Cannot set a read_only field with a default

[bespokebob]

According to the documentation at https://www.django-rest-framework.org/api-guide/validators/#advanced-field-defaults

Using a standard field with read_only=True, but that also includes a default=… argument. This field will be used in the serializer output representation, but cannot be set directly by the user.

This doesn't actually work, though. It passes validation, but the data isn't saved.

class TestReadonlyDefault(TestCase):
    def test_readonly_default(self):
        class TestSerializer(serializers.ModelSerializer):
            char_field = serializers.CharField(read_only=True, default='default')

            class Meta:
                model = OneFieldModel
                fields = ('char_field',)

        serializer = TestSerializer(data={})
        serializer.is_valid()
        instance = serializer.save()
        assert instance.char_field == 'default'

================================================================================================== FAILURES ==================================================================================================
_________________________________________________________________________________ TestReadonlyDefault.test_readonly_default __________________________________________________________________________________
tests/test_model_serializer.py:1350: in test_readonly_default
    assert instance.char_field == 'default'
E   AssertionError: assert '' == 'default'
E     - default
========================================================================================== short test summary info ===========================================================================================
FAILED tests/test_model_serializer.py::TestReadonlyDefault::test_readonly_default - AssertionError: assert '' == 'default'

My actual use case is a little more complicated since I want to use a read_only nested serializer with a default, but it seems like it doesn't work for regular fields either.

[bespokebob]

#7898