[swift-dev] why are we accepting switch statements with duplicate conditions?

[Bob Wilson]

> On Sep 14, 2016, at 10:32 AM, John McCall via swift-dev <swift-dev at swift.org> wrote:
> 
>> On Sep 14, 2016, at 10:25 AM, Ted Kremenek via swift-dev <swift-dev at swift.org <mailto:swift-dev at swift.org>> wrote:
>> 
>> Hi everyone,
>> 
>> Joe Shajrawi nominated an optimizer fix for Swift 3.0.1 where the optimizer was crashing due to duplicate conditions in aswitchstatement:
>> 
>> https://bugs.swift.org/browse/SR-2512 <https://bugs.swift.org/browse/SR-2512>
>> https://github.com/apple/swift/pull/4747 <https://github.com/apple/swift/pull/4747>
>> Here’s an example:
>> 
>> public enum DemoEnum {
>>     case firstCase
>>     case secondCase
>> 
>>     public static func performSwitch(with value: Int) -> DemoEnum {
>>         switch value {
>>         case 0:
>>             return DemoEnum.firstCase
>>         case 0:
>>             return DemoEnum.firstCase
>>         default:
>>             return DemoEnum.secondCase
>>         }
>>     }
>> }
>> I can’t remember why the frontend even accepts this code. Is this intended, or just a defect in the compiler’s validity checking ofswitch statements?
>> 
>> 
> 
> It would be legitimate with, say, a condition on the first case.  But yeah, this is undoubtedly just a defect in the compiler's checking.
> 
> Recall that a switch in Swift doesn't necessarily compile to a primitive switch; it's more like a series of ifs that's a bit more straightforward to optimize matching for.  So the validity checking you'd expect as a matter of course in C doesn't just fall out automatically.
> 
> Also, of course, the compiler does not have primitive knowledge of Int or how it forms literals.
> 
> John.

Joe filed a radar to improve the checking:
<rdar://problem/28301984> reject duplicate conditions when optimizing switch statement
>> Thanks,
>> Ted
>> 
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> 
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