I swear, I only did this for an experiment

evil.m

#import <objc/runtime.h>
#import <Foundation/Foundation.h>

int main(int argc, char *argv[]) {
	@autoreleasepool {
		IMP imp = imp_implementationWithBlock(^id(id self, id block, id arg){
			NSLog(@"self: %@, arg: %@", self, arg);
			return self;
		});
		class_addMethod([NSObject class], @selector (foo), imp, "@@:@");
		NSObject *object = [[NSObject alloc] init];
		id ret = [object performSelector:@selector (foo) withObject:@"Hi!"];
		NSLog(@"ret: %@", ret);
	}
}