[swift-dev] New warning message while switching on an enum

[Jordan Rose]

I think any bridging conversion or upcast would count.

enum Foo {
  case foo(NSFileManager)
  case bar(NSString)
  case baz(Int)
}
func test(x: Foo) {
  switch x {
  case .foo(let obj as NSObject):
    print("totally legal")
  case .bar(let obj as String):
    print("also cool")
  case .baz(let obj as Any):
    print("I'm not sure why, but sure")
  }
}

Jordan

> On May 9, 2017, at 11:29, Robert Widmann <devteam.codafi at gmail.com> wrote:
> 
> Right.  I guess I should have asked: Are there any other kinds of patterns we consider exhaustive that have this structure?  
> 
> ~Robert Widmann
> 
>> On May 9, 2017, at 2:23 PM, Jordan Rose <jordan_rose at apple.com <mailto:jordan_rose at apple.com>> wrote:
>> 
>> "as NSError" isn't a tautology, though—the original type is 'Error'. Additionally, the presence or absence of a warning shouldn't affect exhaustiveness analysis, which is currently an error when you get it wrong. Warnings are things you can build with and fix later.
>> 
>> Jordan
>> 
>> 
>>> On May 9, 2017, at 11:22, Robert Widmann <devteam.codafi at gmail.com <mailto:devteam.codafi at gmail.com>> wrote:
>>> 
>>> We’ll warn if that kind of cast is a tautology, right?  That leaves only the dynamic casts, which can’t be assumed exhaustive.
>>> 
>>> ~Robert Widmann
>>> 
>>>> On May 9, 2017, at 2:13 PM, Jordan Rose <jordan_rose at apple.com <mailto:jordan_rose at apple.com>> wrote:
>>>> 
>>>> It's neither a variable binding nor an expression pattern, right? It has to be compared against the original type to know whether it's exhaustive or not. (Consider "let error as NSError" in a catch clause, which should be considered exhaustive.)
>>>> 
>>>> Jordan
>>>> 
>>>>> On May 9, 2017, at 11:12, Robert Widmann <devteam.codafi at gmail.com <mailto:devteam.codafi at gmail.com>> wrote:
>>>>> 
>>>>> It’s mine, yep.  It looks like it’s classifying the cast in the first pattern as a variable binding instead of an expression pattern.  I’ll push a fix later.
>>>>> 
>>>>> Thanks!
>>>>> 
>>>>>> On May 9, 2017, at 1:52 PM, Jordan Rose <jordan_rose at apple.com <mailto:jordan_rose at apple.com>> wrote:
>>>>>> 
>>>>>> That looks like a bug to me, since of course the first pattern won't always match. I suspect this is Robert's work on trying to make the exhaustive checks better, https://github.com/apple/swift/pull/8908 <https://github.com/apple/swift/pull/8908>. Thanks for catching this!
>>>>>> 
>>>>>> Jordan
>>>>>> 
>>>>>> 
>>>>>>> On May 9, 2017, at 07:09, Pushkar N Kulkarni via swift-dev <swift-dev at swift.org <mailto:swift-dev at swift.org>> wrote:
>>>>>>> 
>>>>>>> Hi there, 
>>>>>>> 
>>>>>>> I see a new warning message for switch statements on enums, like this one: 
>>>>>>> 
>>>>>>> enum Test {
>>>>>>>     case one(Any)
>>>>>>>     case two
>>>>>>> }
>>>>>>> 
>>>>>>> let x: Test = .one("One")
>>>>>>> switch x {
>>>>>>>     case .one(let s as String): print(s)
>>>>>>>     case .one: break
>>>>>>>     case .two: break
>>>>>>> }
>>>>>>> 
>>>>>>> enum.swift:9:10: warning: case is already handled by previous patterns; consider removing it
>>>>>>>     case .one: break 
>>>>>>> 
>>>>>>> 
>>>>>>> I do not see this warning with the 04-24 dev snapshot. 
>>>>>>> 
>>>>>>> The warning goes away with the use of the wildcard pattern in the second case:
>>>>>>> 
>>>>>>> switch x {
>>>>>>>     case .one(let s as String): print(s)
>>>>>>>     case .one(_): break
>>>>>>>     case .two: break
>>>>>>> }
>>>>>>> 
>>>>>>> 
>>>>>>> I am wondering if this change is intentional, though it does make sense to me. Can someone please point me to the related commit?
>>>>>>> 
>>>>>>> Thanks in advance!
>>>>>>> 
>>>>>>> Pushkar N Kulkarni,
>>>>>>> IBM Runtimes
>>>>>>> 
>>>>>>> Simplicity is prerequisite for reliability - Edsger W. Dijkstra
>>>>>>> 
>>>>>>> 
>>>>>>> _______________________________________________
>>>>>>> swift-dev mailing list
>>>>>>> swift-dev at swift.org <mailto:swift-dev at swift.org>
>>>>>>> https://lists.swift.org/mailman/listinfo/swift-dev <https://lists.swift.org/mailman/listinfo/swift-dev>
>>>>>> 
>>>>> 
>>>> 
>>> 
>> 
> 

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