test-proto.m

// clang -o test-proto -Wall -framework Foundation -fmodules -fobjc-arc test-proto.m && test

@import Foundation;

@protocol A <NSObject>
@property (nonatomic, copy, readonly) NSString *a;
@end

@protocol B <NSObject>
@property (nonatomic, copy, readonly) NSString *b;
@end

@interface C : NSObject <A>
@end

@interface D : NSObject <A, B>
@end

@interface E : NSObject
+ (void)doSomethingWithA:(id<A>)a;
+ (id<A>)returnA;
+ (id<A>)returnADoingSomethingWithA:(id<A>)a;
@end

int main(int argc, char **argv) {
  @autoreleasepool {
    C *c1 = [[C alloc] init];
    D *d1 = [[D alloc] init];
    
    [E doSomethingWithA:c1]; // 👍
    [E doSomethingWithA:d1]; // 👍
    
    C *c2 = [E returnA]; // 👍
    D *d2 = [E returnA]; // ❌ warning: initializing 'D *__strong' with an expression of incompatible type 'id<A>'
    
    C *c3 = [E returnADoingSomethingWithA:c1]; // 👍
    D *d3 = [E returnADoingSomethingWithA:d1]; // ❌ warning: initializing 'D *__strong' with an expression of incompatible type 'id<A>'
    
    C *c4 = [E returnADoingSomethingWithA:d1]; // 👍
    D *d4 = [E returnADoingSomethingWithA:c1]; // ❌ warning: initializing 'D *__strong' with an expression of incompatible type 'id<A>'
    
    NSLog(@"%@", c1);
    NSLog(@"%@", d1);
    NSLog(@"%@", c2);
    NSLog(@"%@", d2);
    NSLog(@"%@", c3);
    NSLog(@"%@", d3);
    NSLog(@"%@", c4);
    NSLog(@"%@", d4);
  }
  
  return 0;
}

@implementation C
@synthesize a;
@end

@implementation D
@synthesize a;
@synthesize b;
@end

@implementation E
+ (void)doSomethingWithA:(id<A>)a {
  NSLog(@"%@", a);
}

+ (id<A>)returnA {
  return nil;
}

+ (id<A>)returnADoingSomethingWithA:(id<A>)a {
  return a;
}
@end

Para que luego os riais de mí cuando digo covarianza y contravarianza, cabrones (pero con cariño).

Quién se reía de ti por eso? Que le zurro.

PD: