Bug with type discrimination and string literal types.

[johnfn]

The following code gives an error on the assignment to x:

type Bar = "A" | "B"
type Foo = { key: "a", name: string } | { key: "b", name: Bar }

const x: Foo = { key: "b", name: "A" }

There are multiple weird things that get rid of the error, including:

1. Changing { key: "a", name: string } to { key: "a" }.
2. Changing name: "A" to name: "A" as Bar

[alitaheri]

This issue have been around for a while, it boils down to this:

const obj = {b: 'A'};
type A = 'A';
const a: A = obj.b; // ---> [ts] Type 'string' is not assignable to type '"A"'.

Not much can be done here. typing b as 'A' has too many consequences and breaks way too many code. This issue can't be solved until Expando Objects come to rescue. With those around b can just be typed 'A' and expand to include more literals or morph into string. no code will break and this issue will be solved for good ❤️

[johnfn]

I understand why your code doesn't work, but I still don't see why mine doesn't, as I have explicitly given the type of x to be Foo.

If I wrote const obj: { b: A } = { b: 'A' } in your code, I would expect it to work as well.

[alitaheri]

Right, sorry, I didn't initially understand the issue you reported. I think what you expect the compiler to do is valid. This is a bug indeed 👍

[mhegazy]

Duplicate of #13580