Wrong type for Promise.finally in lib.es2018.promise.d.ts

[Elrendio]

TypeScript Version: 3.4.0-dev.201xxxxx

Search Terms:

• "promise"
• "finally"
• "promise finally"

Code

// A *self-contained* demonstration of the problem follows...
// Test this by running `tsc` on the command-line, rather than through another build tool such as Gulp, Webpack, etc.
let myCmpt = 1
const myPromise = new Promise((fulfill, _reject) => {
	setTimeout(() => fulfill(true), 1000)
}).finally(() => myCmpt--)

Expected behavior:
Code should compile.
Actual behavior:
I get error:

tsBug.ts:4:17 - error TS2322: Type 'number' is not assignable to type 'void'.

4 }).finally(() => myCmpt--)
                  ~~~~~~~~

  ../../../../../usr/local/lib/node_modules/typescript/lib/lib.es2018.promise.d.ts:31:26
    31     finally(onfinally?: (() => void) | undefined | null): Promise<T>
                                ~~~~~~~~~~
    The expected type comes from the return type of this signature.


Found 1 error.

Playground Link:
Couldn't manage to use es2018 with TypeScript Playground.
Related Issues:

[Elrendio]

Patch is actually rather easy, this line /lib/lib.es2018.promise.d.ts#31 should be changed from:

    finally(onfinally?: (() => void) | undefined | null): Promise<T>

to

    finally(onfinally?: (() => any) | undefined | null): Promise<T>

[Elrendio]

I can submit a PR once this issue has been accepted as a bug:
A bug must have an issue tracking it in the issue tracker that has been approved ("Milestone == Community") by the TypeScript team.

[Elrendio]

There's a turnaround using:

let myCmpt = 1
const myPromise = new Promise((fulfill, _reject) => {
	setTimeout(() => fulfill(true), 1000)
}).finally(() => myCmpt--)

I don't understand why it works since number should still not be assignable to void but it does so I'm closing the PR.