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/models.py

@@ -58,7 +58,7 @@ class NullBooleanModel(models.Model):
     nbfield = models.NullBooleanField()
 
 class BooleanModel(models.Model):
-    bfield = models.BooleanField()
+    bfield = models.BooleanField(default=None)
     string = models.CharField(max_length=10, default='abc')
 
 class FksToBooleans(models.Model):
@@ -72,7 +72,7 @@ class RenamedField(models.Model):
 class VerboseNameField(models.Model):
     id = models.AutoField("verbose pk", primary_key=True)
     field1 = models.BigIntegerField("verbose field1")
-    field2 = models.BooleanField("verbose field2")
+    field2 = models.BooleanField("verbose field2", default=False)
     field3 = models.CharField("verbose field3", max_length=10)
     field4 = models.CommaSeparatedIntegerField("verbose field4", max_length=99)
     field5 = models.DateField("verbose field5")

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