Unsoundness in union of object types

[hanayashiki]

Bug Report

🔎 Search Terms

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about union types

⏯ Playground Link

Playground link with relevant code

💻 Code

type V = {
    a: {
        c: string,
    },
    b: number,
} | {
    b: number,
}

const v: V = {
    a: undefined,
    b: 1
}

if ('a' in v) {
    console.log(v.a.c);  // In runtime there would be an error of accessing undefined
}

🙁 Actual behavior

No error is found

🙂 Expected behavior

I know TS is not built to be sound, and it is more a static analytic tool for JavaScript. But is this behavior expected?

In this case, 'a' in v does not indicate v.a is { a: {.c: string } }, because v could be anything containing b: number, but we don't make any assumptions about a, since as long as b: number exists then v is V.

I also find if we initialize v as

const v: V = {
    a: 0,
    b: 1
}

It returns with an error Type 'number' is not assignable to type '{ c: string; }'., even though, it is a subtype of { b: number }.

So, it means "only undefined can be assigned to a"?

It seems a: undefined is a special case for object literals and union types, but it this documented anywhere?

Thanks in advance.

[MartinJohns]

Duplicate of #34975.

[typescript-bot]

This issue has been marked as a 'Duplicate' and has seen no recent activity. It has been automatically closed for house-keeping purposes.