stream: handle sync iterables in merge() multi-source path

The multi-source merge path called `source[Symbol.asyncIterator]()`
directly, which throws for sync-only iterables. The single-source
fast path handled this via for-await-of, but passing multiple
sync sources would fail.

Wrap sync iterators with `PromiseResolve` so they participate in
the `SafePromiseRace` pattern. The wrapping overhead is negligible
relative to the per-iteration promise race.

Author: jasnell

lib/internal/streams/iter/consumers.js

@@ -16,9 +16,11 @@ const {
   ArrayPrototypePush,
   ArrayPrototypeSlice,
   PromisePrototypeThen,
+  PromiseResolve,
   SafePromiseAllReturnVoid,
   SafePromiseRace,
   SymbolAsyncIterator,
+  SymbolIterator,
   TypedArrayPrototypeGetBuffer,
   TypedArrayPrototypeGetByteLength,
   TypedArrayPrototypeGetByteOffset,
@@ -394,11 +396,26 @@ function merge(...args) {
       }
 
       // Multiple sources - race them
-      const states = ArrayPrototypeMap(sources, (source) => ({
-        iterator: source[SymbolAsyncIterator](),
-        done: false,
-        pending: null,
-      }));
+      const states = ArrayPrototypeMap(sources, (source) => {
+        let iterator;
+        if (source[SymbolAsyncIterator]) {
+          iterator = source[SymbolAsyncIterator]();
+        } else if (source[SymbolIterator]) {
+          // Wrap sync iterator to async
+          const syncIter = source[SymbolIterator]();
+          iterator = {
+            next() { return PromiseResolve(syncIter.next()); },
+            return() {
+              return PromiseResolve(syncIter.return?.() ??
+                { __proto__: null, done: true, value: undefined });
+            },
+          };
+        } else {
+          throw new ERR_INVALID_ARG_TYPE(
+            'source', ['AsyncIterable', 'Iterable'], source);
+        }
+        return { iterator, done: false, pending: null };
+      });
 
       const startIterator = (state, index) => {
         if (!state.done && !state.pending) {

test/parallel/test-stream-iter-consumers.js

@@ -317,10 +317,23 @@ Promise.all([
   testMergeSingleSource(),
   testMergeEmpty(),
   testMergeWithAbortSignal(),
+  testMergeSyncSources(),
   testConsumersNonArrayBatch(),
   testConsumersNonArrayBatchSync(),
 ]).then(common.mustCall());
 
+// Regression test: merge() with sync iterable sources
+async function testMergeSyncSources() {
+  const s1 = fromSync('abc');
+  const s2 = fromSync('def');
+  const result = await text(merge(s1, s2));
+  // Both sources should be fully consumed; order may vary
+  assert.strictEqual(result.length, 6);
+  for (const ch of 'abcdef') {
+    assert.ok(result.includes(ch), `missing '${ch}' in '${result}'`);
+  }
+}
+
 // Regression test: consumers should tolerate sources that yield raw
 // Uint8Array or string values instead of Uint8Array[] batches.
 async function testConsumersNonArrayBatch() {