Fixed a regression in forms changed_data

Thanks Loic Bistuer for spotting the regression and the initial
patch. Refs #16612.

tests/forms_tests/tests/forms.py

@@ -1201,6 +1201,32 @@ class FormsTestCase(TestCase):
 <option value="w">whiz</option>
 </select></li>""")
 
+    def test_changed_data(self):
+        class Person(Form):
+            first_name = CharField(initial='Hans')
+            last_name = CharField(initial='Greatel')
+            birthday = DateField(initial=datetime.date(1974, 8, 16))
+
+        p = Person(data={'first_name': 'Hans', 'last_name': 'Scrmbl',
+                         'birthday': '1974-08-16'})
+        self.assertTrue(p.is_valid())
+        self.assertNotIn('first_name', p.changed_data)
+        self.assertIn('last_name', p.changed_data)
+        self.assertNotIn('birthday', p.changed_data)
+
+        # Test that field raising ValidationError is always in changed_data
+        class PedanticField(forms.Field):
+            def to_python(self, value):
+                raise ValidationError('Whatever')
+
+        class Person2(Person):
+            pedantic = PedanticField(initial='whatever', show_hidden_initial=True)
+
+        p = Person2(data={'first_name': 'Hans', 'last_name': 'Scrmbl',
+                         'birthday': '1974-08-16', 'initial-pedantic': 'whatever'})
+        self.assertFalse(p.is_valid())
+        self.assertIn('pedantic', p.changed_data)
+
     def test_boundfield_values(self):
         # It's possible to get to the value which would be used for rendering
         # the widget for a field by using the BoundField's value method.