Missing overload method on union types

[wheeOs]

🔎 Search Terms

overlaod union

🕗 Version & Regression Information

• 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 overload
• I was unable to test this on prior versions because _______

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=5.7.0-dev.20240919#code/FAMwrgdgxgLglgewgAgOYCcFgA4CECeAPACoB8AFAIbrqX4BcyAgjXSaQD7IBKAppQBMkAG3wtaRMgBpkAa14NklCPgCUjALKVshZfhni2ZUsgDewZMnS8YYdCmoSAdNYFgovQlp16DrSaSB5FRQUDK8wrwAtrwQMKrIALwm5haWyFBIAM4wcknIMPjYvAggcgpJiYnIAOTg0PBINcgA-Mjk8vhKWe0R0bEwjMQJyUoqquR9MXEJjFMDANqd3eX4pcjEALoA3GmWkbkYWNj5lKFOqDYdqrvpyHBl5ACERzgJ5neWZ1BOWVeyMleJ2qC02Nz2yAAvhCgU5sGAsgALSaRabxW7pay2exKUK7NKQmQQXgAd2Q3l0Kj8EnYFFU4OhwCAA

💻 Code

function groupBy<T>(array: Array<T>| ReadonlyArray<T>, key: any): Map<any, Array<T>> {
  return array.reduce<Map<any, Array<T>>>((acc, element) => {

    const k = typeof key === 'function' ? (key as (element: T) => any)(element) : element[key as keyof T];
    let group = acc.get(k);
    if (!group) {
      acc.set(k, group = []);
    }
    group.push(element);
    return acc;

  }, new Map<any, Array<T>>());
}

🙁 Actual behavior

getting the errors:

Expected 0 type arguments, but got 1.

Type 'T' is not assignable to type 'Map<any, T[]>'.
Expected 0 type arguments, but got 1.
Property 'get' does not exist on type 'T'.
Property 'set' does not exist on type 'T'.

🙂 Expected behavior

Allowing the use of the 3rd overload method reduce<U> of Array / ReadonlyArray

Additional information about the issue

No response

[jcalz]

There doesn't seem to be a reason to write Array<T> | ReadonlyArray<T> since that's equivalent to ReadonlyArray<T> as shown in this playground link

But I kind of thought #53489 took care of these "union of array" things.

[wheeOs]

@jcalz

There doesn't seem to be a reason to write Array | ReadonlyArray since that's equivalent to ReadonlyArray

true that, thanks for pointing that out. I initially wrote the function for Array<T> , then I had to modify it for ReadonlyArray and it ended up like that, but apparently unnecessarily

[RyanCavanaugh]

You're reporting this issue specifically?

function groupBy<T>(array: Array<T> | ReadonlyArray<T>) {
  array.reduce<any>((acc, element) => {
    throw new Error();
  });
}

[wheeOs]

@RyanCavanaugh yes the code sample can be reduced to that

[Andarist]

But I kind of thought #53489 took care of these "union of array" things.

This was released in TS 5.2 and it's only about fallback signatures. It applies to situations when a signature can be found in a normal way but we can see that TS was able to find some signatures here even before that: TS 5.1 playground.

This case loses the relevant signatures in (somewhat complicated) getUnionSignatures. This function is able to compute some signatures (2 to be precise) in this case though so the referenced improvement from TS 5.2 doesn't even kick-in.