在 ts 中什么是 infer，并实现 Parameters 与 ReturnType

type Parameters<T extends (...args: any[]) => unknown> = T extends (
  ...args: infer R
) => unknown
  ? R
  : never;

type Parameters<T extends (...args: any) => any> = T extends (
  ...args: infer P
) => any
  ? P
  : never;

type A = Parameters<() => void>; // []
type B = Parameters<typeof Array.isArray>; // [any]
type C = Parameters<typeof parseInt>; // [string, (number | undefined)?]
type D = Parameters<typeof Math.max>; // number[]