microsoft / microsoft/TypeScript
Array.from(it: Iterable<X> | Iterable<Y>) fails; […it] works but returns Array<X | Y>; both should return Array<X> | Array<Y>
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Description
### 🔎 Search Terms
Array.from is currently declared as
```ts
from(iterable: Iterable | ArrayLike): T[];
```
This means that unions of distinct iterable types cannot be turned into arrays of the same type.
```ts
let foo: Iterable | Iterable = /* ... */;
let q: Array | Array = Array.from(foo); // Fails type checking, cannot find overload (2769)
```
The array splat operator has a different problem: it tries to create an array that is a union of both types.
```ts
let foo: Iterable | Iterable = /* ... */;
let q: Array | Array = [...foo]; // Fails type checking, not assignable to type (2322)
```
My instinct is that there should be some way to use variadic types here, but tbh I don't really know how.
Does this need new syntax? Or does it work within the existing type system?
### 🕗 Version & Regression Information
Since at least 4.0; prior to 4.0 it still fails but Array.from did not support taking an Iterable.
Still around in 6.0.0-dev.20251009
### ⏯ Playground Link
https://www.typescriptlang.org/play/?ts=5.9.3&ssl=19&ssc=1&pln=11&pc=1#code/PTAEHUFMBsGMHsC2lQBd5oBYoCoE8AHSAZVgCcBLA1UABWgEM8BzM+AVwDsATAGiwoBnUENANQAd0gAjQRVSQAUCEmYKsTKGYUAbpGF4OY0BoadYKdJMoL+gzAzIoz3UNEiPOofEVKVqAHSKymAAmkYI7NCuqGqcANag8ABmIjQUXrFOKBJMggBcISGgoAC0oACCbvCwDKgU8JkY7p7ehCTkVDQS2E6gnPCxGcwmZqDSTgzxxWWVoASMFmgYkAAeRJTInN3ymj4d-jSCeNsMq-wuoPaOltigAKoASgAywhK7SbGQZIIz5VWCFzSeCrZagNYbChbHaxUDcCjJZLfSDbExIAgUdxkUBIursJzCFJtXydajBVDtUAAMXgGAAvKAAJIKMgMaTuAA8glQlE4zAAfKAAD5MllszmcdiIaTffkAbmCyS4sHqjTQZDwFTIrLwVLYiAAFMlaflqbSAJSgADeihKJXcNAAjqatTquTzhoKRa6mBzJdLZaAGT68AFkvqjRaFQBfRXK1WZDXEBZ1SPwU00+CWm12tyQJ0u7W+7m8gXCypFvB+qUysiChkAbQCzeN8AAujHgiocM0PGQvIh4H02RwaF83GZmOwGMxIPxYNB1IkR3oRF4AEQAUVWDEQC3066S2PX4AcqAA5MIAHKQCTroIqADyXzI70Ec60+auqEcClc0jwUAnEHHRhiwfQUAQRBoWES5xwkIdohEAwOGxFMWDYLhuCCEogA
### 💻 Code
```ts
type Foo = Iterable | Iterable;
function tryArrayFrom(foo: Foo) {
let q: Array | Array = Array.from(foo);
}
function trySplat(foo: Foo) {
let q: Array | Array = [...foo];
}
```
### 🙁 Actual behavior
_for tryArrayFrom_
No overload matches this call.
Overload 1 of 4, '(iterable: Iterable | ArrayLike): string[]', gave the following error.
Argument of type 'Foo' is not assignable to parameter of type 'Iterable | ArrayLike'.
Type 'Iterable' is not assignable to type 'Iterable | ArrayLike'.
Type 'Iterable' is not assignable to type 'Iterable'.
The types returned by '[Symbol.iterator]().next(...)' are incompatible between these types.
Type 'IteratorResult' is not assignable to type 'IteratorResult'.
Type 'IteratorYieldResult' is not assignable to type 'IteratorResult'.
Type 'IteratorYieldResult' is not assignable to type 'IteratorYieldResult'.
Type 'number' is not assignable to type 'string'.
Overload 2 of 4, '(arrayLike: ArrayLike): (string | number)[]', gave the following error.
Argument of type 'Foo' is not assignable to parameter of type 'ArrayLike'.
Property 'length' is missing in type 'Iterable' but required in type 'ArrayLike'.
_for trySplat_
Type '(string | number)[]' is not assignable to type 'string[] | number[]'.
Type '(string | number)[]' is not assignable to type 'string[]'.
Type 'string | number' is not assignable to type 'string'.
Type 'number' is not assignable to type 'string'.
### 🙂 Expected behavior
Array.from a union of Iterables should return a union of Arrays.
### Additional information about the issue
_No response_
Contributor guide
Research direction
Start by reproducing both examples in the linked TypeScript Playground and compare the declared Array.from overload with spread behavior. Investigate how unions of Iterable and Iterable are inferred, then determine whether existing type-system mechanisms can produce Array | Array without new syntax. Done means both examples type-check with the expected union type and relevant regressions are covered.
Written by the indexing model from the issue text.
Assessment
- Tech stack
- typescript
- Domain
- compilers
- Issue type
- Feature
- Difficulty
- 5/5
- Estimated time
- Over a week
- Activity status
- Stale
- Clarity
- Mostly clear
- Newbie friendliness
- 35/100