[Typescript] Socket from socket.io cannot be unioned with Socket from socket.io-client

[evanpurkhiser]

Describe the bug

In 3.x of socket.io I could do the following:

import {Socket as ServerSocket} from 'socket.io';
import {Socket as ClientSocket} from 'socket.io-client';

function doStuff(ws: ClientSocket | ServerSocket) => {
	ws.on('some-event', () => {
		// ...
	});
}

In 4.x this now produces an error on the ws.on call:

[tsserver 2349] [E] This expression is not callable.
  Each member of the union type '(<Ev extends string>(ev: Ev, listener: ReservedOrUserListener<SocketReservedEvents, DefaultEventsMap, Ev>) => Socket<...>) | (<Ev extends string>(ev: Ev, listener: FallbackToUntypedListener<...>) => Socket<...>)' has signatures, but none of those signatures are compatible with each other.

To Reproduce

Socket.IO server / client version: 4.0.0

See above

Expected behavior
I realize this was a major version bump between 3 and 4, so I can understand if this issue needs to be closed.

This also might be more of a typescript problem than anything else. I know the templated emitter feature was added in 4 (which I am looking forward to using once I get around this!) which I assume is what has made the method signatures incompatible?

I noticed I can "fix" this by doing

const doStuff = (ws: ClientSocket | ServerSocket) => {
  if (ws instanceof ClientSocket) {
    ws.on('some-event', () => {
      // ...
    });
  }

  if (ws instanceof ServerSocket) {
    ws.on('some-event', () => {
      // ...
    });
  }
};

which is pretty goofy.

Additional context
I am on Typescript 4.0.2

[darrachequesne]

I could indeed reproduce the issue. Looks like it's related to 0107510, but I'm not sure how we can fix this.

[darrachequesne]

For future readers:

It seems this was fixed in TypeScript between 4.1 and 4.2:

$ npm i [email protected]

changed 1 package, and audited 86 packages in 904ms

2 packages are looking for funding
  run `npm fund` for details

found 0 vulnerabilities

$ npx tsc
test.ts:5:8 - error TS2349: This expression is not callable.
  Each member of the union type '(<Ev extends string>(ev: Ev, listener: FallbackToUntypedListener<Ev extends "connect" | "connect_error" | "disconnect" ? SocketReservedEvents[Ev] : Ev extends string ? (...args: any[]) => void : never>) => Socket<...>) | (<Ev extends string>(ev: Ev, listener: FallbackToUntypedListener<...>) => Socket<...>)' has signatures, but none of those signatures are compatible with each other.

5     ws.on('some-event', () => {
         ~~


Found 1 error.

$ npm i [email protected]

changed 1 package, and audited 86 packages in 924ms

2 packages are looking for funding
  run `npm fund` for details

found 0 vulnerabilities

$ npx tsc
$

Please reopen if needed!