From 84adbfa467ec235fd7caeadfe1c669e69a6f5452 Mon Sep 17 00:00:00 2001
From: Mike Rolig <mrolig@google.com>
Date: Mon, 26 Feb 2024 10:46:04 -0800
Subject: [PATCH 1/3] [Emval] Update co_await to move promise 'val' when
 possible

---
 system/include/emscripten/val.h | 13 ++++++++++---
 1 file changed, 10 insertions(+), 3 deletions(-)

diff --git a/system/include/emscripten/val.h b/system/include/emscripten/val.h
index 9721f66f7999e..30377ee91ce1a 100644
--- a/system/include/emscripten/val.h
+++ b/system/include/emscripten/val.h
@@ -629,7 +629,8 @@ class val {
 
 #if __cplusplus >= 202002L
   class awaiter;
-  awaiter operator co_await() const;
+  awaiter operator co_await() const&;
+  awaiter operator co_await() &&;
 
   class promise_type;
 #endif
@@ -717,6 +718,8 @@ class val::awaiter {
   constexpr static std::size_t STATE_RESULT = 2;
 
 public:
+  awaiter(val&& promise)
+    : state(std::in_place_index<STATE_PROMISE>, std::move(promise)) {}
   awaiter(const val& promise)
     : state(std::in_place_index<STATE_PROMISE>, promise) {}
 
@@ -746,7 +749,11 @@ class val::awaiter {
   val await_resume() { return std::move(std::get<STATE_RESULT>(state)); }
 };
 
-inline val::awaiter val::operator co_await() const {
+inline val::awaiter val::operator co_await() && {
+  return {std::move(*this)};
+}
+
+inline val::awaiter val::operator co_await() const& {
   return {*this};
 }
 
@@ -768,7 +775,7 @@ class val::promise_type {
   }
 
   // Return the stored promise as the actual return value of the coroutine.
-  val get_return_object() { return promise; }
+  val get_return_object() { return std::move(promise); }
 
   // For similarity with JS async functions, our coroutines are eagerly evaluated.
   auto initial_suspend() noexcept { return std::suspend_never{}; }

From ae1b41c8885ce517a08c0a30364d7cf980915bdb Mon Sep 17 00:00:00 2001
From: Mike Rolig <mrolig@google.com>
Date: Mon, 26 Feb 2024 10:57:26 -0800
Subject: [PATCH 2/3] Move in typed_awaiter.

---
 test/embind/test_val_coro.cpp | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/test/embind/test_val_coro.cpp b/test/embind/test_val_coro.cpp
index 442667e2a56cd..3ba54da64c76a 100644
--- a/test/embind/test_val_coro.cpp
+++ b/test/embind/test_val_coro.cpp
@@ -26,14 +26,14 @@ class typed_promise: public val {
 public:
   typed_promise(val&& promise): val(std::move(promise)) {}
 
-  auto operator co_await() const {
+  auto operator co_await() && {
     struct typed_awaiter: public val::awaiter {
       T await_resume() {
         return val::awaiter::await_resume().template as<T>();
       }
     };
 
-    return typed_awaiter(*this);
+    return typed_awaiter(std::move(*this));
   }
 };
 

From 17c023b7de4ce0826b8c778758b70fe032e1f02a Mon Sep 17 00:00:00 2001
From: Mike Rolig <mrolig@google.com>
Date: Mon, 26 Feb 2024 11:10:26 -0800
Subject: [PATCH 3/3] Explicitly test the copy code path for co_await.

---
 test/embind/test_val_coro.cpp | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/test/embind/test_val_coro.cpp b/test/embind/test_val_coro.cpp
index 3ba54da64c76a..9dc5c59425d78 100644
--- a/test/embind/test_val_coro.cpp
+++ b/test/embind/test_val_coro.cpp
@@ -46,6 +46,12 @@ val asyncCoro() {
   // check that awaiting a subclassed promise works and returns the correct type
   int x = co_await typed_promise<int>(promise_sleep(1, 23));
   assert(x == 23);
+  // check awaiting the same promise twice using copy-path of co_await.
+  val twice = promise_sleep(1, 30);
+  val first = co_await twice;
+  val second = co_await twice;
+  assert(first.as<int>() == 30);
+  assert(second.as<int>() == 30);
   // check that returning value works (checked by JS in tests)
   co_return 34;
 }