Fix joinPromises for Promise<void>.

c++/src/kj/async-inl.h

@@ -547,6 +547,24 @@ private:
   Array<ExceptionOr<T>> resultParts;
 };
 
+template <>
+class ArrayJoinPromiseNode<void> final: public ArrayJoinPromiseNodeBase {
+public:
+  ArrayJoinPromiseNode(Array<Own<PromiseNode>> promises,
+                       Array<ExceptionOr<_::Void>> resultParts)
+      : ArrayJoinPromiseNodeBase(
+            kj::mv(promises), resultParts.begin(), sizeof(ExceptionOr<_::Void>)),
+        resultParts(kj::mv(resultParts)) {}
+
+protected:
+  void getNoError(ExceptionOrValue& output) noexcept override {
+    output.as<_::Void>() = _::Void();
+  }
+
+private:
+  Array<ExceptionOr<_::Void>> resultParts;
+};
+
 // -------------------------------------------------------------------
 
 class EagerPromiseNodeBase: public PromiseNode, protected Event {
@@ -758,6 +776,12 @@ Promise<Array<T>> joinPromises(Array<Promise<T>>&& promises) {
       heapArray<_::ExceptionOr<T>>(promises.size())));
 }
 
+inline Promise<void> joinPromises(Array<Promise<void>>&& promises) {
+  return Promise<void>(false, kj::heap<_::ArrayJoinPromiseNode<void>>(
+      KJ_MAP(p, promises) { return kj::mv(p.node); },
+      heapArray<_::ExceptionOr<_::Void>>(promises.size())));
+}
+
 // =======================================================================================
 
 namespace _ {  // private

c++/src/kj/async-prelude.h

@@ -40,6 +40,7 @@ class WaitScope;
 
 template <typename T>
 Promise<Array<T>> joinPromises(Array<Promise<T>>&& promises);
+Promise<void> joinPromises(Array<Promise<void>>&& promises);
 
 namespace _ {  // private
 
@@ -181,6 +182,7 @@ private:
   friend class TaskSetImpl;
   template <typename U>
   friend Promise<Array<U>> kj::joinPromises(Array<Promise<U>>&& promises);
+  friend Promise<void> kj::joinPromises(Array<Promise<void>>&& promises);
 };
 
 void detach(kj::Promise<void>&& promise);

c++/src/kj/async-test.c++

@@ -531,6 +531,20 @@ TEST(Async, ArrayJoin) {
   EXPECT_EQ(789, result[2]);
 }
 
+TEST(Async, ArrayJoinVoid) {
+  EventLoop loop;
+  WaitScope waitScope(loop);
+
+  auto builder = heapArrayBuilder<Promise<void>>(3);
+  builder.add(READY_NOW);
+  builder.add(READY_NOW);
+  builder.add(READY_NOW);
+
+  Promise<void> promise = joinPromises(builder.finish());
+
+  promise.wait(waitScope);
+}
+
 class ErrorHandlerImpl: public TaskSet::ErrorHandler {
 public:
   uint exceptionCount = 0;

c++/src/kj/async.h

@@ -294,6 +294,7 @@ private:
   friend class _::NeverDone;
   template <typename U>
   friend Promise<Array<U>> joinPromises(Array<Promise<U>>&& promises);
+  friend Promise<void> joinPromises(Array<Promise<void>>&& promises);
 };
 
 template <typename T>