Elementwise elaboration fails to be found for properties mixed with incompatible index signatures using intersections

🔎 Search Terms

elementwise elaboration errors write type index signature intersection

🕗 Version & Regression Information

• This is the behavior in every version I tried

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=5.5.0-dev.20240428#code/MYewdgzgLgBFCm0BcMDeMDaBrF0BOAlmAOYC6KYArgLYBG8eMAvjAGRowBmIIuUhJZjAC8HbrxgAiWgEM8k5gG4AUEA

💻 Code

const test: { [k: string]: number } & { foo: string } = { foo: "bar" };

🙁 Actual behavior

test variable gets highlighted

🙂 Expected behavior

I'd expect foo property in the source to get highlighted

Additional information about the issue

I don't really expect this to get any meaningful priority ;p it's just something interesting that I noticed while investigating some legitimate question as to why an error is not on the property itself in a more complex situation. Such intersections are flawed on principle so 🤷‍♂️

This happens because elaborateElementwise uses getBestMatchIndexedAccessTypeOrUndefined and that prefers concrete properties on intersections over index signatures. The problem is that you can read such dubious properties from the type (and it has a matching type in the example above so it doesn't become an elementwise candidate) but you can't quite write to them.

Reproduce the example in the linked TypeScript Playground, then inspect elaborateElementwise and getBestMatchIndexedAccessTypeOrUndefined, which the issue identifies as the relevant path. Confirm how the intersection's concrete property is preferred over its index signature, and consider the issue done when the source foo property is highlighted rather than only the test variable.