Regression in 4.8 where string union type widens to string

[me4502]

Bug Report

🔎 Search Terms

string union widening, string union regression, union widening

🕗 Version & Regression Information

This regression occurred in 4.8.0, and can be reproduced with the 4.8.0-beta, and nightly (4.9.0-dev.20220904) tag in TS Playground. This works with 4.7.4.

• This changed between versions 4.7.4 and 4.8.0-beta

I feel like this might be related to the Improved inference from binding patterns change

⏯ Playground Link

Playground link with relevant code

To reproduce, hover over the "isWorking" variable to see that the type definition includes a union of the strings. Switch to 4.8.0-beta, and note that the type definition is now just string.

💻 Code

interface Guard<T> {
    (val: unknown): val is T;
}

type ObjectGuard<T> = {
    [key in keyof T]: Guard<T[key]>;
};

function isObject(val: unknown): val is Record<string, unknown> {
    return val !== undefined && val !== null && typeof val === 'object' && !Array.isArray(val);
}

function isObjectOfShape<T>(
    value: unknown,
    shape: ObjectGuard<T>,
): value is T {
    if (!isObject(value)) {
        return false;
    }
    let validShape = true;

    for (const key in shape) {
        const guard = shape[key];
        if (guard && !guard(value[key])) {
            return false;
        }
    }

    return validShape;
}

function createObjectGuard<T>(guard: ObjectGuard<T>) {
    return (val: unknown): val is T => isObjectOfShape(val, guard);
}

function asLiteral<T extends (string | boolean | number)[]>(...literals: T) {
    return (val: unknown): val is T[number] => {
        return literals.includes(val as T[number]);
    };
}

// See type of `isWorking` - should include the type key as a union of strings
const isWorking = createObjectGuard({
//    ^?
    type: asLiteral('these', 'should', 'be', 'a', 'union'),
});

🙁 Actual behavior

String union type is widened to string.

🙂 Expected behavior

String union type remains a union, rather than widening

[typescript-bot]

👋 Hi, I'm the Repro bot. I can help narrow down and track compiler bugs across releases! This comment reflects the current state of the repro in the issue body running against the nightly TypeScript.

---

Issue body code block by @me4502

⚠️ Assertions:

• const isWorking: (val: unknown) => val is { type: string; }

Historical Information

Version	Reproduction Outputs
4.8.2	⚠️ Assertions: const isWorking: (val: unknown) => val is { type: string; }
4.4.2, 4.5.2, 4.6.2, 4.7.2	⚠️ Assertions: const isWorking: (val: unknown) => val is { type: "these" | "should" | "be" | "a" | "union"; }

[typescript-bot]

The change between 4.7.2 and 4.8.2 occurred at a2b785b.

[andrewbranch]

@typescript-bot agrees with your guess about the cause, which is strange and definitely unintentional since there is no binding pattern in the repro 🤔

[HolgerJeromin]

I think the milestone should be adjusted because that ship has sailed... :)

[andrewbranch]

Thanks. I forgot to update this issue, but we decided that the fix looked a little risky to ship in a patch when weighed against the small number of reports of this issue we’ve gotten.