[promises] Fix handling of `absl::Status` by `TrySeq` (#34162)

src/core/lib/promise/poll.h

@@ -29,15 +29,13 @@ namespace grpc_core {
 // A type that signals a Promise is still pending and not yet completed.
 // Allows writing 'return Pending{}' and with automatic conversions gets
 // upgraded to a Poll<> object.
-struct Pending {
+struct Pending {};
-  constexpr bool operator==(Pending) const { return true; }
+inline bool operator==(const Pending&, const Pending&) { return true; }
-};
 
 // A type that contains no value. Useful for simulating 'void' in promises that
 // always need to return some kind of value.
-struct Empty {
+struct Empty {};
-  constexpr bool operator==(Empty) const { return true; }
+inline bool operator==(const Empty&, const Empty&) { return true; }
-};
 
 // The result of polling a Promise once.
 //

src/core/lib/promise/try_join.h

@@ -51,14 +51,20 @@ struct TryJoinTraits {
   static bool IsOk(const absl::StatusOr<T>& x) {
     return x.ok();
   }
+  static bool IsOk(const absl::Status& x) { return x.ok(); }
   template <typename T>
   static T Unwrapped(absl::StatusOr<T> x) {
     return std::move(*x);
   }
+  static Empty Unwrapped(absl::Status) { return Empty{}; }
   template <typename R, typename T>
   static R EarlyReturn(absl::StatusOr<T> x) {
     return x.status();
   }
+  template <typename R>
+  static R EarlyReturn(absl::Status x) {
+    return x;
+  }
 };
 
 // Implementation of TryJoin combinator.

test/core/promise/try_join_test.cc

@@ -76,6 +76,12 @@ TEST(TryJoinTest, Join2Fail2P) {
   EXPECT_EQ(TryJoin(instant_fail<int>(), pending<int>())(), (fail<int, int>()));
 }
 
+TEST(TryJoinTest, JoinStatus) {
+  EXPECT_EQ(TryJoin([]() { return absl::OkStatus(); },
+                    []() { return absl::OkStatus(); })(),
+            ok(Empty{}, Empty{}));
+}
+
 }  // namespace grpc_core
 
 int main(int argc, char** argv) {