类型默认值

interface Todo1 {
  title: string
  description?: string
  completed: boolean
}
type A<T, K extends keyof T = keyof T> = T
type B = A<Todo>
type C = A<Todo, 'title'>

如何理解K extends keyof T = keyof T, K继承T的key集合，并且K的默认值是T的key集合，B C两种均对