Bug with union types

[johnfn]

TypeScript Version: 3.1.0-dev.20180714

Search Terms:

union

Code

function g(x: "a" | "b") {
    const f: { type: "a" } | { type: "b" } = {
        type: x
    };
}

Expected behavior:

No error.

Actual behavior:

Type '"a"' is not assignable to type '"b"'.

Playground Link:

http://www.typescriptlang.org/play/index.html#src=function%20g(x%3A%20%22a%22%20%7C%20%22b%22)%20%7B%0A%20%20%20%20const%20f%3A%20%7B%20type%3A%20%22a%22%20%7D%20%7C%20%7B%20type%3A%20%22b%22%20%7D%20%3D%20%7B%0A%20%20%20%20%20%20%20%20type%3A%20x%0A%20%20%20%20%7D%3B%0A%7D%0A

Related Issues:

I'm pretty sure this already exists, but I have no clue what to search for. :(

[AlCalzone]

Does this work for you?

function g<T extends "a" | "b">(x: T) {
    const f: { type: T } = {
        type: x
    };
    return f;
}

const a = g("a"); // { type: "a" }
const b = g("b"); // { type: "b" }
const c = g("c"); // error

[jcalz]

Duplicate of #12052

[typescript-bot]

Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.