Another thing Darin didn’t mention but I think people should be careful about:
The move constructor for std::optional does not clear the is-set flag (while 
the one for WTF::Optional did).

As a result, you will be having a very bad time if you do a use-after-move of a 
std::optional. Please make sure to use std::exchange() instead of WTFMove() if 
you want to leave to std::optional in a clean state for reuse later.

Chris Dumez

> On Jun 1, 2021, at 8:54 PM, Darin Adler via webkit-dev 
> <webkit-dev@lists.webkit.org> wrote:
> Hi folks.
> We’re getting rid of the WTF::Optional class template, because, hooray, 
> std::optional is supported quite well by all the C++17 compilers we use, and 
> we don’t have to keep using our own special version. Generally we don’t want 
> to reimplement the C++ standard library when there is not a significant 
> benefit, and this is one of those times.
> Here are a few considerations:
> 1) Since https://commits.webkit.org/238290@main, if you use Optional<> by 
> mistake instead of std::optional<>, your code won’t compile. (Unless you are 
> writing code for ANGLE, which has its own separate Optional<>.)
> 2) If you want to use std::optional, include the C++ standard header, 
> <optional>, or something that includes it. In a lot of cases, adding an 
> include will not be required since it’s included by widely-used headers like 
> WTFString.h and Vector.h, so if you include one of those are covered. Another 
> way to think about this is that if your base class already uses 
> std::optional, then you don’t need to include it.
> 3) Once the patch in https://bugs.webkit.org/show_bug.cgi?id=226437 lands, 
> includes of <wtf/Forward.h> won’t forward declare optional, and includes of 
> <wtf/Optional.h> won’t do anything at all.
> — Darin
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