anything is assignable to T[P] where T extends null/undefined/void/object/never

[zpdDG4gta8XKpMCd]

case 1

export function test<T extends object, P extends keyof T>(
 whatever: T, name: P
): T[P] {
 const result = Math.random() > 0.5 ? whatever[name] : 'Hello!'; // <-- expected T[P] | string, actual T[P]
 return result; // <-- returning unsound result
}

case 2

export function oneOrAnother<A, B>(one: A, another: B): A | B {
 return Math.random() > 0.5 ? one : another;
}

export function test<T extends object, P extends keyof T>(
 whatever: T, name: P
): T[P] {
 const result = oneOrAnother(whatever[name], 'Hello!'); // <-- result is clearly string | T[P] as expected
 return result; // <-- expected a type error, actual returning string | T[P] instead of T[P] no problem
}

[MrAndersen1]

You probably already found this out but these issues arise from extending object.
What one gains from extending object and why these issues arise when you do it is beyond me however.

[sandersn]

Here's a smaller repro:

function test<T extends object, P extends keyof T>(s: string, tp: T[P]): void {
 tp = s; // should error
}