relates to:

• SR-5255 Make this initialization pattern efficient

Issue Description:

@import Foundation;
@interface Foo : NSObject
+ (nonnull Foo *)fooWithPowerLevel:(NSInteger)level;
@end
@interface Bar : Foo
@end

@implementation Foo
+ (nonnull Foo *)fooWithPowerLevel:(NSInteger)level {
  (void)powerLevel; // hey, it's just an example
  return [[Foo alloc] init];
}
@end
@implementation Bar
@end

extension Foo {
  convenience init(overNineThousand: Bool) {
    // Uh oh!
    self.init(powerLevel: overNineThousand ? 9001 : 2)
  }
}

print(Bar(overNineThousand: true)) // creates a Foo!

A "non-convenience factory initializer" is a factory initializer that can't be inherited. You can't define these in Swift, but you can in Objective-C (by specifying an explicit return type instead of using instancetype). Swift's convenience initializers are inherited (as are Objective-C's), and so they shouldn't be allowed to delegate to non-convenience factory initializers (unless the class is final).

However…fixing this would be a source-breaking change, and we don't currently have a way to define what people want in Swift (a non-inherited factory initializer). Meanwhile, everything works fine if they only use the convenience initializer on the original class.

If we don't think we're going to get factory initializers in Swift soon, we should consider adding a run-time check that the Self type matches the type the factory initializer is going to create.