From fcc81c034cbef0e3fbcea1835a131d43359e8692 Mon Sep 17 00:00:00 2001
From: Chris Tate <chris@ctate.dev>
Date: Sat, 24 Oct 2026 17:59:56 -0500
Subject: [PATCH] Read the final result's value only for sync pipeline sources

for await reads the value of a done result only through the
async-from-sync wrapper; an async iterator's done result is not read.
The source's generator read it for both, so an async iterator whose
done result has a throwing value getter rejected the pipeline. It now
reads and awaits the final value only when the iterator is sync.
---
 .../src/frontend/lowering/lower-stream.ts     | 19 ++++++++---
 .../7504-pipeline-untyped-source-errors.mjs   | 34 +++++++++++++++++++
 2 files changed, 48 insertions(+), 5 deletions(-)

diff --git a/packages/compiler/src/frontend/lowering/lower-stream.ts b/packages/compiler/src/frontend/lowering/lower-stream.ts
index d3883427a..d67cf257e 100644
--- a/packages/compiler/src/frontend/lowering/lower-stream.ts
+++ b/packages/compiler/src/frontend/lowering/lower-stream.ts
@@ -4085,14 +4085,23 @@ function lowerDynIterableSource(
                   type: BOOL,
                   loc,
                 },
-                // the final value is awaited too: a rejection rejects the
-                // pump, and the finished iterator is not closed
+                // a sync iterator's final value is awaited too: a rejection
+                // rejects the pump, and the finished iterator is not closed.
+                // An async iterator's final value is not read.
                 then: [
-                  { kind: "assign", localId: valueId, value: get(step(), "value"), loc },
                   {
                     kind: "if",
-                    cond: awaitsValue(),
-                    then: [{ kind: "exprStmt", expr: awaitDyn(value()), loc }],
+                    cond: sync(),
+                    then: [
+                      { kind: "assign", localId: valueId, value: get(step(), "value"), loc },
+                      {
+                        kind: "if",
+                        cond: awaitsValue(),
+                        then: [{ kind: "exprStmt", expr: awaitDyn(value()), loc }],
+                        else_: null,
+                        loc,
+                      },
+                    ],
                     else_: null,
                     loc,
                   },
diff --git a/tests/corpus/7504-pipeline-untyped-source-errors.mjs b/tests/corpus/7504-pipeline-untyped-source-errors.mjs
index 5d6faded1..9dd671606 100644
--- a/tests/corpus/7504-pipeline-untyped-source-errors.mjs
+++ b/tests/corpus/7504-pipeline-untyped-source-errors.mjs
@@ -135,3 +135,37 @@ for (const [label, source] of [["finalRejects", finalRejects], ["finalResolves",
     console.log(label, "rejected", err.message);
   }
 }
+
+// the final result's value is read for a sync iterator only: an async
+// iterator's done result is not read, so a throwing value getter there is
+// never called
+function finalResult(label, i) {
+  if (i === 1) return { value: `${label}1`, done: false };
+  return {
+    done: true,
+    get value() {
+      throw new Error(`${label} final value read`);
+    },
+  };
+}
+function asyncFinal() {
+  let i = 0;
+  return { [Symbol.asyncIterator]() { return { async next() { i++; return finalResult("async", i); } }; } };
+}
+function syncFinal() {
+  let i = 0;
+  return { [Symbol.iterator]() { return { next() { i++; return finalResult("sync", i); } }; } };
+}
+async function runFinal(label, source) {
+  try {
+    await pipelineP(
+      pick(source),
+      new Writable({ objectMode: true, write(c, _e, cb) { console.log(label, "write", c); cb(); } }),
+    );
+    console.log(label, "resolved");
+  } catch (err) {
+    console.log(label, "rejected", err.message);
+  }
+}
+await runFinal("async final", asyncFinal());
+await runFinal("sync final", syncFinal());