为什么objc运行时函数`class_addMethod()`在目标类为'NSObject`时将实现添加为实例和类方法？

当我使用objc运行时函数class_addMethod()向NSObject的实例选择器注入一个实现时,它实际上将实现注入实例选择器和类选择器:

@implementation HelloWorldClass
- (void) helloWorld{
    NSLog(@"hello world from instance method -helloWorld");
}
@end

// ====
// Do method injection when the application did finish launching.
Class sourceClass = objc_getClass("HelloWorldClass");
Class targetClass = objc_getClass("NSObject");
SEL helloWorldSelector = @selector(helloWorld);

Method method = class_getInstanceMethod(sourceClass, helloWorldSelector);
IMP imp = method_getImplementation(method);
const char *methodTypeEncoding = method_getTypeEncoding(method);

class_addMethod(targetClass, helloWorldSelector, imp, methodTypeEncoding);

现在我们只声明helloWorldvia Objc Category 的接口,并将helloWorld消息调用到NSObject实例和类:

// Declare the interface for `helloWorld
@interface NSObject (HelloWorld)
+ (void) helloWorld;
- (void) helloWorld;
@end


// Send the `helloWorld` message to NSObject class
NSLog(@"Send the `helloWorld` message to NSObject class");
[NSObject helloWorld];

// Send the `helloWorld` message to NSObject instance
NSLog(@"Send the `helloWorld` message to NSObject instance");
[[NSObject new] helloWorld];

虽然您只是将helloWorld实现注入到NSObject实例选择器中class_addMethod(),但是在注入之后解析了类和实例消息:

=> Send the `helloWorld` message to NSObject class
=> hello world from instance method -helloWorld
=> Send the `helloWorld` message to NSObject instance
=> hello world from instance method -helloWorld

经过测试,我发现class_addMethod()只有当目标类为class_addMethod()is 时才将实现添加到类和实例选择器中NSObject.

它是objc-runtime还是Cocoa的bug？