[c++14] Make latch impl simpler with c++14 (#31434)

* [c++14] Make latch impl simpler with c++14

* fix

src/core/lib/promise/latch.h

@@ -17,6 +17,8 @@
 
 #include <grpc/support/port_platform.h>
 
+#include <type_traits>
+
 #include <grpc/support/log.h>
 
 #include "src/core/lib/promise/intra_activity_waiter.h"
@@ -31,23 +33,6 @@ namespace grpc_core {
 template <typename T>
 class Latch {
  public:
-  // This is the type of the promise returned by Wait.
-  class WaitPromise {
-   public:
-    Poll<T*> operator()() const {
-      if (latch_->has_value_) {
-        return &latch_->value_;
-      } else {
-        return latch_->waiter_.pending();
-      }
-    }
-
-   private:
-    friend class Latch;
-    explicit WaitPromise(Latch* latch) : latch_(latch) {}
-    Latch* latch_;
-  };
-
   Latch() = default;
   Latch(const Latch&) = delete;
   Latch& operator=(const Latch&) = delete;
@@ -67,11 +52,17 @@ class Latch {
   }
 
   // Produce a promise to wait for a value from this latch.
-  WaitPromise Wait() {
+  auto Wait() {
 #ifndef NDEBUG
     has_had_waiters_ = true;
 #endif
-    return WaitPromise(this);
+    return [this]() -> Poll<T*> {
+      if (has_value_) {
+        return &value_;
+      } else {
+        return waiter_.pending();
+      }
+    };
   }
 
   // Set the value of the latch. Can only be called once.
@@ -98,6 +89,9 @@ class Latch {
   IntraActivityWaiter waiter_;
 };
 
+template <typename T>
+using LatchWaitPromise = decltype(std::declval<Latch<T>>().Wait());
+
 }  // namespace grpc_core
 
 #endif  // GRPC_CORE_LIB_PROMISE_LATCH_H

src/core/lib/surface/call.cc

@@ -2454,7 +2454,7 @@ class ClientPromiseBasedCall final : public PromiseBasedCall {
       ABSL_GUARDED_BY(mu());
   absl::optional<PipeReceiver<MessageHandle>::NextType> outstanding_recv_
       ABSL_GUARDED_BY(mu());
-  absl::optional<Latch<ServerMetadata*>::WaitPromise>
+  absl::optional<LatchWaitPromise<ServerMetadata*>>
       server_initial_metadata_ready_;
   absl::optional<grpc_compression_algorithm> incoming_compression_algorithm_;
   Completion recv_initial_metadata_completion_ ABSL_GUARDED_BY(mu());
@@ -2537,7 +2537,7 @@ void ClientPromiseBasedCall::CommitBatch(const grpc_op* ops, size_t nops,
       case GRPC_OP_RECV_INITIAL_METADATA: {
         recv_initial_metadata_ =
             op.data.recv_initial_metadata.recv_initial_metadata;
-        server_initial_metadata_ready_ = server_initial_metadata_.Wait();
+        server_initial_metadata_ready_.emplace(server_initial_metadata_.Wait());
         recv_initial_metadata_completion_ =
             AddOpToCompletion(completion, PendingOp::kReceiveInitialMetadata);
       } break;