An index signature parameter type with types extends string or number.

An_index_signature_parameter_type_must_be_string_or_number_1023:
with this Feature, I can't make this code work:

export class Events<ED extends EventsDef, EventName extends keyof ED = keyof ED> {
    private eventMap = new Map<EventName, Event<ED[EventName]>>();

    constructor() {
        new Proxy(this, {
            get: (target, p) => {
                if (p in target) {
                    return target[p];
                } else {
                    this.getEvent(p as EventName);
                }
            }
        })
    }

    private getEvent(eventName: EventName) {
        return this.eventMap.get(eventName) || this.generateEvent(eventName);
    }

    private generateEvent(eventName: EventName) {
        let event = new Event<ED[EventName]>();
        this.eventMap.set(eventName, event);
        return event;
    }

                     // error here
    readonly [eventName: EventName]: Event<ED[EventName]>;
}

Start with the reproducing TypeScript snippet and the diagnostic “An index signature parameter type must be string or number” in this issue. Trace how the typechecker handles the generic EventName index signature, then establish the intended behavior and add coverage showing when this form should be accepted or rejected.