[1.7.x] Fixed #23615 -- Validate that a Model instance's "check" attribute is a method.

The "check" name is a reserved word used by Django's check framework,
and cannot be redefined as something else other than a method, or the check
framework will raise an error.

This change amends the django.core.checks.model_check.check_all_models()
function, so that it verifies that a model instance's attribute "check"
is actually a method. This new check is assigned the id "models.E020".

Conflicts:
	docs/ref/checks.txt

Backport of a5c77417a6 from master

4 files changed, 83 insertions, 9 deletions

diff --git a/django/core/checks/model_checks.py b/django/core/checks/model_checks.py
index 3f5a36b41b..3d0d56af58 100644
--- a/django/core/checks/model_checks.py
+++ b/django/core/checks/model_checks.py
@@ -1,28 +1,43 @@
 # -*- coding: utf-8 -*-
 from __future__ import unicode_literals
 
-from itertools import chain
+import inspect
 import types
 
 from django.apps import apps
-
-from . import Error, Tags, register
+from django.core.checks import Error, Tags, register
 
 
 @register(Tags.models)
 def check_all_models(app_configs=None, **kwargs):
-    errors = [model.check(**kwargs)
-        for model in apps.get_models()
-        if app_configs is None or model._meta.app_config in app_configs]
-    return list(chain(*errors))
+    errors = []
+    for model in apps.get_models():
+        if app_configs is None or model._meta.app_config in app_configs:
+            if not inspect.ismethod(model.check):
+                errors.append(
+                    Error(
+                        "The '%s.check()' class method is "
+                        "currently overridden by %r." % (
+                            model.__name__, model.check),
+                        hint=None,
+                        obj=model,
+                        id='models.E020'
+                    )
+                )
+            else:
+                errors.extend(model.check(**kwargs))
+    return errors
 
 
 @register(Tags.models, Tags.signals)
 def check_model_signals(app_configs=None, **kwargs):
-    """Ensure lazily referenced model signals senders are installed."""
+    """
+    Ensure lazily referenced model signals senders are installed.
+    """
+    # Avoid circular import
     from django.db import models
-    errors = []
 
+    errors = []
     for name in dir(models.signals):
         obj = getattr(models.signals, name)
         if isinstance(obj, models.signals.ModelSignal):
diff --git a/docs/ref/checks.txt b/docs/ref/checks.txt
index ca37de4385..0a2f3a7557 100644
--- a/docs/ref/checks.txt
+++ b/docs/ref/checks.txt
@@ -56,6 +56,7 @@ Models
 * **models.E015**: ``ordering`` refers to the non-existent field
   ``<field name>``.
 * **models.E017**: Proxy model ``<model>`` contains model fields.
+* **models.E020**: The ``<model>.check()`` class method is currently overridden.
 
 Fields
 ~~~~~~
diff --git a/docs/releases/1.7.1.txt b/docs/releases/1.7.1.txt
index 4334525877..bda016a5c2 100644
--- a/docs/releases/1.7.1.txt
+++ b/docs/releases/1.7.1.txt
@@ -119,3 +119,6 @@ Bugfixes
 
 * Fixed a crash while parsing cookies containing invalid content
   (:ticket:`23638`).
+
+* The system check framework now raises error **models.E020** when the
+  class method ``Model.check()`` is unreachable (:ticket:`23615`).
diff --git a/tests/check_framework/tests.py b/tests/check_framework/tests.py
index 47936c3a4e..55fcc80f58 100644
--- a/tests/check_framework/tests.py
+++ b/tests/check_framework/tests.py
@@ -8,11 +8,13 @@ from django.apps import apps
 from django.conf import settings
 from django.core import checks
 from django.core.checks import Error, Warning
+from django.core.checks.model_checks import check_all_models
 from django.core.checks.registry import CheckRegistry
 from django.core.checks.compatibility.django_1_6_0 import check_1_6_compatibility
 from django.core.checks.compatibility.django_1_7_0 import check_1_7_compatibility
 from django.core.management.base import CommandError
 from django.core.management import call_command
+from django.db import models
 from django.db.models.fields import NOT_PROVIDED
 from django.test import TestCase
 from django.test.utils import override_settings, override_system_checks
@@ -300,3 +302,56 @@ class SilencingCheckTests(TestCase):
 
         self.assertEqual(out.getvalue(), 'System check identified no issues (1 silenced).\n')
         self.assertEqual(err.getvalue(), '')
+
+
+class CheckFrameworkReservedNamesTests(TestCase):
+
+    def setUp(self):
+        self.current_models = apps.all_models[__package__]
+        self.saved_models = set(self.current_models)
+
+    def tearDown(self):
+        for model in (set(self.current_models) - self.saved_models):
+            del self.current_models[model]
+        apps.clear_cache()
+
+    @override_settings(SILENCED_SYSTEM_CHECKS=['models.E020'])
+    def test_model_check_method_not_shadowed(self):
+        class ModelWithAttributeCalledCheck(models.Model):
+            check = 42
+
+        class ModelWithFieldCalledCheck(models.Model):
+            check = models.IntegerField()
+
+        class ModelWithRelatedManagerCalledCheck(models.Model):
+            pass
+
+        class ModelWithDescriptorCalledCheck(models.Model):
+            check = models.ForeignKey(ModelWithRelatedManagerCalledCheck)
+            article = models.ForeignKey(ModelWithRelatedManagerCalledCheck, related_name='check')
+
+        expected = [
+            Error(
+                "The 'ModelWithAttributeCalledCheck.check()' class method is "
+                "currently overridden by 42.",
+                hint=None,
+                obj=ModelWithAttributeCalledCheck,
+                id='models.E020'
+            ),
+            Error(
+                "The 'ModelWithRelatedManagerCalledCheck.check()' class method is "
+                "currently overridden by %r." % ModelWithRelatedManagerCalledCheck.check,
+                hint=None,
+                obj=ModelWithRelatedManagerCalledCheck,
+                id='models.E020'
+            ),
+            Error(
+                "The 'ModelWithDescriptorCalledCheck.check()' class method is "
+                "currently overridden by %r." % ModelWithDescriptorCalledCheck.check,
+                hint=None,
+                obj=ModelWithDescriptorCalledCheck,
+                id='models.E020'
+            ),
+        ]
+
+        self.assertEqual(check_all_models(), expected)