From 279d42e5624436b28d70178e95e4098abbab6c6a Mon Sep 17 00:00:00 2001
From: Malte Ubl <malte@vercel.com>
Date: Tue, 27 Oct 2026 21:11:41 -0700
Subject: [PATCH] Lower a discarded `undefined` catch handler
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`Promise<void>.catch(() => undefined)` tripped the IR validator's
bare-unitLit rule: the handler body lowered as a bare expression, and
`undefined` has no standalone IR value. The handler's value is
discarded, so lower the body as a statement — the same rule a
discarded `undefined;` statement already follows.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
---
 .../src/frontend/lowering/lower-calls.ts      |  5 ++-
 tests/corpus/promise-void-catch-undefined.ts  | 31 +++++++++++++++++++
 2 files changed, 35 insertions(+), 1 deletion(-)
 create mode 100644 tests/corpus/promise-void-catch-undefined.ts

diff --git a/packages/compiler/src/frontend/lowering/lower-calls.ts b/packages/compiler/src/frontend/lowering/lower-calls.ts
index 6229917a2..45d470c60 100644
--- a/packages/compiler/src/frontend/lowering/lower-calls.ts
+++ b/packages/compiler/src/frontend/lowering/lower-calls.ts
@@ -6908,7 +6908,10 @@ export function lowerPromiseMethodCall(
       if (ts.isBlock(hb)) {
         catchBody = lowerer.lowerStmts(hb.statements.nodes);
       } else if (R.kind === "void") {
-        catchBody = [{ kind: "exprStmt", expr: lowerer.lowerExpr(hb), loc: locOf(hb) }];
+        // The handler's value is discarded, so the body lowers as a
+        // statement: `() => undefined` is a pure no-op (a bare unit
+        // literal has no IR value), not an expression to evaluate.
+        catchBody = [lowerer.lowerExprStatement(hb)];
       } else {
         // Bare-expression handler: `(e) => v` (promise results flatten
         // through the async-return path, like any `return v`).
diff --git a/tests/corpus/promise-void-catch-undefined.ts b/tests/corpus/promise-void-catch-undefined.ts
new file mode 100644
index 000000000..ee7285d8a
--- /dev/null
+++ b/tests/corpus/promise-void-catch-undefined.ts
@@ -0,0 +1,31 @@
+// `Promise<void>.catch(() => undefined)` — the handler's value is
+// discarded and the recovered promise settles as void. The handler runs
+// exactly once, and the chain continues on the next turn whether the
+// receiver fulfilled or rejected.
+
+const ok: Promise<void> = Promise.resolve();
+const bad: Promise<void> = Promise.reject(new Error("boom"));
+
+ok.catch(() => undefined).then(() => console.log("fulfilled recovered"));
+
+bad
+  .catch(() => {
+    console.log("rejected recovered");
+  })
+  .then(() => console.log("rejected continued"));
+
+let runs = 0;
+
+function recover(): Promise<void> {
+  runs++;
+  return runs % 2 === 1 ? Promise.reject(new Error("odd")) : Promise.resolve();
+}
+
+async function run(): Promise<void> {
+  await recover().catch(() => undefined);
+  console.log("awaited recover", runs);
+  await recover().catch(() => undefined);
+  console.log("awaited recover", runs);
+}
+
+void run().then(() => console.log("done"));