Any reason why DANGEROUS operations like this work and surprisingly does not work in ONLY one edge case? #55763

// 1. Scenario - A Dangerous operation which TS allows
function mutate1(obj: Record<string, unknown>) {
    obj.key = 1; // this is dangerous. No error
}

const obj1 = {key: 'some key'};

mutate1(obj1); // Should TS error out here as it errors out when passing a readonly to non readonly

console.log(obj1.key.toLocaleLowerCase()); // EXCEPTION: key is now number NOT string. 
// -------------------------- END ---------


// 2. Scenario - Here TS prevents error from happening but don't know why
function mutate2<Args extends Record<string, unknown>>(obj: Args) {
    // Why does this error out?
    obj.key = 1; // This errors out. So NOTHING can be mutated here

    if('key' in obj && typeof obj.key === 'number') {
        // NO obj keys can EVER be changed not matter the typeguard. Why????
        obj.key = 2; // It still error's out inspite of all the typeguard
    }
}

const obj2 = {key: 'some key'};

mutate2(obj2);

console.log(obj2.key.toLocaleLowerCase()); // key WILL be string here. PERFECT
// -------------------------- END ---------



/**
 * 3. 
 * 
 * Following Scenario 2. This surprisingly DOES NOT prevent error from happening
 * don't know why
 */
function mutate3<Args extends Record<string, unknown>[]>(obj: Args) {
    if(obj[0]) {
        obj[0].key = 1; // This DOES NOT errors out. Whyyy?
    }
}

const obj3 = [{key: 'some key'}];

mutate3(obj3);

console.log(obj3[0]?.key.toLocaleLowerCase()); // EXCEPTION: key is now number NOT string. 
// -------------------------- END ---------

// 5. Scenario - this works as expected
const readonlyArr: readonly string[] = ['first'];

const mutatingArr: unknown[] = readonlyArr; // This errors out correctly
// --------- END -----

// 4. Scenario - can easily change readonly object keys
const readonlyObj = {k: 2} as const;

const mutatingObj: Record<string, unknown> = readonlyObj;
mutatingObj.k = 'string'; // No error here as well

readonlyObj.k.toFixed(2); // This WILL THROW as 'k' is no longer a number now
// ------------ END ------------