnsibleUndefinedVariable, UndefinedError):
-                raise
+            except (AnsibleUndefinedVariable, UndefinedError) as e:
+                raise AnsibleUndefinedVariable(e)
             except Exception, e:
                 if self._fail_on_lookup_errors:
                     raise
@@ -337,7 +337,7 @@ class Templar:
             return res
         except (UndefinedError, AnsibleUndefinedVariable), e:
             if fail_on_undefined:
-                raise
+                raise AnsibleUndefinedVariable(e)
             else:
                 #TODO: return warning about undefined var
                 return data