A stricter variation of https://gist.github.com/JasonKleban/50cee44960c225ac1993c922563aa540 . This version changes function signatures in the case of `T extends void` to take no data parameter, freeing the non-void case to require its data parameter.

LiteEventStricter.ts

interface ILiteEvent<T> {
    on(handler: T extends void
        ? { (): void }
        : { (data: T): void }): void;
    off(handler: T extends void
        ? { (): void }
        : { (data: T): void }): void;
}

class LiteEvent<T> implements ILiteEvent<T> {
    private handlers: (T extends void
        ? { (): void }
        : { (data: T): void })[] = [];

    public on(handler: T extends void
        ? { (): void }
        : { (data: T): void }): void {
        this.handlers.push(handler);
    }

    public off(handler: T extends void
        ? { (): void }
        : { (data: T): void }): void {
        this.handlers = this.handlers.filter(h => h !== handler);
    }

    public trigger: T extends void
        ? { (): void }
        : { (data: T): void } =
        ((data?: T) => {
            this.handlers.slice(0).forEach(h => h(data));
        }) as any;

    public expose() : ILiteEvent<T> {
        return this;
    }
}

LiteEventStricterExample.ts

class Security{
    private readonly onLogin = new LiteEvent<string>();
    private readonly onLogout = new LiteEvent<void>();

    public get LoggedIn() { return this.onLogin.expose(); } 
    public get LoggedOut() { return this.onLogout.expose(); }

    // ... onLogin.trigger('bob');
}

function Init() {
    var security = new Security();

    var loggedOut = () => { /* ... */ }

    security.LoggedIn.on((username) => { /* ... */ });
    security.LoggedOut.on(loggedOut);

    // ...

    security.LoggedOut.off(loggedOut);
}

Might not be necessary - the simpler version without the optionality on the data parameters seems to work ok with void but microsoft/TypeScript#19259 ?