undefined | string property of object incorrectly narrowed to string in union type

[everhardt]

Bug Report

🔎 Search Terms

union undefined incorrectly narrowed

🕗 Version & Regression Information

This is the behavior in every version I tried (back to 3.3.3), and I reviewed the FAQ for entries about bugs, undefined and unions.

⏯ Playground Link

Playground link with relevant code

💻 Code

let a: string | undefined;

type B_1 = { a: string, c: 'test', d: 'yeah' };
type B_2 = { c: 'test'}
type B = B_1 | B_2;

// this should complain that
// - property a should not not exist
// - or property a should be string and property d is missing
const b: B = { a, c: 'test'};

if ('a' in b) {
    // because of this issue, b is narrowed to B_1 here, while a is undefined and d is not present
    console.log(b);
}

🙁 Actual behavior

const b: B = { a, c: 'test'}; does not throw a TS error.

🙂 Expected behavior

const b: B = { a, c: 'test'}; throws a TS error.

[MartinJohns]

The assignment is allowed, because { a, c: 'test' } is a valid B (specifically, it's a valid B_2). Excess property checks is not performed for union types, see #51873.

This unfortunately leads to an intentional unsoundness using the in operator, see #34975.

[everhardt]

Thanks for your quick reply! I think this issue can then be closed as it doesn't add much to the two issues you mentioned. Sorry for wasting your time.