Using Objective-C generics with type constraints on forward-declared classes?

generics.m

#import <Foundation/Foundation.h>

@interface Base : NSObject
@end

@interface GenericCollection<T: Base *> : NSObject
@end

@class Derived2;

@interface Derived1 : Base
@property (strong) GenericCollection<Derived2 *> *collection;
@end

@interface Derived2 : Base
@property (strong) GenericCollection<Derived1 *> *collection;
@end

Deferring the property declaration to a class extension works:

#import <Foundation/Foundation.h>

@interface Base : NSObject
@end

@interface GenericCollection<T: Base *> : NSObject
@end

@interface Derived1 : Base
@end

@interface Derived2 : Base
@property (strong) GenericCollection<Derived1 *> *collection;
@end

@interface Derived1 ()
@property (strong) GenericCollection<Derived2 *> *collection;
@end

In a real-world setting it would likely require introducing an additional header file that both Derived1.h and Derived2.h import.