keyof NonNullable<T> cannot be used to index type T

TypeScript Version: 2.9.0-dev.20180412

Code

function assign<T extends object>(to: T, from: T | undefined) {
	for (const p in from!)
		to[p] = from![p];
	return to;
}

Expected behavior:

No error.

Actual behavior:

src/a.ts(3,9): error TS2536: Type 'keyof NonNullable<T>' cannot be used to index type 'T'.

Related issues: #19461

[weswigham]

Partial dupe of #22934 ? In any case, also should also be "fixed" by #22971

The root cause is still that NonNullable<T> and T are (rightly) unrelated in this direction, but when T extends object, one expects NonNullable<T> to just be T.

Possibly related:

function f<T extends string | undefined>(t: T): NonNullable<T> {
 if (t === undefined) throw "bad";
 return t;
}

[weswigham]

@Andy-MS I think that's just because narrowing doesn't apply higher order types right now.

[dhruvrajvanshi]

Hey everyone. I was trying to figure this issue out and I realized that keyof <Higher order type> reduces to string | number | symbol which is causing this problem.

If this issue is worth fixing, I suggest adding the following rule to the isTypeRelated relation.

keyof X extends keyof Y
when
 Y extends X

This rule should be applied before the rule that reduces keyof Y to string | number | symbol because that loses the type relationship between X and Y

Example of the first rule being applied:

function f<X, Y extends X>(keyofX: keyof X, keyofY: keyof Y){
 keyofY = keyofX; // fine because keyof X is a subset of keyof Y
 keyofX = keyofY; // error because Y may have more keys than X
}

Right now, an error is raised, but for the wrong reasons.

It tries to assign string | number | symbol to keyofX after reducing keyof Y which is fine, but would miss out on edge cases like this.

The original issue doesn't occur anymore but this does

function f1<T>(keyofT: keyof T, keyofNonNullT: keyof NonNullable<T>) {
	keyofT = keyofNonNullT // should not raise error but does on the latest master
}

EDIT:

Okay, I implemented the new rule but turns out this isn't quite enough to fix the second example because
NonNullable<T> extends T is false. I think this would require another rule for conditional types.

[dhruvrajvanshi]

Update:

I implemented a special case for checks like
(keyof isRelatedTo keyof ( extends ? ... : ...))
right before the checker gives up and reduces the source type to string | number | symbol which works as expected.

Is that kind of logic too much of a special case for the type checker? NonNullable has become kinda pervasive so I don't know.