[swift-users] inout params seem to have undefined behavior

[Jens Alfke]

> On Jun 13, 2016, at 1:43 PM, Joe Groff <jgroff at apple.com> wrote:
> 
> I see, missed that part. That's a bug—`acopy` should remain a distinct copy of `a`.

I’m guessing that when `a` is copied into `acopy`, the same array contents are shared between them, then when `a` is mutated it gets a new copy of the contents. Then the assignment to `b` writes into the original array contents now held by `acopy`.

Still seems like it’s caused by having aliased `inout` variables, but the effect is nastier. I can imagine horrible bugs where `acopy` gets passed around somewhere else and then changes, breaking the something-else that’s holding it. It could be hard to track that back to its original cause, especially if there are two different codebases involved.

—Jens
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