I have implemented it.
It just isn't used that much. So no real implementation experience either way. It hasn't been a problem, but neither optional<T&> nor assignment come up that much at all, so inconclusive.
If anything, I'd say
Imagine that vector.front() returns an optional<T&>...
As the developer writing vector, I'd like assignment to rebind.
As the developer using vector.front(), I'd like assignment to not rebind.
auto f = vector.front();
if (f)
f = 17; // current optional would not be able to assign from an rvalue
That that doesn't compile is actually a pro, not a con. If you want "assign-through" behavior, you'd write e.g.
if (f.has_value()) {
f.value() = 17;
}
(That is: `f` is an optional that holds a reference; 17 isn't a reference; therefore 17 can't be put into the optional. But f.value() is the reference held by the optional, and you can assign 17 into the variable referred to by that reference.)
Initially (before reaching your code snippet), I was going to say "oh yeah, it would be convenient if `vector.front() = 17` would Just Work"; but you (Tony) aren't even proposing that that should work. You're only showing a more complicated syntax with `if`, where the only difference between the two snippets is which one better reflects our mental model of `optional`. :)
my $.02,
–Arthur