Type inference fails with nested arrow function callback with block

[Rugvip]

Bug Report

🔎 Search Terms

callback, block, inference, double

🕗 Version & Regression Information

This is the behavior in every version I tried, and I reviewed the FAQ for entries about arrow functions

⏯ Playground Link

https://www.typescriptlang.org/play?#code/GYVwdgxgLglg9mABAJwKYAdlwDwBUB8AFAPoQLAwDmAXIgN4BQii6AhsqwLYDOtuA3E0QRWAG1EAjVhADWtQmw48+ASkQBefIgXsuvRLjWbEYEJwmpkggL4raANzgwAJvRSooIZEmsMGaTDhCRmZFPVoARgAaIRFxKVl5Iy06d09vRFYNLSyAakQIxGsoxAB6UsRwGTA4AHcwBltBAKxgoTDlApjmOMlpOURiCWy3NHSkLOM8gqKS8sQEUQBPRGcYYGBLVEhURBhuRCgAC12qGrRXDsaVZoxWkJZdTujYsT7E7WTMkenosorFis1hstjs9gdjrtRP0FsBEAADVJjLxIAB06KK8Ou-CAA

💻 Code

function repro<T>(_config: {
  params: T;
  callback: (params: T) => (params: T) => number;
}): void { return }

repro({
  params: 1,
  callback: () => { return a => a + 1 }, // a is unknown
});
repro({
  params: 1,
  callback: _b => { return a => a + 1 }, // a is a number, only difference is the ignored outer param
});
repro({
  params: 1,
  callback: () => a => a + 1, // a is a number, only difference is the lack of `{ return ... }`
});

🙁 Actual behavior

The type of a ends up being unknown, but only if no parameter is declared and a block is used in the outer callback.

🙂 Expected behavior

The type of the inner callback parameter a should be known in all 3 cases.

[Urtgard]

In my project I stumbled over a similiar case:

class Test {
  array = [1, 2, 3]
  optional?: number

  public methodError(): void {
    if (!this.optional) return;

    this.array.forEach((x) => {
      x = this.optional; // Type 'number | undefined' is not assignable to type 'number'. Type 'undefined' is not assignable to type 'number'.(2322)
    });
  }

  public methodOk(): void {
    this.array.forEach((x) => {
      if (!this.optional) return;
      x = this.optional;
    });
  }
}

Let me know if this is different problem, then I'll open a new issue.

[Andarist]

@Urtgard your problem is different and it's a not a bug, take a look at the inline explanation that I have added in this TS playground

[nix6839]

function fn<ToInferred>(
  callback: (unused: string) => {
    key: ToInferred;
    keyCallback: (key: ToInferred) => number;
  },
) {}

fn(() => ({
  key: 5,
  keyCallback: (key) => {
    return key;
  },
}));

fn((unused) => ({
  key: 5,
  // @ts-expect-error key is unknown
  keyCallback: (key) => {
    return key;
  },
}));

Same issue here... I hope it will be fixed at v4.9.2