[promises] A few tweaks to poll (#32588)

Looking for something else I made some test additions, code tweaks to
make `Poll<>` better.

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src/core/lib/promise/poll.h

@@ -88,6 +88,8 @@ class Poll {
   Poll(U value) : ready_(true) {
     Construct(&value_, std::move(value));
   }
+  // NOLINTNEXTLINE(google-explicit-constructor)
+  Poll(T&& value) : ready_(true) { Construct(&value_, std::forward<T>(value)); }
   ~Poll() {
     if (ready_) Destruct(&value_);
   }
@@ -137,9 +139,55 @@ class Poll {
   };
 };
 
+template <>
+class Poll<Empty> {
+ public:
+  // NOLINTNEXTLINE(google-explicit-constructor)
+  Poll(Pending) : ready_(false) {}
+  Poll() : ready_(false) {}
+  Poll(const Poll& other) = default;
+  Poll(Poll&& other) noexcept = default;
+  Poll& operator=(const Poll& other) = default;
+  Poll& operator=(Poll&& other) = default;
+  // NOLINTNEXTLINE(google-explicit-constructor)
+  Poll(Empty) : ready_(true) {}
+  ~Poll() = default;
+
+  bool pending() const { return !ready_; }
+  bool ready() const { return ready_; }
+
+  Empty value() const {
+    GPR_DEBUG_ASSERT(ready());
+    return Empty{};
+  }
+
+  Empty* value_if_ready() {
+    static Empty value;
+    if (ready()) return &value;
+    return nullptr;
+  }
+
+  const Empty* value_if_ready() const {
+    static Empty value;
+    if (ready()) return &value;
+    return nullptr;
+  }
+
+ private:
+  // Flag indicating readiness.
+  bool ready_;
+};
+
+// Ensure degenerate cases are not defined:
+
+// Can't poll for a Pending
 template <>
 class Poll<Pending>;
 
+// Can't poll for a poll
+template <class T>
+class Poll<Poll<T>>;
+
 template <typename T>
 bool operator==(const Poll<T>& a, const Poll<T>& b) {
   if (a.pending() && b.pending()) return true;

test/core/promise/poll_test.cc

@@ -20,24 +20,46 @@
 
 namespace grpc_core {
 
+static_assert(sizeof(Poll<Empty>) == sizeof(bool),
+              "Poll<Empty> should be just a bool");
+
 TEST(PollTest, IsItPoll) {
   EXPECT_EQ(PollTraits<Poll<int>>::is_poll(), true);
   EXPECT_EQ(PollTraits<Poll<bool>>::is_poll(), true);
+  EXPECT_EQ(PollTraits<Poll<Empty>>::is_poll(), true);
   EXPECT_EQ(PollTraits<Poll<std::unique_ptr<int>>>::is_poll(), true);
   EXPECT_EQ(PollTraits<int>::is_poll(), false);
   EXPECT_EQ(PollTraits<bool>::is_poll(), false);
+  EXPECT_EQ(PollTraits<Empty>::is_poll(), false);
   EXPECT_EQ(PollTraits<std::unique_ptr<int>>::is_poll(), false);
 }
 
 TEST(PollTest, Pending) {
   Poll<int> i = Pending();
   EXPECT_TRUE(i.pending());
+  Poll<Empty> j = Pending();
+  EXPECT_TRUE(j.pending());
 }
 
 TEST(PollTest, Ready) {
   Poll<int> i = 1;
   EXPECT_TRUE(i.ready());
   EXPECT_EQ(i.value(), 1);
+  Poll<Empty> j = Empty();
+  EXPECT_TRUE(j.ready());
+}
+
+TEST(PollTest, CanMove) {
+  Poll<std::shared_ptr<int>> x = std::make_shared<int>(3);
+  Poll<std::shared_ptr<int>> y = std::make_shared<int>(4);
+  y = std::move(x);
+  Poll<std::shared_ptr<int>> z = std::move(y);
+  EXPECT_EQ(*z.value(), 3);
+}
+
+TEST(PollTest, ImplicitConstructor) {
+  Poll<std::shared_ptr<int>> x(std::make_unique<int>(3));
+  EXPECT_EQ(*x.value(), 3);
 }
 
 }  // namespace grpc_core