Fixed #12990, Refs #27694 -- Added JSONField model field.

Thanks to Adam Johnson, Carlton Gibson, Mariusz Felisiak, and Raphael
Michel for mentoring this Google Summer of Code 2019 project and
everyone else who helped with the patch.

Special thanks to Mads Jensen, Nick Pope, and Simon Charette for
extensive reviews.

Co-authored-by: Mariusz Felisiak <felisiak.mariusz@gmail.com>

tests/invalid_models_tests/test_ordinary_fields.py

@@ -3,7 +3,9 @@ import uuid
 
 from django.core.checks import Error, Warning as DjangoWarning
 from django.db import connection, models
-from django.test import SimpleTestCase, TestCase, skipIfDBFeature
+from django.test import (
+    SimpleTestCase, TestCase, skipIfDBFeature, skipUnlessDBFeature,
+)
 from django.test.utils import isolate_apps, override_settings
 from django.utils.functional import lazy
 from django.utils.timezone import now
@@ -793,3 +795,47 @@ class UUIDFieldTests(TestCase):
             )
 
         self.assertEqual(Model._meta.get_field('field').check(), [])
+
+
+@isolate_apps('invalid_models_tests')
+@skipUnlessDBFeature('supports_json_field')
+class JSONFieldTests(TestCase):
+    def test_invalid_default(self):
+        class Model(models.Model):
+            field = models.JSONField(default={})
+
+        self.assertEqual(Model._meta.get_field('field').check(), [
+            DjangoWarning(
+                msg=(
+                    "JSONField default should be a callable instead of an "
+                    "instance so that it's not shared between all field "
+                    "instances."
+                ),
+                hint=(
+                    'Use a callable instead, e.g., use `dict` instead of `{}`.'
+                ),
+                obj=Model._meta.get_field('field'),
+                id='fields.E010',
+            )
+        ])
+
+    def test_valid_default(self):
+        class Model(models.Model):
+            field = models.JSONField(default=dict)
+
+        self.assertEqual(Model._meta.get_field('field').check(), [])
+
+    def test_valid_default_none(self):
+        class Model(models.Model):
+            field = models.JSONField(default=None)
+
+        self.assertEqual(Model._meta.get_field('field').check(), [])
+
+    def test_valid_callable_default(self):
+        def callable_default():
+            return {'it': 'works'}
+
+        class Model(models.Model):
+            field = models.JSONField(default=callable_default)
+
+        self.assertEqual(Model._meta.get_field('field').check(), [])