[1.5.x] Fixed #18574 -- Make BaseFormSet.is_valid call its underlying forms' is_valid

Thanks Simon Charette for the report and the initial patch.
Backport of 66dfcc10b from master.

django/forms/formsets.py

@@ -267,7 +267,7 @@ class BaseFormSet(object):
 
     def is_valid(self):
         """
-        Returns True if form.errors is empty for every form in self.forms.
+        Returns True if every form in self.forms is valid.
         """
         if not self.is_bound:
             return False
@@ -282,8 +282,7 @@ class BaseFormSet(object):
                     # This form is going to be deleted so any of its errors
                     # should not cause the entire formset to be invalid.
                     continue
-            if bool(self.errors[i]):
-                forms_valid = False
+            forms_valid &= form.is_valid()
         return forms_valid and not bool(self.non_form_errors())
 
     def full_clean(self):

tests/regressiontests/forms/tests/formsets.py

@@ -856,6 +856,27 @@ class FormsFormsetTestCase(TestCase):
         formset = FavoriteDrinksFormSet(error_class=CustomErrorList)
         self.assertEqual(formset.forms[0].error_class, CustomErrorList)
 
+    def test_formset_calls_forms_is_valid(self):
+        # Regression tests for #18574 -- make sure formsets call
+        # is_valid() on each form.
+
+        class AnotherChoice(Choice):
+            def is_valid(self):
+                self.is_valid_called = True
+                return super(AnotherChoice, self).is_valid()
+
+        AnotherChoiceFormSet = formset_factory(AnotherChoice)
+        data = {
+            'choices-TOTAL_FORMS': '1',  # number of forms rendered
+            'choices-INITIAL_FORMS': '0',  # number of forms with initial data
+            'choices-MAX_NUM_FORMS': '0',  # max number of forms
+            'choices-0-choice': 'Calexico',
+            'choices-0-votes': '100',
+        }
+        formset = AnotherChoiceFormSet(data, auto_id=False, prefix='choices')
+        self.assertTrue(formset.is_valid())
+        self.assertTrue(all([form.is_valid_called for form in formset.forms]))
+
 
 data = {
     'choices-TOTAL_FORMS': '1', # the number of forms rendered