[1.11.x] Fixed #27974 -- Kept resolved templates constant during one rendering cycle.

Thanks Florian Apolloner for the initial patch.

Backport of 002fe07622 from master

tests/template_tests/syntax_tests/test_include.py

@@ -311,3 +311,23 @@ class IncludeTests(SimpleTestCase):
             "Recursion!  A1  Recursion!  B1   B2   B3  Recursion!  C1",
             t.render(Context({'comments': comments})).replace(' ', '').replace('\n', ' ').strip(),
         )
+
+    def test_include_cache(self):
+        """
+        {% include %} keeps resolved templates constant (#27974). The
+        CounterNode object in the {% counter %} template tag is created once
+        if caching works properly. Each iteration increases the counter instead
+        of restarting it.
+
+        This works as a regression test only if the cached loader
+        isn't used, so the @setup decorator isn't used.
+        """
+        engine = Engine(loaders=[
+            ('django.template.loaders.locmem.Loader', {
+                'template': '{% for x in vars %}{% include "include" %}{% endfor %}',
+                'include': '{% include "next" %}',
+                'next': '{% load custom %}{% counter %}'
+            }),
+        ], libraries={'custom': 'template_tests.templatetags.custom'})
+        output = engine.render_to_string('template', dict(vars=range(9)))
+        self.assertEqual(output, '012345678')