I do not see the use-case, as it easily can be implemented with wrapper
class/struct and operator():

struct S {

    enum E {
        VALUE_1,
        VALUE_2
    };

    E value_;

    S operator()(int state, double value) {
        switch (value_) {
            case E::VALUE_1 : {
                std::cout << "VALUE_1 int:" << state << " dbl:" << value <<
std::endl;
                return S{E::VALUE_2};
            }
            case E::VALUE_2 : {
                std::cout << "VALUE_1 int:" << state << " dbl:" << value <<
std::endl;
                return S{E::VALUE_1};
            }
            default: throw std::domain_error("Undefined value");
        }
    }

};




int main() {
    S state{S::VALUE_1};


    state = state(1, 3.14);
    state = state(2, 10.0);

// Additional syntax sugar
    state = state
        (1, 3.14)
        (2, 10);

}

On Fri, Apr 1, 2022 at 7:04 PM Gergely Nagy via Std-Proposals <
std-proposals_at_[hidden]> wrote:

> Hi,
>
> Well, state machines are not that uncommon. The self-mutating part is not
> much weirder than the self-mutating nature of a normal class.
>
> Besides state machines, it can be useful for VM-like things.
>
> The point is that maintaining switch statements together with the enum is
> error-prone above a level.
>
> If "out-of-the-box" self-mutation is a problem, it can be made an opt-in
> feature, like for lambdas:
>
> enum class DispatchStuff(int Arg1, double Arg2) *mutable* : int {
>
> Of course there are nuances, like can it be templated for the arguments,
> etc...
>
> Thanks,
> Gergely
>
>
>
> Jason McKesson via Std-Proposals <std-proposals_at_[hidden]> ezt
> írta (időpont: 2022. ápr. 1., P, 18:53):
>
>> Is this a circumstance that comes up often enough to *require* a
>> language feature like this? I mean yes, it simplifies these specific
>> cases, but are these cases prevalent enough to be worthy of being
>> simplified?
>>
>> I would much rather see the code spelled out, so that it is pretty
>> obvious what is going on when you "call" such a value. Especially if
>> it's going to be self-mutating.
>> --
>> Std-Proposals mailing list
>> Std-Proposals_at_[hidden]
>> https://lists.isocpp.org/mailman/listinfo.cgi/std-proposals
>>
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>