generic.ts

```
1. T extends (...args: any) => any 为约束条件，约束 T 的类型为 `(...args: any) => any`
2. T extends (...args: any) => infer R ? R : any 取值，如果 `T extends (...args: any) => R` 那么返回值为 `R` 否则为 `any` 
```
type ReturnType<T extends (...args: any) => any> = T extends (...args: any) => infer R ? R : any;

```
如果 T 是Promise 返回值为 `U` 否则 为 `never`
```
type PromiseType<T> = T extends Promise<infer U> ? U : never;
                                        
                                        
type Optional<T, K extends keyof T> = Pick<T, Exclude<keyof T, K>>;