More flexible handling of circular types with "typeof function" inference about typescript

[ahejlsberg]

This is sort of a bug, but really more of a possible improvement. At any rate, during type inference in the instantiation of the Parameters<T> conditional type, we attempt to resolve the return type of bar without really needing to, which leads to a circularity. It's a pretty easy fix, I'll put up a PR.

[fatcerberus]

I would like to be able to infer type of a function A that calls another function B, returning its value, relying on parameters of function A (not sure if the description is good, see the code snippet).

I'm not 100% sure, but based on your description I think you're looking for #40179?

[finom]

@fatcerberus I think I'd still need to refer to the function type: paramsof typeof bar what would cause the same problem.

[finom]

@ahejlsberg that's amazing! Honestly, I didn't expect to get this reaction seeing so many issues at this repo ❤️

[finom]

@ahejlsberg that's perfect! Thank you! I think many people would be happy to show their appreciation to you by sponsoring but unfortunately your sponsoring page is not set up. This update changes a lot in my code and solves a problem that I was having for very long time. Sorry for delayed reply, I didn't know how TS versioning works and just installed the PR version ❤️

[finom]

@ahejlsberg I'm doing some tests but still getting similar problems. Could you also check the following and more complex example:

function withSchmuck<T, R>(arg: T, handler: (arg2: T) => R) {
  console.log(arg);
  return handler;
}

const foo = withSchmuck('hello', (arg) => {
  return bar(arg);
});

function bar(arg: Parameters<typeof foo>[0]) {
   console.log(arg);
}

The arg of bar should have type of string but TS says this is the same error as I got before. As in the previous case, nothing is trying to infer the return type, only arguments.