[swift-users] Redundant superclass constraint

[Slava Pestov]

Hi Phil,

I think the warning is bogus in this case. Do you mind filing a bug?

Slava

> On Oct 24, 2017, at 11:00 PM, Phil Kirby via swift-users <swift-users at swift.org> wrote:
> 
> Original StackOverflow post:
> 
> https://stackoverflow.com/questions/46924554/redundant-superclass-constraint-in-swift-4 <https://stackoverflow.com/questions/46924554/redundant-superclass-constraint-in-swift-4>
> 
>  <https://stackoverflow.com/questions/46924554/redundant-superclass-constraint-in-swift-4#>	
> I'm getting a Redundant superclass constraint... warning in swift4: (paste in a playground)
> 
> import CoreData
> 
> class Item : NSManagedObject {}
> 
> protocol DataSourceProtocol {
>     associatedtype DataSourceItem : NSManagedObject
> }
> 
> protocol DataSourceProtocolProvider : class { }
> 
> extension DataSourceProtocolProvider {
>     func createDataSource<T: DataSourceProtocol>(dataSource: T)  
>          where T.DataSourceItem == Item {
>     }
> }
> On the createDataSource<T: DataSourceProtocol> declaration I get the following warning:
> 
> Redundant superclass constraint 'T.DataSourceItem' : 'NSManagedObject'
> 
> I thought that you could specify that an associatedtype could be used with the == operator to constrain the associatedtype to a specific type. I want to have a func createDataSource<T: DataSourceProtocol>(dataSource:T) where the DataSourceItem is an Item.
> 
> If I replace the == operator with : then the warning goes away:
> 
> extension DataSourceProtocolProvider {
>     func createDataSource<T: DataSourceProtocol>(dataSource: T)  
>          where T.DataSourceItem : Item {
>     }
> }
> This happens to be a completely different context now. This constraint specifies that I want to have a func createDataSource<T: DataSourceProtocol>(dataSource:T) where the DataSourceItem is a subclass of Item. Which isn't the same thing as DataSourceItem is an Item object. Also, the code runs fine with == so am I just not understanding how constraints work?
> 
> 
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