Check for missing property on union type causes failure in subsequent property checks

🔎 Search Terms

"missing property", "property check", "type union", "control flow analysis"

🕗 Version & Regression Information

• This is the behavior in every version I tried*, and I reviewed the FAQ for entries about property checks

*There was a minor change between 4.8 and 4.9 that changed the types but did not change the behavior: inference improved from never to incorrectUnionElement & Record<"key", unknown>.

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=5.4.5#code/FASwdgLgpgTgZgQwMZQARIPYFsAOBGVAb2FVNRigQBMMwAbAT1QQC5UwBXLAI1gG4SZCtVqNU3Npx79gAX2ChIsRCnTYcAJiKDSwmvSat2XXjAHzgEBjjSZcqALw7UAHzW48zt3c0K4HMCQIEFpUGgB9AGdsKAgAC3AAcwAKAEptMjUwSIh3HEciWWZIvIFnEDhUZIAiBGrUcDz04kzMnwA6BHaIDABlCBgktIFWsgB6MdQAPQB+ZwtMiqrq7nrGn2bnNvV27m6+gaHUkdHUCem5zIWyJZqkNbAmjNOOpH3+wbAU463xydn5gpFpUanUGo8Ns9Rh0uj0PkcTqNzgCrkCbiCVg8ni0Xjs9nDDl9hr9SMjLmR5LIgA

💻 Code

interface comp1 {
    readonly a: number;
    readonly b: number;
}

interface comp2 {
    readonly a: number;
}

type comp =
    | comp1
    | comp2

function do_something() {
    const comp = {} as comp;

    if ("a" in comp) {
        comp.a.toString();
        // ^? comp
    }

    if ("b" in comp) {
        comp.b.toString();
        // ^? comp1
    }

    if ("c" in comp) {
        comp.c.toString();
        // ^? comp & Record<"c", unknown>
    }

    if ("a" in comp) {
        comp.a.toString();
        // ^? comp2
    }

    if ("b" in comp) {
        comp.b.toString();
        // ^? comp2 & Record<"b", unknown>
    }
}

🙁 Actual behavior

Every property check after the ("c" in comp) one fails to narrow correctly if the key is one that is only defined for some of the element types of the type union. Attempting to access the property after one of these checks results in unknown. As well, the narrowed variable in subsequent conditionals is an arbitrary(?) union element.

🙂 Expected behavior

The conditionals that come after the ("c" in comp) conditional should have the same behavior as the ones before it, as they are the exact same code.

Additional information about the issue

This was initially noticed in a more complicated context with assertions, that I can include here as a secondary example. This one is even more strange, as it is a check for a property that is only defined on some of the elements of the union and it blocks its own duplicate check later.

Start with the provided TypeScript Playground repro and compare the union narrowing before and after the "c" in comp check. Trace the control-flow analysis and property-check handling involved in narrowing union types. Done means later checks narrow the same way as identical checks before the missing-property test, including the secondary example.