Should `with` return a value from the lambda?

https://news.ycombinator.com/item?id=35464152#35464934

```cpp
auto x = state.with([](auto& state) { return state.x; });
```

If you're mutating the protected state
```cpp
state.with([](auto& v) { v++; });
```
the current `void with(F&&)` is fine, but if you're reading the state,
```cpp
count += state.with([](auto& v) { return v; });
```
is certainly nicer than
```cpp
state.with([&count](auto& v) { count += v; });
```
though I guess it needs an extra copy.

Considerations:
- Can we enforce that the return value is copied out instead of returned by reference? Should we?
- What about the try variants? I guess then we have to return a std::optional to show whether there is a return value at all?

If we decide against this, can/should we enforce that the lambda return type is void?