Refs #36913 -- Maintained error message determinism in MultipleChoiceField.validate().

Used Django's OrderedSet datastructure instead of set() in MultipleChoiceField.validate()
to prevent submission ordering from being discarded during validation.

Thanks to Jacob Walls, JaeHyuck Sa, Jake Howard and Simon Charette for
the reviews.

Author: anjaniacatus

django/forms/fields.py

@@ -40,6 +40,7 @@
 )
 from django.utils import formats
 from django.utils.choices import normalize_choices
+from django.utils.datastructures import OrderedSet
 from django.utils.dateparse import parse_datetime, parse_duration
 from django.utils.duration import duration_string
 from django.utils.ipv6 import MAX_IPV6_ADDRESS_LENGTH, clean_ipv6_address
@@ -965,8 +966,8 @@ def validate(self, value):
         if self.required and not value:
             raise ValidationError(self.error_messages["required"], code="required")
         # Validate that each value in the value list is in self.choices.
-        # Use set() to avoid redundant validation.
-        for val in set(value):
+        # Avoid redundant validation, and keep elements ordered.
+        for val in OrderedSet(value):
             if not self.valid_value(val):
                 raise ValidationError(
                     self.error_messages["invalid_choice"],

tests/forms_tests/field_tests/test_multiplechoicefield.py

@@ -91,5 +91,5 @@ def test_validate_duplicated_values(self):
 
     def test_validate_duplicated_invalid_values(self):
         f = MultipleChoiceField(choices=[("1", "one"), ("2", "Two")])
-        with self.assertRaises(ValidationError):
-            f.validate(["1", "1", "invalid", "invalid"])
+        with self.assertRaisesMessage(ValidationError, "invalid-one"):
+            f.validate(["invalid-one", "invalid-one", "invalid-two"])