fix(types): improved void handling for emit()

These type-level changes should improve the emit signature when `void` is used and when union types are used.

Author: aleclarson

src/internal/types.ts

@@ -1,5 +1,6 @@
 import type { ReadonlySignal } from "@preact/signals";
 import type { Patch } from "immer";
+import { EventParameters } from "src/listener.js";
 import type { SigmaType } from "../framework.js";
 import type { Draft, Immutable } from "../immer.js";
 import { sigmaEventsBrand, sigmaRefBrand, sigmaStateBrand } from "./symbols.js";
@@ -73,9 +74,7 @@ export type EventMethods<TEvents extends AnyEvents | undefined> = [undefined] ex
       /** Registers a typed event listener and returns an unsubscribe function. */
       on<TEvent extends string & keyof TEvents>(
         name: TEvent,
-        listener: [TEvents[TEvent]] extends [void]
-          ? () => void
-          : (payload: TEvents[TEvent]) => void,
+        listener: (...[detail]: EventParameters<TEvents[TEvent]>) => void,
       ): Cleanup;
     };
 
@@ -98,7 +97,7 @@ export type Emit<T extends SigmaDefinition, TOverrides extends Partial<SigmaDefi
     ? [TEvents] extends [AnyEvents]
       ? <TEvent extends string & keyof TEvents>(
           name: TEvent,
-          ...args: [TEvents[TEvent]] extends [void] ? [] : [payload: TEvents[TEvent]]
+          ...[detail]: EventParameters<TEvents[TEvent]>
         ) => void
       : never
     : never;

src/listener.ts

@@ -51,6 +51,13 @@ export type InferEventType<TTarget extends EventTarget> =
   | (InferListener<TTarget> extends { __eventType?: infer TEvent } ? string & TEvent : never)
   | (string & {});
 
+/** Infers the detail parameter for a typed emit. */
+export type EventParameters<T> = [void] extends [T]
+  ? [detail?: T extends void ? undefined : T]
+  : [undefined] extends [T]
+    ? [detail?: T]
+    : [detail: T];
+
 /**
  * A standalone typed event hub with `emit(...)` and `on(...)` methods and full
  * `EventTarget`, `listen(...)`, and `useListener(...)` compatibility.
@@ -66,11 +73,9 @@ export class SigmaTarget<TEvents extends AnyEvents = {}> extends EventTarget {
    */
   emit<TEvent extends string & keyof TEvents>(
     name: TEvent,
-    ...args: [TEvents[TEvent]] extends [void] ? [] : [payload: TEvents[TEvent]]
+    ...[detail]: EventParameters<TEvents[TEvent]>
   ) {
-    this.dispatchEvent(
-      args.length === 0 ? new Event(name) : new CustomEvent(name, { detail: args[0] }),
-    );
+    this.dispatchEvent(detail === undefined ? new Event(name) : new CustomEvent(name, { detail }));
   }
 
   /**