Wrong type inferred with the new genric typed arrays

[pfumagalli]

🔎 Search Terms

typed array
Uint8Array
ArrayBufferLike

🕗 Version & Regression Information

• This changed between versions 5.6 and 5.7

⏯ Playground Link

No response

💻 Code

This is a very simple file that exports a Uint8Array parser created by a factory method:

export type Parser<T> = (input: string) => T

function createUint8ArrayParser(): Parser<Uint8Array> {
  return (_input: unknown): Uint8Array => {
    throw new Error('Not implemented')
  }
}

export const parser = createUint8ArrayParser()

🙁 Actual behavior

The resulting .d.ts file looks as follows:

export type Parser<T> = (input: string) => T;
export declare const parser: Parser<Uint8Array<ArrayBufferLike>>;

The type returned for parser includes the generic version of Uint8Array which basically mean that anything depending on this export must also be at least on version 5.7.

When importing this in a project using TypeScript 5.6 we get an error:

TS(2315): Type 'Uint8Array' is not generic.

🙂 Expected behavior

The type returned for parser should be Parser<Uint8Array> and not Parser<Uint8Array<ArrayBufferLink>>

export type Parser<T> = (input: string) => T;
export declare const parser: Parser<Uint8Array>;

Additional information about the issue

No response

[jcalz]

Doesn't seem like a bug, per se, since I assume whenever TS adds or modifies types in its libraries, the emitted code might be incompatible with older versions of TS (like, I'm guessing when TS added Awaited, things that produced that stopped being compatible with prev versions, etc). Maybe you could work around it like

export type MyUint8Array = Uint8Array;
function createUint8ArrayParser(): Parser<MyUint8Array> {
    return (_input: unknown): Uint8Array => {
        throw new Error('Not implemented')
    }
}

Playground link

[RyanCavanaugh]

Backcompat like this is not an invariant, but we should add a rule to downlevel-dts for this case

[typescript-bot]

This issue has been marked as "Not a Defect" and has seen no recent activity. It has been automatically closed for house-keeping purposes.