From 764232fa13c0e22ae2a4a4a4d8631692c9408784 Mon Sep 17 00:00:00 2001
From: Craig Tiller <ctiller@google.com>
Date: Thu, 26 Aug 2021 14:34:00 -0700
Subject: [PATCH] Fixes for map (#27079)

* fixes for map

* fix bad merge

* Automated change: Fix sanity tests

* Automated change: Fix sanity tests

Co-authored-by: ctiller <ctiller@users.noreply.github.com>
---
 build_autogenerated.yaml                      |  1 +
 src/core/lib/promise/detail/promise_factory.h |  9 +++-----
 src/core/lib/promise/detail/promise_like.h    |  4 ++++
 src/core/lib/promise/map.h                    | 23 +++++++++++++++++--
 test/core/promise/map_test.cc                 |  6 +++++
 5 files changed, 35 insertions(+), 8 deletions(-)

diff --git a/build_autogenerated.yaml b/build_autogenerated.yaml
index f51c2b83a00..4ea31f35dd4 100644
--- a/build_autogenerated.yaml
+++ b/build_autogenerated.yaml
@@ -4224,6 +4224,7 @@ targets:
   - src/core/ext/upb-generated/google/rpc/status.upb.h
   - src/core/lib/gpr/alloc.h
   - src/core/lib/gpr/env.h
+  - src/core/lib/gpr/log_internal.h
   - src/core/lib/gpr/murmur_hash.h
   - src/core/lib/gpr/spinlock.h
   - src/core/lib/gpr/string.h
diff --git a/src/core/lib/promise/detail/promise_factory.h b/src/core/lib/promise/detail/promise_factory.h
index b6db4eb0f51..0b4d89a95f8 100644
--- a/src/core/lib/promise/detail/promise_factory.h
+++ b/src/core/lib/promise/detail/promise_factory.h
@@ -62,10 +62,6 @@ struct IsVoidCallable<F, absl::void_t<decltype(std::declval<F>()())>> {
   static constexpr bool value = true;
 };
 
-// T -> T, const T& -> T
-template <typename T>
-using RemoveCVRef = absl::remove_cv_t<absl::remove_reference_t<T>>;
-
 // Given F(A,B,C,...), what's the return type?
 template <typename T, typename Ignored = void>
 struct ResultOfT;
@@ -86,6 +82,7 @@ class Curried {
  public:
   Curried(F&& f, Arg&& arg)
       : f_(std::forward<F>(f)), arg_(std::forward<Arg>(arg)) {}
+  Curried(const F& f, Arg&& arg) : f_(f), arg_(std::forward<Arg>(arg)) {}
   using Result = decltype(std::declval<F>()(std::declval<Arg>()));
   Result operator()() { return f_(arg_); }
 
@@ -98,9 +95,9 @@ class Curried {
 // capturing A.
 template <typename A, typename F>
 absl::enable_if_t<!IsVoidCallable<ResultOf<F(A)>>::value,
-                  PromiseLike<Curried<F, A>>>
+                  PromiseLike<Curried<RemoveCVRef<F>, A>>>
 PromiseFactoryImpl(F&& f, A&& arg) {
-  return Curried<F, A>(std::forward<F>(f), std::forward<A>(arg));
+  return Curried<RemoveCVRef<F>, A>(std::forward<F>(f), std::forward<A>(arg));
 }
 
 // Promote a callable() -> T|Poll<T> to a PromiseFactory(A) -> Promise<T>
diff --git a/src/core/lib/promise/detail/promise_like.h b/src/core/lib/promise/detail/promise_like.h
index eeaf768dbf3..57507141102 100644
--- a/src/core/lib/promise/detail/promise_like.h
+++ b/src/core/lib/promise/detail/promise_like.h
@@ -74,6 +74,10 @@ class PromiseLike {
   using Result = typename PollTraits<decltype(WrapInPoll(f_()))>::Type;
 };
 
+// T -> T, const T& -> T
+template <typename T>
+using RemoveCVRef = absl::remove_cv_t<absl::remove_reference_t<T>>;
+
 }  // namespace promise_detail
 }  // namespace grpc_core
 
diff --git a/src/core/lib/promise/map.h b/src/core/lib/promise/map.h
index a5b3e9e8377..aba0f259ecb 100644
--- a/src/core/lib/promise/map.h
+++ b/src/core/lib/promise/map.h
@@ -17,6 +17,8 @@
 
 #include <grpc/impl/codegen/port_platform.h>
 
+#include <tuple>
+
 #include "absl/types/variant.h"
 #include "src/core/lib/promise/detail/promise_like.h"
 #include "src/core/lib/promise/poll.h"
@@ -34,10 +36,12 @@ class Map {
   Map(Promise promise, Fn fn)
       : promise_(std::move(promise)), fn_(std::move(fn)) {}
 
-  using Result = typename PromiseLike<Promise>::Result;
+  using PromiseResult = typename PromiseLike<Promise>::Result;
+  using Result =
+      RemoveCVRef<decltype(std::declval<Fn>()(std::declval<PromiseResult>()))>;
 
   Poll<Result> operator()() {
-    Poll<Result> r = promise_();
+    Poll<PromiseResult> r = promise_();
     if (auto* p = absl::get_if<kPollReadyIdx>(&r)) {
       return fn_(std::move(*p));
     }
@@ -59,6 +63,21 @@ promise_detail::Map<Promise, Fn> Map(Promise promise, Fn fn) {
   return promise_detail::Map<Promise, Fn>(std::move(promise), std::move(fn));
 }
 
+// Callable that takes a tuple and returns one element
+template <size_t kElem>
+struct JustElem {
+  template <typename... A>
+  auto operator()(std::tuple<A...>&& t) const
+      -> decltype(std::get<kElem>(std::forward<std::tuple<A...>>(t))) {
+    return std::get<kElem>(std::forward<std::tuple<A...>>(t));
+  }
+  template <typename... A>
+  auto operator()(const std::tuple<A...>& t) const
+      -> decltype(std::get<kElem>(t)) {
+    return std::get<kElem>(t);
+  }
+};
+
 }  // namespace grpc_core
 
 #endif  // GRPC_CORE_LIB_PROMISE_MAP_H
diff --git a/test/core/promise/map_test.cc b/test/core/promise/map_test.cc
index 0dfc50d6258..51bf047459e 100644
--- a/test/core/promise/map_test.cc
+++ b/test/core/promise/map_test.cc
@@ -23,6 +23,12 @@ TEST(MapTest, Works) {
   EXPECT_EQ(x(), Poll<int>(21));
 }
 
+TEST(MapTest, JustElem) {
+  std::tuple<int, double> t(1, 3.2);
+  EXPECT_EQ(JustElem<1>()(t), 3.2);
+  EXPECT_EQ(JustElem<0>()(t), 1);
+}
+
 }  // namespace grpc_core
 
 int main(int argc, char** argv) {