From cd38fd2248fefeddcff452670bac65d07e9a129f Mon Sep 17 00:00:00 2001
From: Chris Tate <chris@ctate.dev>
Date: Wed, 21 Oct 2026 11:03:19 -0500
Subject: [PATCH] Bridge events whose typed listeners take no parameters

An event that typed code only listened to with a listener taking no
parameters had no bridge entry: the scan marked a listener as typed only
when one of its parameters was not a checked-dynamic value. Emitting the
event through an `any` reference then threw instead of calling the
listener. A TypeScript listener with no parameters now marks the event as
typed. A JavaScript function without parameters still registers as before,
since it can read `arguments`.
---
 .../frontend/lowering/lower-event-emitter.ts  |  8 ++++++-
 ...-emitter-untyped-zero-arg-listener.ts.json |  6 +++++
 .../6941-emitter-untyped-zero-arg-listener.ts | 23 +++++++++++++++++++
 ...-emitter-untyped-zero-arg-listener.ts.json |  1 +
 4 files changed, 37 insertions(+), 1 deletion(-)
 create mode 100644 packages/compiler/test/ts7/baselines/order-parity/tests/corpus/6941-emitter-untyped-zero-arg-listener.ts.json
 create mode 100644 tests/corpus/6941-emitter-untyped-zero-arg-listener.ts
 create mode 100644 tests/harness/corpus-costs/6941-emitter-untyped-zero-arg-listener.ts.json

diff --git a/packages/compiler/src/frontend/lowering/lower-event-emitter.ts b/packages/compiler/src/frontend/lowering/lower-event-emitter.ts
index 6b79666b1..d84f84f4e 100644
--- a/packages/compiler/src/frontend/lowering/lower-event-emitter.ts
+++ b/packages/compiler/src/frontend/lowering/lower-event-emitter.ts
@@ -338,9 +338,15 @@ function emitterEvents(lowerer: Lowerer): Map<string, EventSig> {
           const cbCt = lowerer.typeOf(node.arguments.nodes[1]);
           const cbT =
             lowerer.mapTypeOf(cbCt) ?? dynFallbackType(lowerer, node.arguments.nodes[1], cbCt);
+          // A TypeScript listener with no parameters is typed too: it
+          // registers without the adapter, so an untyped emit reaches it
+          // only through the event's fixed ABI. A JavaScript function
+          // without parameters can still read `arguments`.
           if (
             cbT?.kind !== "dyn" &&
-            (cbT?.kind !== "func" || cbT.params.some((p) => p.kind !== "dyn"))
+            (cbT?.kind !== "func" ||
+              (cbT.params.length === 0 && !isJsSourceFile(sf)) ||
+              cbT.params.some((p) => p.kind !== "dyn"))
           ) {
             sigOf(name).typedListener = true;
           }
diff --git a/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/6941-emitter-untyped-zero-arg-listener.ts.json b/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/6941-emitter-untyped-zero-arg-listener.ts.json
new file mode 100644
index 000000000..5e0afea43
--- /dev/null
+++ b/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/6941-emitter-untyped-zero-arg-listener.ts.json
@@ -0,0 +1,6 @@
+{
+ "order": [
+  "<repo>/tests/corpus/6941-emitter-untyped-zero-arg-listener.ts"
+ ],
+ "diags": []
+}
diff --git a/tests/corpus/6941-emitter-untyped-zero-arg-listener.ts b/tests/corpus/6941-emitter-untyped-zero-arg-listener.ts
new file mode 100644
index 000000000..89c4dc70f
--- /dev/null
+++ b/tests/corpus/6941-emitter-untyped-zero-arg-listener.ts
@@ -0,0 +1,23 @@
+// Emitting through an `any` reference reaches listeners that typed code
+// registered with no parameters, for an event no typed code emits: they
+// run once per emit, whatever arguments the untyped emit passes.
+
+import { EventEmitter } from "node:events";
+
+const emitter = new EventEmitter();
+let ready = 0;
+emitter.on("ready", () => {
+  ready++;
+  console.log("ready", ready);
+});
+emitter.once("closed", function () {
+  console.log("closed once");
+});
+
+const loose: any = emitter;
+console.log(loose.emit("ready"));
+console.log(loose.emit("ready", 1, "two"));
+console.log(loose.emit("closed", { reason: "done" }));
+console.log(loose.emit("closed"));
+console.log(loose.emit("missing"));
+console.log(emitter.listenerCount("ready"), emitter.listenerCount("closed"), ready);
diff --git a/tests/harness/corpus-costs/6941-emitter-untyped-zero-arg-listener.ts.json b/tests/harness/corpus-costs/6941-emitter-untyped-zero-arg-listener.ts.json
new file mode 100644
index 000000000..21b784250
--- /dev/null
+++ b/tests/harness/corpus-costs/6941-emitter-untyped-zero-arg-listener.ts.json
@@ -0,0 +1 @@
+{ "plain": 2800, "san": 3400 }