no need to overload return_value

Summary: No need to overload coroutine `promise_type::return_value` to handle initializer list expressions - just template and default the parameter.

Reviewed By: Orvid

Differential Revision: D26508901

fbshipit-source-id: 61c4a2225355b0044638cd89b9153d599317a453

folly/Expected.h

@@ -1357,7 +1357,7 @@ struct Promise {
     return {};
   }
   std::experimental::suspend_never final_suspend() const noexcept { return {}; }
-  template <typename U>
+  template <typename U = Value>
   void return_value(U&& u) {
     value_->emplace(static_cast<U&&>(u));
   }

folly/Optional.h

@@ -642,7 +642,7 @@ struct OptionalPromise {
     return {};
   }
   std::experimental::suspend_never final_suspend() const noexcept { return {}; }
-  template <typename U>
+  template <typename U = Value>
   void return_value(U&& u) {
     *value_ = static_cast<U&&>(u);
   }

folly/experimental/coro/BlockingWait.h

@@ -102,7 +102,7 @@ class BlockingWaitPromise final : public BlockingWaitPromiseBase {
   }
 
   template <
-      typename U,
+      typename U = T,
       std::enable_if_t<std::is_convertible<U, T>::value, int> = 0>
   void return_value(U&& value) noexcept(
       std::is_nothrow_constructible<T, U&&>::value) {

folly/experimental/coro/Task.h

@@ -159,9 +159,7 @@ class TaskPromise : public TaskPromiseBase {
         exception_wrapper::from_exception_ptr(std::current_exception()));
   }
 
-  void return_value(T&& t) { result_.emplace(static_cast<T&&>(t)); }
-
-  template <typename U>
+  template <typename U = T>
   void return_value(U&& value) {
     if constexpr (std::is_same_v<remove_cvref_t<U>, Try<StorageType>>) {
       DCHECK(value.hasValue() || value.hasException());

folly/experimental/coro/detail/InlineTask.h

@@ -107,20 +107,13 @@ class InlineTaskPromise : public InlineTaskPromiseBase {
   InlineTask<T> get_return_object() noexcept;
 
   template <
-      typename Value,
+      typename Value = T,
       std::enable_if_t<std::is_convertible<Value&&, T>::value, int> = 0>
   void return_value(Value&& value) noexcept(
       std::is_nothrow_constructible<T, Value&&>::value) {
     result_.emplace(static_cast<Value&&>(value));
   }
 
-  // Also provide non-template overload for T&& so that we can do
-  // 'co_return {arg1, arg2}' as shorthand for 'co_return T{arg1, arg2}'.
-  void return_value(T&& value) noexcept(
-      std::is_nothrow_move_constructible<T>::value) {
-    result_.emplace(static_cast<T&&>(value));
-  }
-
   void unhandled_exception() noexcept {
     result_.emplaceException(
         folly::exception_wrapper::from_exception_ptr(std::current_exception()));

folly/experimental/coro/test/CoroBenchmarkAllocator.cpp

@@ -87,7 +87,7 @@ class InlineTask {
    public:
     InlineTask get_return_object() { return InlineTask(this); }
 
-    template <typename U>
+    template <typename U = T>
     void return_value(U&& value) {
       *valuePtr_ = std::forward<U>(value);
     }
@@ -196,7 +196,7 @@ class InlineTaskAllocator {
       return InlineTaskAllocator(this);
     }
 
-    template <typename U>
+    template <typename U = T>
     void return_value(U&& value) {
       *valuePtr_ = std::forward<U>(value);
     }

folly/experimental/coro/test/CoroBenchmarkNRVO.cpp

@@ -96,7 +96,7 @@ class InlineTask {
    public:
     InlineTask get_return_object() { return InlineTask(this); }
 
-    template <typename U>
+    template <typename U = T>
     void return_value(U&& value) {
       *valuePtr_ = std::forward<U>(value);
     }