[libcxx-commits] [libcxx] [libc++] Implement P2988R12: `std::optional<T&>` (PR #155202)

[William Tran-Viet via libcxx-commits]

smallp-o-p wrote:

> Oh... I forgot the following partial specialization. Looks like that it isn't added yet.
> 
> ```c++
> template<class T>
> constexpr bool ranges::enable_borrowed_range<optional<T&>> = true;
> ```
> 
> I think we should test that
> 
>     * `ranges::enable_borrowed_range<optional<T&>>` is `true` for all `T`, even if `T` is an array of unknown bound or a function type;
> 
>     * `ranges::borrow_range<optional<T&>>` is `true` if and only if `ranges::range<optional<T&>>` is `true`. It should be `false` when `T` is a function type.

I believe `ranges::borrow_range<optional<T&>>` should also be false if `T` is an array type, at least until LWG4308 is accepted, which I currently don't think should be tackled in this PR.

https://github.com/llvm/llvm-project/pull/155202