QFutureInterface: takeResult() should either require a type to be default-constructible or return std::optional

The function has the following signature now:

template<typename T>
T QFutureInterface<T>::takeResult()

And somewhere in the function's body we have a code like this:

if (...) {
    return {};
}

Which means that the type should be default-constructible. So, we have the following options:

• Add static assert (std::is_default_constructible_v<T>) at the beginning of this function.
• Return std::optional<T> instead of T.

The documentation should be extended in both cases.

I think that the second option is better because:

• This is not mandatory functional requirement for the type.
• You don't need to create a default constructed object.
• std::optional was designed for cases like this.

Perhaps, in the future, we could use std::expected or QExpected (once/if we have it implemented) to propagate an error message/code.