Partial generic types get resolved completely incorrectly

[Mudloop]

🔎 Search Terms

incorrect generic resolution

🕗 Version & Regression Information

I don't think this is caused by a new update.

⏯ Playground Link

https://tsplay.dev/weG9Xm

💻 Code

export interface Item1 { kind: "item1" }
export interface Item2 { kind: "item2" }
export interface Item3 { kind: 'item3' }
export type Item = Item1 | Item2

const Guard1 = (o: unknown): o is Item => true
const Guard2 = <M extends Item>(o: M | Item3): o is M => true
export function Test1<M extends Item>(src: M | Item3) {
    if (!Guard1(src)) {
        // these are flipped :
        src					// Item3 | M
        src.kind            // item3
    }

    if (!Guard2(src)) {
        // and so are these :
        src					// Item3
        src.kind            // "item1" | "item2" | "item3"

        // And src isn't even typeof src :
        const test: typeof src = src;
        // Type 'Item1 | Item2 | Item3' is not assignable to type 'Item3'.
    }
}

🙁 Actual behavior

Looks like there's two resolvers that come up with opposing results, and both are used.

🙂 Expected behavior

Consistent type resolution.
Edit: or potentially an error message to indicate it's not resolvable.

Additional information about the issue

No response

[Mudloop]

If partially generic types are too complex to resolve, I think an error would be preferable over a state where the value and its type no longer line up.