Thank you for the clarification.
> On 8. Aug 2017, at 18:39, Guillaume Lessard <gless...@tffenterprises.com>
> wrote:
>
>
>> On Aug 8, 2017, at 09:10, Martin R <martinr...@gmail.com> wrote:
>>
>> In that example the tuple is a (stored) property of a class, not a global
>> variable. And why does it crash for a global variable, but not for a local
>> variable in a function?
>
> In the case of a local variable in a function, the compiler can statically
> prove that there is no simultaneous access, and using `swap` is allowed. With
> a global variable, the compiler can’t statically prove exclusive access.
> (it seems silly with your simple example, but the system doesn’t try static
> enforcement with global variables.)
>
> Here’s another modification which traps at runtime:
>
> ***
> import Dispatch
>
> func foo() {
> var tuple = (1, 2)
>
> let q = DispatchQueue(label: "queue")
> q.async { swap(&tuple.0, &tuple.1) }
> q.sync {}
>
> print(tuple)
> }
>
> foo()
> ***
>
> Guillaume Lessard
>
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