Fixed #21707 -- Added helpful error message when using {{ block.super }} in base template.

Thanks mitar for the suggestion.

django/template/loader_tags.py

@@ -68,6 +68,11 @@ class BlockNode(Node):
         return result
 
     def super(self):
+        if not hasattr(self, 'context'):
+            raise TemplateSyntaxError(
+                "'%s' object has no attribute 'context'. Did you use "
+                "{{ block.super }} in a base template?" % self.__class__.__name__
+            )
         render_context = self.context.render_context
         if (BLOCK_CONTEXT_KEY in render_context and
                 render_context[BLOCK_CONTEXT_KEY].get_block(self.name) is not None):

tests/template_tests/tests.py

@@ -881,6 +881,9 @@ class TemplateTests(TestCase):
             # Raise exception for custom tags used in child with {% load %} tag in parent, not in child
             'exception04': ("{% extends 'inheritance17' %}{% block first %}{% echo 400 %}5678{% endblock %}", {}, template.TemplateSyntaxError),
 
+            # Raise exception for block.super used in base template
+            'exception05': ("{% block first %}{{ block.super }}{% endblock %}", {}, template.TemplateSyntaxError),
+
             ### FILTER TAG ############################################################
             'filter01': ('{% filter upper %}{% endfilter %}', {}, ''),
             'filter02': ('{% filter upper %}django{% endfilter %}', {}, 'DJANGO'),