typescript - 请教TS的泛型条件约束问题？

Question

演示代码：https://tsplay.dev/N5jo0m

interface Base {    name: string;    age: number;};interface FixedInstance extends Base {}interface FollowInstance extends Base {}type NameType = "fixed"|"follow";type FixedName = "fixed";type FollowName = "follow";const data: NameType = 'fixed';type myType = typeof data;type isFixed = myType extends FixedName ? true : false;   // truetype isFollow = myType extends FollowName ? true : false;   // false// -----------cut---function sellect<T extends NameType, U = T extends FixedName ? FixedInstance : FollowInstance>(name: T, data: U) {    return {        name, data    };}const { name: myName, data: myData } = sellect("fixed", { name: "levi", age: 18 } as FollowInstance);type dataType = typeof myData;type dataisFixed = myType extends FixedName ? true : false;   // truetype dataisFollow = myType extends FollowName ? true : false;   // false

从cut以上，可以看到都是正确的，问题在函数sellect

• 我需要根据第一个参数name: T去判断第二个参数data的类型
• 提供的name是fixed就限制data是FixedInstance，否则就限制为FollowInstance

问题1：我在sellect参数传参的时候故意 as FollowInstance，在TS中并没有报错
问题2：在拿到的结果中dataisFixed是true，但是我传过去的是FollowInstance

Answer 1

修改如下

interface Base {    name: string;    age: number;};interface FixedInstance extends Base {}interface FollowInstance extends FixedInstance {    sex: 1|2;}type NameType = "fixed"|"follow";type FixedName = "fixed";type FollowName = "follow";const data: NameType = 'fixed';type myType = typeof data;type isFixed = myType extends FixedName ? true : false;   // truetype isFollow = myType extends FollowName ? true : false;   // false// -----------cut---function sellect<T extends NameType>(name: T, data: T extends FixedName ? FixedInstance : FollowInstance) {    return {        name, data    } as const;}const infodata: FixedInstance = { name: "levi", age: 18 };const { name: myName, data: myData } = sellect("follow", infodata);type dataType = typeof myData;type dataisFixed = myType extends FixedName ? true : false;   // falsetype dataisFollow = myType extends FollowName ? true : false;   // false