Re: [swift-users] Why does casting to Any make my forced downcast not crash?

[Joe Groff via swift-users]

> On Mar 15, 2017, at 4:47 AM, Kim Burgestrand  wrote:
> 
> On Tue, 14 Mar 2017 at 19:50 Joe Groff  > wrote:
> 
> It shouldn't. Please file a bug, if you haven't already.
> 
> Will do! I'll file a bug that casting to `Any` first causes different 
> behavior.
> 
> Follow-up question, looking at the expression `name as! T`, is it expected to 
> cause a nil-unwrapping runtime error when `name == nil` and `T == 
> Optional`? Mind you, `(nil as! Optional)` does not cause an 
> error.

If you're casting to an Optional destination type, then the cast ought to 
succeed and produce a `nil` result if the input is nil.

-Joe

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Re: [swift-users] Why does casting to Any make my forced downcast not crash?

[Kim Burgestrand via swift-users]

Oh, wow, the reported bug is quite similar but not exactly.

Anyway, I reported the problem here https://bugs.swift.org/browse/SR-4248.
I guess the discussion will continue in there.

Thanks again!

On Wed, 15 Mar 2017 at 12:51 Kim Burgestrand  wrote:

> Found the answer to my follow-up question! It's a known and reported bug:
> https://bugs.swift.org/browse/SR-912
>
> Thanks!
>
> On Wed, 15 Mar 2017 at 12:47 Kim Burgestrand  wrote:
>
> On Tue, 14 Mar 2017 at 19:50 Joe Groff  wrote:
>
>
> It shouldn't. Please file a bug, if you haven't already.
>
>
> Will do! I'll file a bug that casting to `Any` first causes different
> behavior.
>
> Follow-up question, looking at the expression `name as! T`, is it
> expected to cause a nil-unwrapping runtime error when `name == nil` and `T
> == Optional`? Mind you, `(nil as! Optional)` does not
> cause an error.
>
> If the nil-unwrapping error is expected, how can I write it so that it
> doesn't, and instead returns nil?
>
>
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Re: [swift-users] Why does casting to Any make my forced downcast not crash?

[Kim Burgestrand via swift-users]

On Tue, 14 Mar 2017 at 19:50 Joe Groff  wrote:

>
> It shouldn't. Please file a bug, if you haven't already.
>

Will do! I'll file a bug that casting to `Any` first causes different
behavior.

Follow-up question, looking at the expression `name as! T`, is it expected
to cause a nil-unwrapping runtime error when `name == nil` and `T ==
Optional`? Mind you, `(nil as! Optional)` does not cause an
error.

If the nil-unwrapping error is expected, how can I write it so that it
doesn't, and instead returns nil?
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Re: [swift-users] Why does casting to Any make my forced downcast not crash?

[Joe Groff via swift-users]

> On Mar 13, 2017, at 5:13 AM, Kim Burgestrand via swift-users 
>  wrote:
> 
> Here are two implementations that I'd expect to be equivalent, but 
> something's different because the first example crashes, whereas the second 
> example does not crash.
> 
> What's the difference between these two? Why does casting to Any before the 
> forced downcast to T have any effect?

It shouldn't. Please file a bug, if you haven't already.

-Joe

> Example 1: (`unexpectedly found nil while unwrapping an Optional value`)
> ```
> func cast(_ name: String?) -> T {
> return name as! T
> }
> 
> debugPrint((cast(nil) as String?) as Any))
> ```
> 
> Example 2: (prints `nil`)
> ```
> func cast(_ name: String?) -> T {
> return (name as Any) as! T
> }
> 
> 
> debugPrint((cast(nil) as String?) as Any)
> ```
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[swift-users] Why does casting to Any make my forced downcast not crash?

[Kim Burgestrand via swift-users]

Here are two implementations that I'd expect to be equivalent, but
something's different because the first example crashes, whereas the second
example does not crash.

What's the difference between these two? Why does casting to Any before the
forced downcast to T have any effect?

Example 1: (`unexpectedly found nil while unwrapping an Optional value`)
```
func cast(_ name: String?) -> T {
return name as! T
}

debugPrint((cast(nil) as String?) as Any))
```

Example 2: (prints `nil`)
```
func cast(_ name: String?) -> T {
return (name as Any) as! T
}


debugPrint((cast(nil) as String?) as Any)
```
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