unions of objects are annoying

[DetachHead]

Bug Report

🔎 Search Terms

Object union

🕗 Version & Regression Information

4.4.0-dev.20210526

⏯ Playground Link

Will add one later, writing this on my phone

💻 Code

interface Foo {
    foo: number
}
interface Bar {
    bar: string
}
declare const baz: Foo | Bar

const qux = baz.bar //error: bar doesn't exist on Foo|Bar

🙁 Actual behavior

The type is narrowed to the intersection of the two types, making it impossible to narrow it without casting it

🙂 Expected behavior

It should narrow to something like

{
    foo?: number
    bar?: string
}

[MartinJohns]

That would easily lead to unsoundness:

const bar = { bar: "", foo: "" };
const baz: Foo | Bar = bar

---

This also looks like a duplicate of #36194.

[DetachHead]

In that example it would correctly fail because foo can only be number | undefined, not string. Is that the unsoundness you're referring to?

[canonic-epicure]

No, current behavior is correct. Unions are sum types. You don't know which variant of the union this value represents. Need to perform "case" matching on it (so called "pattern matching") to know that. You are asking to reduce the type-safety.

[DetachHead]

ah i see now. my suggestion would mean {} is a valid value, which is wrong