StateMachine with std::variant, getting the custom template deduction right - c++17

Question

With the following code how do I write the custom template deduction correctly?
template<class R, class State, class... Ts> struct visitor : Ts... { using Ts::operator()...; };
template<class R, class State, class... Ts> visitor(State, Ts...)->visitor<class R, State,Ts...>;
using Event = std::variant<HeartBeat, ConfigurationRead>;
using State = std::variant<Idle, Starting, Running, Error>;
void case3()
{
Event e;
State currentState;
State newState = std::visit( visitor{
[](Idle&& state, HeartBeat event) {std::cout << "Currently stopped, heartbeat received.\n"; return Error{}; }
}, currentState, e);
}
I have looked at several examples, but I cannot find one which uses the return of the std::visit.

Answer 1

Your visitor does not need to deduce and encode the return type R -- one will be inferred by std::visit itself: deduced from the callable or fixed to a specified template argument (see below). That being said, it could be simplified to:
template <typename... Ts> struct visitor : Ts... { using Ts::operator()...; };
template <typename... Ts> visitor(Ts...) -> visitor<Ts...>;
In c++17 each visitor must, however, return the same type. You could avoid any type mismatch by, e.g., explicitly specifying one for each lambda expression:
State newState = std::visit( visitor{
[](Idle state, HeartBeat event) -> State { return Error{}; },
// ~~~~^
[](auto state, auto event) -> State { return state; }
// ~~~~^
}, currentState, e);
DEMO
In c++20, the return type can be specified as the first template argument of std::visit:
State newState = std::visit<State>( visitor{
// ~~~~^
[](Idle state, HeartBeat event) { return Error{}; },
[](auto state, auto event) { return state; }
}, currentState, e);