[webkit-dev] Watch out for std::optional's move constructor

[Alex Christensen]

This one and the many others like it are fragile, relying on undefined behavior, and should be replaced by std::exchange.  Such a change was made in https://trac.webkit.org/changeset/198755/webkit and we probably need many more like that, but we are getting away with relying on undefined behavior which works for us in most places.

> On Dec 17, 2018, at 11:24 AM, Chris Dumez <cdumez at apple.com> wrote:
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> 
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>> On Dec 17, 2018, at 11:10 AM, Chris Dumez <cdumez at apple.com <mailto:cdumez at apple.com>> wrote:
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>>> On Dec 17, 2018, at 10:27 AM, Alex Christensen <achristensen at apple.com <mailto:achristensen at apple.com>> wrote:
>>> 
>>>>>>> On Dec 14, 2018, at 1:37 PM, Chris Dumez <cdumez at apple.com <mailto:cdumez at apple.com>> wrote:
>>>>>> 
>>>>>>> 
>>>>>>> As far as I know, our convention in WebKit so far for our types has been that types getting moved-out are left in a valid “empty” state.
>>> This is not necessarily true.  When we move out of an object to pass into a function parameter, for example, the state of the moved-from object depends on the behavior of the callee.  If the callee function uses the object, we often have behavior that leaves the object in an “empty” state of some kind, but we are definitely relying on fragile undefined behavior when we do so because changing the callee to not use the parameter changes the state of the caller.  We should never assume that WTFMove or std::move leaves the object in an empty state.  That is always a bug that needs to be replaced by std::exchange.
>> 
>> Feel like we’re taking about different things. I am talking about move constructors (and assignment operators), which have a well defined behavior in WebKit. And it seems you are talking about WTFMove(), which despite the name does not “move” anything, it is merely a cast.
>> In the case you’re talking about the caller does NOT call the move constructor, it merely does a cast so I do not think your comment invalidates my statement. Note that in my patch, I was nearly WTFMove()ing the data member and assigning it to a local variable right away, calling the move constructor.
> 
> Also note that may of us already rely on our move constructors’ behavior, just search for WTFMove(m_responseCompletionHandler) in:
> https://trac.webkit.org/changeset/236463/webkit <https://trac.webkit.org/changeset/236463/webkit>
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