Typescript allows assignment of wrong type to property on union type about typescript HOT 10 CLOSED

type NumericValue = { kind: 'number'; value: number; };
type StringValue = { kind: 'string'; value: string; };
type Val = NumericValue | StringValue;

type JsVal<T extends Val> = T["value"];

function f1<T extends Val>(): JsVal<T> {
    return 123;
}

function f2<T extends Val>(obj: T): JsVal<T> {
    return 123;
}

const o: Val = { kind: "string", value: "hello" };
const s1: string = f1();  // Error!
const s2: string = f2(o); // No error!

type JsVal<T extends Val> = T extends { value: infer U } ? U :never;
function myFunction<T extends Val>(obj: T): JsVal<T> {
    const v: JsVal<T> = 123; // is error
    return "ok"; // is error
}

type NumericValue = { kind: 'number'; value: number; };
type StringValue = { kind: 'string'; value: string; };
type Val = NumericValue | StringValue;

function myFunction<T extends Val>(obj: T) {
    obj.value = 123;         // should error (?)
}

const o: Val = { kind: "string", value: "hello" };
myFunction(o);                // OR this assignment should error
type T = (typeof o)["value"]; // string
console.log(o.value);         // [LOG]: 123

type NumericValue = { kind: 'number'; value: number; };
type StringValue = { kind: 'string'; value: string; };
type Val = NumericValue | StringValue;
function myFunction(obj: Val) {
    obj.value = 123;
}

type NumericValue = { kind: 'number'; value: number; };
type StringValue = { kind: 'string'; value: string; };
type Val = NumericValue | StringValue;

type R1 = number | string;
type R2<T extends Val> = T["value"];

function f1(): R1 {
    return 123;
}

function f2(obj: Val): R1 {
    return 123;
}

function f3<T extends Val>(): R2<T> {
    return 123;
}

function f4<T extends Val>(obj: T): R2<T> {
    return 123;
}

const o: Val = { kind: "string", value: "hello" };

const s1: string = f1();  // Error!
const s2: string = f2(o); // Error!
const s3: string = f3();  // Error!
const s4: string = f4(o); // No error!