Checklist

• I have verified that that issue exists against the master branch of Django REST framework.
• I have searched for similar issues in both open and closed tickets and cannot find a duplicate.
• This is not a usage question. (Those should be directed to the discussion group instead.)
• This cannot be dealt with as a third party library. (We prefer new functionality to be in the form of third party libraries where possible.)
• I have reduced the issue to the simplest possible case.
• I have included a failing test as a pull request. (If you are unable to do so we can still accept the issue.)

Steps to reproduce

class ExampleSerializer(serializers.Serializer):
    a = serializers.CharField(required=True)
    b = serializers.CharField(required=False)

class AnotherSerializer(serializers.Serializer):
    c = serializers.CharField(required=True)
    d = serializers.CharField(required=False)

class ExampleViewSet(ModelViewSet):
    serializer_class = ExampleSerializer

    @detail_route(methods=['post'], serializer_class=AnotherSerializer)
    def custom_action(self, request, pk):
        return super(ExampleSerializer, self).retrieve(self, request)

Expected behavior

Fields c and d from AnotherSerializer in generated schema.

Actual behavior

Fields a and b from ExampleSerializer - serializer of viewset itself.