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Why are generics not deducing nested objects correctly? #57658
Description
🔎 Search Terms
Generic derivation
🕗 Version & Regression Information
- This is a crash
- This changed between versions ______ and _______
- This changed in commit or PR _______
- This is the behavior in every version I tried, and I reviewed the FAQ for entries about _________
- I was unable to test this on prior versions because _______
⏯ Playground Link
💻 Code
// Your code here
type AnyRecord = Record<string, any>;
type fn = <T extends { a: AnyRecord }>(params: { a: T["a"] }) => T["a"];
const b: fn = (params) => {
return params.a
};
const c = b({
a: {
a1: 123,
},
});🙁 Actual behavior
Why isn't c deduced to the type I actually passed in?
🙂 Expected behavior
c is deduced to the type I actually passed in
Additional information about the issue
No response
Activity
Andarist commentedon Mar 6, 2024
Indexed accesses like this are never used as inference sources - the information at this location is at best partial information for what
Tcan be. That said, there is an open PR that would make this inference work: #53017You can just infer into the whole
Tlike this: TS playgroundOr you can rewrite this signature using reverse-mapped types: TS playground
Using the technique above you can even benefit from excess property check in the upcoming TS 5.4: TS playground
masterYi2333 commentedon Mar 8, 2024
thanks