[swift-users] Tuple of two or more instances to withExtendedLifetime

[Andrew Trick]

Yes, withExtendedLifetime is a generic API that guarantees the lifetime of any non-trivial value. i.e. if the tuple contains a reference, the lifetime of that reference will be extended.

That said, there is a theoretical bug in the optimizer that you will be much more likely to hit in practice if you pass a tuple to this API. I still think it’s extremely unlikely that the  code will be miscompiled though, and I’m sure it will be fixed shortly ;)

https://bugs.swift.org/browse/SR-6608: DeadCodeElimination removes fix_lifetime

-Andy

> On Dec 13, 2017, at 10:05 AM, Jordan Rose <jordan_rose at apple.com> wrote:
> 
> Good question and good thought! I believe the answer is yes, this will work as intended, but I'd like to get someone more versed in SIL than me to verify that. Andy?
> 
> Jordan
> 
> 
>> On Dec 13, 2017, at 01:28, Jens Persson via swift-users <swift-users at swift.org> wrote:
>> 
>> There are no restrictions on the type of the first argument to withExtendedLiftetime, even though it makes sense only for reference types.
>> 
>> So both the below variants will compile, and my question is if the first one will work as expected (extending the lifetime of both a and b) or if the second variant must be used instead.
>> 
>> 1:
>> withExtendedLifetime((a, b)) {
>>    ...
>> }
>> 
>> 2:
>> withExtendedLifetime(a) {
>>    withExtendedLifetime(b) {
>>        ...
>>    }
>> }
>> 
>> /Jens
>> 
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