public <U> Class<? extends U> asSubclass(Class<U> clazz)这个函数加泛型以后Class<Number> x = String.class.asSubclass(Number.class);为啥这样写就错

public <U> Class<? extends U> asSubclass(Class<U> clazz)这个函数加泛型以后Class<Number> x = String.class.asSubclass(Number.class);为啥这样写就错