union type {x, y} | {x, z} sometimes incorrectly allows {x, y, z}

[felix9]

Bug Report

🔎 Search Terms

union type

🕗 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 XY<T> = {
    x: T;
    y: T;
};
type XZ<U> = {
    x: U;
    z: U;
};

// This is correctly an error
const a: XY<boolean> | XZ<string> = { x: true, y: true, z: 'z' };

// This is incorrectly allowed
const b: XY<number> | XZ<string> = { x: 1, y: 1, z: 'z' };

[DanielRosenwasser]

I think this is a duplicate of #20863.

[felix9]

It's similar? but I'm not sure I understand why it behaves differently when I change the type from number to boolean.

this irregularity makes me completely confused about union types, I don't feel like I can predict at all what will and won't work.

[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.