Fixed #20895 -- Made check management command warn if a BooleanField does not have a default value

Thanks to Collin Anderson for the suggestion and Tim Graham for
reviewing the patch.

tests/model_fields/tests.py

@@ -12,7 +12,7 @@ from django.db.models.fields import (
     AutoField, BigIntegerField, BinaryField, BooleanField, CharField,
     CommaSeparatedIntegerField, DateField, DateTimeField, DecimalField,
     EmailField, FilePathField, FloatField, IntegerField, IPAddressField,
-    GenericIPAddressField, NullBooleanField, PositiveIntegerField,
+    GenericIPAddressField, NOT_PROVIDED, NullBooleanField, PositiveIntegerField,
     PositiveSmallIntegerField, SlugField, SmallIntegerField, TextField,
     TimeField, URLField)
 from django.db.models.fields.files import FileField, ImageField
@@ -275,10 +275,23 @@ class BooleanFieldTests(unittest.TestCase):
         Check that a BooleanField defaults to None -- which isn't
         a valid value (#15124).
         """
-        b = BooleanModel()
-        self.assertIsNone(b.bfield)
-        with self.assertRaises(IntegrityError):
-            b.save()
+        # Patch the boolean field's default value. We give it a default
+        # value when defining the model to satisfy the check tests
+        # #20895.
+        boolean_field = BooleanModel._meta.get_field('bfield')
+        self.assertTrue(boolean_field.has_default())
+        old_default = boolean_field.default
+        try:
+            boolean_field.default = NOT_PROVIDED
+            # check patch was succcessful
+            self.assertFalse(boolean_field.has_default())
+            b = BooleanModel()
+            self.assertIsNone(b.bfield)
+            with self.assertRaises(IntegrityError):
+                b.save()
+        finally:
+            boolean_field.default = old_default
+
         nb = NullBooleanModel()
         self.assertIsNone(nb.nbfield)
         nb.save()           # no error