See issue #621 - this patch makes it so that Python can properly call C++ or BP methods with one or more output parameters as well as C++ methods that have both output parameters and a return value. Further, it makes it so that in Python you can override and implement methods that have both return values and output parameters. A side effect of this patch is that so far it appears to have fixed #622.

Note that when combining out params and a return value, the return value needs to come last in the type annotation info, e.g. given a C++ base class method like this:

    UFUNCTION(BlueprintCallable, BlueprintNativeEvent) float Func(int i, int& oi);

you'd implement it in Python like this:

    def Func(self, i:int) -> (int, float):
        return 5, 12.34 # 5 is the out param, 12.34 is the return value

In terms of lines of code, the biggest change is moving property creation to new_property_from_pyobject since it is needed in more places (and was getting unwieldy anyway).

Given that this patch affects some pretty fundamental code, I also created a project that runs through a large set of test scenarios: https://github.com/dfb/uepy_test_prjs/tree/master/output_params