Unsafe declaration of resolve function in Promise constructor

[ikokostya]

TypeScript Version: typescript@2.8.0-dev.20180217

Search Terms:

Promise, resolve

Code

function f(): Promise<number> {
    return new Promise((resolve) => {
        resolve();
    });
}

tsconfig.json:

{
    "compilerOptions": {
        "module": "commonjs",
        "moduleResolution": "node",
        "target": "es2017",
        "lib": ["es2017"],
        "alwaysStrict": true,
        "noEmitOnError": true,
        "pretty": true,
        "strict": true
    }
}

Expected behavior:

Compile error: undefined is not assignable to number.

Actual behavior:

No errors.

Playground Link: http://www.typescriptlang.org/play/index.html#src=function%20f()%3A%20Promise%3Cnumber%3E%20%7B%0A%20%20%20%20return%20new%20Promise((resolve)%20%3D%3E%20%7B%0A%20%20%20%20%20%20%20%20resolve()%3B%0A%20%20%20%20%7D)%3B%0A%7D

Looks like this happens, because resolve() function in Promise executor takes optional argument https://github.com/Microsoft/TypeScript/blob/b3edc8f9f4d9cf4203c4c4493e4f0f3dc96c845d/lib/lib.es2015.promise.d.ts#L33

The provided example is correctly checked by Flow: https://flow.org/try/#0PQKgBAAgZgNg9gdzCYAoVUCuA7AxgFwEs5swoAKASgC4wAFAJzgFtCBnAUwB5tNmAjDgwB8YAN6owUsAw75MDUtg5JGLdh3LlZbODABuHSmAC8oidMsyOug5soBuSdIC+j1C9RA

---

I trying to change declaration (i.e. make value parameter of resolve function is required) and the following code stop working:

function bar(): Promise<void> {
    return new Promise((resolve) => {
        resolve(); // Expected 1 arguments, but got 0.
    });
}

In Flow this code also works without error https://flow.org/try/#0PQKgBAAgZgNg9gdzCYAoVUCuA7AxgFwEs5swAjAQwCcAKASgC4wAFKuAW0IGcBTAHgBucQgBMAfGADeqMLLBUe+TFVLYeSVh248aNBVzgwBPOmAC8E6XOvyeBozroBuGXIC+z1G6A

[rbuckton]

We have already tried to address this, but unfortunately this resulted in either Promise no longer being extensible or other breaks.

[ikokostya]

@mhegazy @rbuckton Can you provide more detailed description why this is Design Limitation? From my point of view need ad-hoc solution to allow call resolve() without undefined argument for Promise<void> type.

[RyanCavanaugh]

@ikokostya #22772 (comment) goes into a fair bit of detail on this

[typescript-bot]

Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.