Forward at most four emit arguments through EventEmitter method values

The dispatch instantiates one bridged emit per argument count and emitter
class; capping it at four keeps programs that only read emitter members
through computed keys from growing by the full table.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/content/limitations/builtin-apis/eventemitter.mdx

@@ -6,7 +6,7 @@
   Through an untyped value, `listeners()` and `rawListeners()` of an event with a listener that has typed parameters are unsupported, and `emit('error')` of an object that is not an Error with no `error` listener is unsupported.
   Untyped `newListener` and `removeListener` listeners receive `undefined` in place of a listener registered with typed parameters.
   An untyped `emit` on, or an untyped read of `emit` from, an instance of a class that overrides `emit` is unsupported.
-  Calling `EventEmitter.prototype.emit` as a function value on a compiled emitter with more than 16 arguments after the event name is unsupported.
+  Calling `EventEmitter.prototype.emit` as a function value on a compiled emitter with more than 4 arguments after the event name is unsupported.
   Calling EventEmitter.prototype's methods with an instance of a compiled class that does not extend EventEmitter as the receiver throws a `TypeError`.
   Through an untyped value, a stream's `emit` and listener-introspection methods read as `undefined`.
   `events.once` accepts EventEmitter and stream instances with a literal event name and an optional object-literal `{ signal }`; a stream's `data` event, other options values, and EventTarget emitters are unsupported.

packages/compiler/src/frontend/lowering/emitter-method-values.ts

@@ -40,7 +40,7 @@ export const EMITTER_PROTOTYPE_METHODS: ReadonlySet<string> = new Set([
 
 /** `emit` forwards this many arguments after the event name; a call with
  * more fences. */
-const EMIT_ARGUMENTS = 16;
+const EMIT_ARGUMENTS = 4;
 
 /** The runtime's SCR_EE_M_* order: each method's name and the argument
  * counts its call forwards. Node's methods that look at

tests/corpus/6600-emitter-method-values-untyped.ts

@@ -40,7 +40,7 @@ console.log(o.removeAllListeners.call(o, "c") === o, o.eventNames());
 o.removeAllListeners.call(o);
 console.log(o.eventNames());
 o.on("many", (...args: unknown[]) => console.log("many:", args.length, args.join(",")));
-console.log(emit("many", 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11));
+console.log(emit("many", 1, 2, 3, 4));
 console.log(o.setMaxListeners.call(o, 3) === o, o.getMaxListeners.call(o));
 
 // off through a detached method value

tests/corpus/6601-emitter-method-values.cjs

@@ -38,7 +38,7 @@ const plain = new EventEmitter();
 const emit = plain.emit;
 plain.on("hi", (a, b, c) => console.log("hi", a, b, c));
 console.log(emit.call(plain, "hi", 1, "two", [3]));
-console.log(emit.call(plain, "hi", 1, 2, 3, 4, 5, 6, 7, 8, 9));
+console.log(emit.call(plain, "hi", 1, 2, 3, 4));
 const fn = () => console.log("never");
 plain.once("x", fn);
 const off = plain.off;