What is the equivalent of C++'s std::optional<T> and std::nullopt in D?

[Timoses]

On Tuesday, 3 April 2018 at 02:10:09 UTC, helxi wrote:
> For reference: 
> https://en.cppreference.com/w/cpp/utility/optional

See also this topic recently discussed:
https://forum.dlang.org/post/p9qvb8$1j1n$1@digitalmars.com