Why are generics not deducing nested objects correctly?

[masterYi2333]

🔎 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

https://www.typescriptlang.org/play?ts=5.3.3#code/C4TwDgpgBAggdiAShAxgewE4BMoF4rLrYA8AzsBgJZwDmANFAIYIB8A3AFCiRQBmceKMQAqUCAA9gEOFlJQA3kwBcsBIUw4AviwAUYRhkYBbUisWMVwgNoAiRjYC6UTQEo8LKNbuPOHdHHIoACMVfkE9A2NSN1wPeQ4oRKgMCGAAVwwBfUMTADpGDk1ff0CUQSCdeMSLBQSkxgBGFQaAJgBmOjrNTtdOIA

💻 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

[Andarist]

Indexed accesses like this are never used as inference sources - the information at this location is at best partial information for what T can be. That said, there is an open PR that would make this inference work: #53017

You can just infer into the whole T like this: TS playground

Or 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]

thanks