Unable to return Promise<void>

[RomainMuller]

While methods are fine to return void, they cannot return a Promise<void>, as JSII interprets this as Promise<unknown type>.

Reproduction:

export class PromiseOfVoid {
  async function returnsVoidPromise(): Promise<void> {
    return;
  }
}

[drandell]

Is there a workaround for this in place?

[Chriscbr]

I also run into this error with Promises that return undefined or null:

export class PromiseOfUndefined {
  async function returnsUndefinedPromise(): Promise<undefined> {
    return;
  }
}

export class PromiseOfNull {
  async function returnsNullPromise(): Promise<null> {
    return;
  }
}

Using jsii 1.67.0.

A possible workaround is to design around the API like so, though it might look unidiomatic for TypeScript/JavaScript users:

export interface Nothing {}

export class PromiseOfNothing {
  async function returnsNothingPromise(): Promise<Nothing> {
    return;
  }
}

[RomainMuller]

The undefined and null types are not legal at this stage... which is why these don't work. If they were accepted, they'd likely be treated as synonyms with unknown as these don't otherwise exist in most of our target languages.

Promise<void> should however work, and is currently broken due to a missed edge-case in the compiler I guess. You can use Promise<unknown> in the meantime, as this works.