Lose generic when spreading T | undefined where T is a generic object type

[yfzhe]

🔎 Search Terms

spread, union

🕗 Version & Regression Information

There are 2 problems. The first was changed between versions 4.0.5 and 4.1.5, and the second exists in every version of TS I tried in the Playground.

⏯ Playground Link

https://www.typescriptlang.org/play/?#code/GYVwdgxgLglg9mABMOcA8AVRBTAHlbMAEwGdE4AjAK22gD4AKKALkSwB9FwjtgYxsRAJSIA3gChEiAE7YoIaUlGIAdGqgAaZKlYAWAEyIAvgG5xR8eNCRYCRBQCG0zDnyFS5arSiMWbRJzcvPyCIhJSsvKKYqrqxmZGQA

💻 Code

// spread and add a property
function foo<T extends object>(t: T | undefined) {
  return { ...t, foo: 42 };
}

// just spread
function bar<T extends object>(t: T | undefined) {
  return { ...t };
}

🙁 Actual behavior

1. The result type of foo is { foo: number }.
2. The result type of bar is {}.

🙂 Expected behavior

1. The result type of foo is(T & { foo: number }) | { foo: number }.
2. The result type of bar is T.

Additional information about the issue

I'm not sure if this is same with #58617. But as there are some previous versions of TS work as I expected, I tend to treat it as a different issue and a bug.

[ahejlsberg]

I don't think there is a bug here. Spreading an undefined value creates no properties, so { ...t } produces {} when t is undefined. Thus, the result is T | {}, which reduces to just {} by subtype reduction.

[yfzhe]

I'm confused a little on the situation of foo. I think it is also because of subtype reduction which the type of { ...t, foo: number } is { foo: number }. But as we see it was (T & { foo: number }) | { foo: number } in some old versions of TS, did there happen some "breaking changes" on subtype reduction?

[typescript-bot]

This issue has been marked as "Working as Intended" and has seen no recent activity. It has been automatically closed for house-keeping purposes.