[libcxx] r323971 - Remove <experimental/optional>; use <optional> instead. See https://libcxx.llvm.org/TS_deprecation.html

[Marshall Clow via cfe-commits]

On Thu, Feb 22, 2018 at 7:58 AM, Nico Weber <thakis at chromium.org> wrote:

> I have a small personal project where I used to use this. I tried
> switching to <optional> instead, but that apparently requires -std=c++17.
> With that, things build fine with my locally-built clang, but latest Xcode
> clang doesn't accept that flag yet. So I tried -std=c++1z, but latest Xcode
> (9.2) doesn't even include the <optional> header yet. So now I have no way
> of being able to build my project with both trunk clang and Xcode clang.
> Maybe a one-year deprecation period is too short?
>

Nico --

I'm sorry you were caught out by this.

(It's not a huge deal, I have a optional<> implementation in my project for
> platforms that don't ship it yet, but things used to be fine on mac until
> this change at least. It's also not an important project, I just thought
> I'd point out that this makes life more complicated than it would be if the
> deletion period was longer.)
>

Yes, but it also makes life less complicated also.

In fact, optional was the poster child for removing stuff from experimental.
experimental::optional and std::optional have diverged, and are
significantly different today (and only going to get more different in the
future)

The cost for someone to move to std::optional will never be lower than it
is now. (they're only going to become more different).

I really didn't (and don't) want to maintain two closely related (but
different, and slowly diverging) code bases - and (repeatedly) explain to
people the difference between them.

As for Apple not shipping std::optional, I too am dismayed by that, but I
have no control over that. :-(  I don't know when Apple will ship
std::optional (they don't share their product plans with me), but it
requires an update to libc++.dylib, and in the past they have gone *years*
without updating the dylib.   Hopefully they will update before LLVM 7
ships in August.

-- Marshall
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