Refs #26479 -- Documented is/is not if tag operator behavior for nonexistent variables.

docs/ref/templates/builtins.txt

@@ -523,8 +523,12 @@ Not contained within. This is the negation of the ``in`` operator.
 
 Object identity. Tests if two values are the same object. Example::
 
-    {% if value is None %}
-      This will output if and only if value is None.
+    {% if somevar is True %}
+      This appears if and only if somevar is True.
+    {% endif %}
+
+    {% if somevar is None %}
+      This appears if somevar is None, or if somevar is not found in the context.
     {% endif %}
 
 ``is not`` operator
@@ -532,11 +536,16 @@ Object identity. Tests if two values are the same object. Example::
 
 .. versionadded:: 1.10
 
-Tests if two values are not the same object. This is the negation of
-the ``is`` operator. Example::
+Negated object identity. Tests if two values are not the same object. This is
+the negation of the ``is`` operator. Example::
+
+    {% if somevar is not True %}
+      This appears if somevar is not True, or if somevar is not found in the
+      context.
+    {% endif %}
 
-    {% if value is not None %}
-      This will output if and only if value is not None.
+    {% if somevar is not None %}
+      This appears if and only if somevar is not None.
     {% endif %}
 
 Filters

tests/template_tests/syntax_tests/test_if.py

@@ -564,6 +564,16 @@ class IfTagTests(SimpleTestCase):
         output = self.engine.render_to_string('template', {'foo': 1})
         self.assertEqual(output, 'no')
 
+    @setup({'template': '{% if foo is bar %}yes{% else %}no{% endif %}'})
+    def test_if_is_variable_missing(self):
+        output = self.engine.render_to_string('template', {'foo': 1})
+        self.assertEqual(output, 'no')
+
+    @setup({'template': '{% if foo is bar %}yes{% else %}no{% endif %}'})
+    def test_if_is_both_variables_missing(self):
+        output = self.engine.render_to_string('template', {})
+        self.assertEqual(output, 'yes')
+
     @setup({'template': '{% if foo is not None %}yes{% else %}no{% endif %}'})
     def test_if_is_not_match(self):
         # For this to act as a regression test, it's important not to use
@@ -575,3 +585,13 @@ class IfTagTests(SimpleTestCase):
     def test_if_is_not_no_match(self):
         output = self.engine.render_to_string('template', {'foo': None})
         self.assertEqual(output, 'no')
+
+    @setup({'template': '{% if foo is not bar %}yes{% else %}no{% endif %}'})
+    def test_if_is_not_variable_missing(self):
+        output = self.engine.render_to_string('template', {'foo': False})
+        self.assertEqual(output, 'yes')
+
+    @setup({'template': '{% if foo is not bar %}yes{% else %}no{% endif %}'})
+    def test_if_is_not_both_variables_missing(self):
+        output = self.engine.render_to_string('template', {})
+        self.assertEqual(output, 'no')