为什么std :: optional :: operator =（U &&）要求U是非标量类型？

Question

For optional's template<class U = T> optional<T>& operator=(U&& v); the standard demands that (see [optional.assign]/3.16 ):

This function shall not participate in overload resolution unless ... conjunction_v<is_scalar<T>, is_same<T, decay_t<U>>> is false ...

Why do we have to exclude case when assigning a scalar of type U == T ?

Answer 1

This exists to support:

optional<int> o(42);
o = {}; // <== we want this to reset o

We have a bunch of assignment overloads , which take:

1. nullopt_t
2. optional const&
3. optional&&
4. U&&
5. optional<U> const&
6. optional<U>&&

For scalars, specifically, #4 would be a standard conversion whereas anything else would be a user-defined conversion - so it would be the best match. However, the result of that would be assigning o to be engaged with a value of 0 . That would mean that o = {} could potentially mean different things depending on the type of T . Hence, we exclude scalars.

For non-scalars, #4 and #3 would be equivalent (both user-defined conversions), and #3 would win by being a non-template. No problem there.