Fix #3833: Add fallback to untyped event listener

[MaximeKjaer]

The kind of change this PR does introduce

• a bug fix
• a new feature
• an update to the documentation
• a code change that improves performance
• other

Current behavior

Due to a compiler bug, we had the following issue (see #3833):

import { Server } from "socket.io";

const io = new Server(3000, {});

// this works fine
io.on("connection", (socket) => {});

enum Event {
    CONNECTION = "connection",
}
// Fails to compile!
// Argument of type '(socket:Socket) => void' is not assignable to parameter of type 'never'.
// ts(2345)
io.on(Event.CONNECTION, (socket) => {});

New behavior

The above code now compiles. However, in the line io.on(Event.CONNECTION, (socket) => {});, the lambda parameter socket is inferred to be of type any instead of Socket if an enum member is used.

Note that this only affects the situation where an enum member is used: If a string literal is used, the correct type (Socket) is still inferred.

Also note that inferring any in this situation is what was already happening in 3.x. So we are essentially reverting to that, at least until the compiler bug is fixed.

Other information (e.g. related issues)

Fixes #3833

[westy92]

@darrachequesne can you please take a look at this? This fixes a regression in 4.0.0 that is preventing me from updating.

[westy92]

Thank you! 🙏