[1.11.x] Fixed #27722 -- Reallowed using django.Template in {% include %}.

Backport of fe2d288434 from master

django/template/loader_tags.py

@@ -202,6 +202,9 @@ class IncludeNode(Node):
                 if template is None:
                     template = context.template.engine.get_template(template_name)
                     cache[template_name] = template
+            # Use the base.Template of a backends.django.Template.
+            elif hasattr(template, 'template'):
+                template = template.template
             values = {
                 name: var.resolve(context)
                 for name, var in six.iteritems(self.extra_context)

tests/template_tests/syntax_tests/test_include.py

@@ -1,7 +1,7 @@
 import warnings
 
 from django.template import (
-    Context, Engine, TemplateDoesNotExist, TemplateSyntaxError,
+    Context, Engine, TemplateDoesNotExist, TemplateSyntaxError, loader,
 )
 from django.test import SimpleTestCase, ignore_warnings
 from django.utils.deprecation import RemovedInDjango21Warning
@@ -280,6 +280,11 @@ class IncludeTests(SimpleTestCase):
         output = outer_tmpl.render(ctx)
         self.assertEqual(output, 'This worked!')
 
+    def test_include_from_loader_get_template(self):
+        tmpl = loader.get_template('include_tpl.html')  # {% include tmpl %}
+        output = tmpl.render({'tmpl': loader.get_template('index.html')})
+        self.assertEqual(output, 'index\n\n')
+
     def test_include_immediate_missing(self):
         """
         #16417 -- Include tags pointing to missing templates should not raise

tests/template_tests/templates/include_tpl.html

@@ -0,0 +1 @@
+{% include tmpl %}