main.m

#import <Foundation/Foundation.h>


@protocol SomeProtocol <NSObject>
- (void)someMethod;
@end


@protocol MyProtocol <SomeProtocol>
- (void)myMethod;
@end


@interface NSObject (MyProtocol)
- (void)myMethod;
@end

@implementation NSObject (MyProtocol)
- (void)myMethod
{
	[(id<SomeProtocol>)self someMethod];
}
@end


int main(int argc, const char * argv[]) {
	@autoreleasepool
	{
		id<SomeProtocol> object = nil;
		
		[object someMethod];
		
		[(id<MyProtocol>)object myMethod];
		
		[object myMethod]; // error
	}
	return 0;
}