[swift-evolution] Problems with generics - should be fixed for Xcode 9?

[Charlie Monroe]

This IMHO isn't a bug as the behavior is not completely defined. Joanna mentioned that Args is not generic, but it actually is, once you address it from global namespace - as it inherits the T type (while not used anywhere):

NSStringFromClass(Event<NSString>.Args.self)
NSStringFromClass(Event<NSDate>.Args.self)

-->

_TtGCC14__lldb_expr_375Event4ArgsCSo8NSString__
_TtGCC14__lldb_expr_375Event4ArgsCSo6NSDate__

Which is the root of the problem - you are currently not actually using the same class under different event types. It then makes a static property an issue as you are not able to distinguish these two scenarios with current language:

- is the stored property the same for all variations of the Args class?
- or does each type has its own stored property?

I can think of cases for both. And this is (from what I understood is the core of the problem).

@Joanna - in this particular case, I don't see the benefit of Args.empty over Args() as you currently create a new instance all the time. If the arguments are not intended to be generic, much better solution is to do something like:

class EventArgs {
	public static let empty = Args()
}

class Event<T> {
	typealias Args = EventArgs
}


> On Sep 17, 2017, at 11:59 PM, Félix Cloutier via swift-evolution <swift-evolution at swift.org> wrote:
> 
> I always thought that this was a bug, but I can't find it on bugs.swift.org, so I'd love to know if it's meant to stay this way or not.
> 
>> Le 16 sept. 2017 à 06:32, Joanna Carter via swift-evolution <swift-evolution at swift.org> a écrit :
>> 
>> Greetings
>> 
>> Old chestnut, sort of partially solved but still problems.
>> 
>> Now we can nest types in generic types, what I want is :
>> 
>> class Event<typeT>
>> {
>> class Args
>> {
>>   public static let empty = Args() // error : Static stored properties not supported in generic types
>> }
>> }
>> 
>> But the static let is not directly in the generic class.
>> 
>> So, I end up doing another convoluted workaround in the shape of :
>> 
>> class Event<typeT>
>> {
>> class Args
>> {
>>   public static var empty: Args
>>   {
>>     return Args()
>>   }
>> }
>> }
>> 
>> The main difference is that I have to create a new instance on every call to Args.empty instead of returning the same one.
>> 
>> Is this an oversight or as intended, with the hope of fixing it in a future version
>> 
>> Joanna
>> 
>> --
>> Joanna Carter
>> Carter Consulting
>> 
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> 
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