It is not an oversight. It's just nobody has proposed updates in the
standard library after C++20 language was updated to allow changing active
union member (P1330).

Regards,
Maciej


czw., 24 wrz 2020 o 10:13 Michael Scire via Std-Proposals <
std-proposals_at_[hidden]> napisał(a):

> The rules surrounding constexpr for std::optional seem inconsistent --
> like there are a lot of functions that are "just" missing the keyword due
> to oversight.
>
> For example, `optional(std::nullopt_t)` is constexpr, but `optional
> &operator=(std::nullopt_t)` is not.
>
> This leads to the following (https://godbolt.org/z/8a9Ga3):
>
> // Okay
> ```
> constexpr std::optional<int> ValidOptional = []{
> std::optional<int> opt(std::nullopt);
> opt = std::optional<int>(std::nullopt);
> return opt;
> }();
> ```
>
> // Compiler error
> ```
> constexpr std::optional<int> InvalidOptional = []{
> std::optional<int> opt(std::nullopt);
> opt = std::nullopt;
> return opt;
> }();
> ```
>
> Other oversights:
> `template<typename U> optional(U &&)` is constexpr, but `template<typename
> U> optional &operator=(U &&)` is not.
>
> Things that are less obviously oversights, but might want to change:
> `emplace` is not constexpr, even though the constructors it would call are.
> `reset` is not constexpr.
> `swap(optional &)` is not constexpr, nor is std::swap for optionals.
> (std::swap is constexpr for other types).
> copy constructor/move constructor for template<typename T> optional<U> are
> not constexpr, even though optional(U &&) is.
> If the following changed, copy/move assignment operators would probably
> also want to be constexpr.
>
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