id and protocol conformance

protocol_id.m

// when specify id as the type, you can use NSObject methods.
// when you specify as a custom protocol, you cannot.
// UNLESS you say that your custom protocol includes <NSObject>


#import <Foundation/Foundation.h>

@protocol CustomProtocol // <NSObject> // uncommenting <NSObject> fixes the issue

@end

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        // insert code here...
        NSLog(@"Hello, World!");
        
        id thing1;
        
        if ([thing1 isKindOfClass:[NSString class]]) {
            // works fine
        }
        
        id <CustomProtocol> thing2;
        
        if ([thing2 isKindOfClass:[NSString class]]) {
            // does not compile
        }
    }
    return 0;
}