From 0b5683001175f2e321c90f6608ade719c0a34fab Mon Sep 17 00:00:00 2001
From: Brian Gesiak <modocache@gmail.com>
Date: Mon, 8 Oct 2018 03:08:39 +0000
Subject: [PATCH] [coro]Pass rvalue reference for named local variable to
 return_value

Summary:
Addressing https://bugs.llvm.org/show_bug.cgi?id=37265.

Implements [class.copy]/33 of coroutines TS.

When the criteria for elision of a copy/move operation are met, but not
for an exception-declaration, and the object to be copied is designated by an
lvalue, or when the expression in a return or co_return statement is a
(possibly parenthesized) id-expression that names an object with automatic
storage duration declared in the body or parameter-declaration-clause of the
innermost enclosing function or lambda-expression, overload resolution to select
the constructor for the copy or the return_value overload to call is first
performed as if the object were designated by an rvalue.

Patch by Tanoy Sinha!

Reviewers: modocache, GorNishanov

Reviewed By: modocache, GorNishanov

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D51741

llvm-svn: 343949
---
 clang/lib/Sema/SemaCoroutine.cpp     | 13 +++++++
 clang/test/SemaCXX/coroutine-rvo.cpp | 69 ++++++++++++++++++++++++++++++++++++
 2 files changed, 82 insertions(+)
 create mode 100644 clang/test/SemaCXX/coroutine-rvo.cpp

diff --git a/clang/lib/Sema/SemaCoroutine.cpp b/clang/lib/Sema/SemaCoroutine.cpp
index 826cd48..9c9d266 100644
--- a/clang/lib/Sema/SemaCoroutine.cpp
+++ b/clang/lib/Sema/SemaCoroutine.cpp
@@ -841,6 +841,19 @@ StmtResult Sema::BuildCoreturnStmt(SourceLocation Loc, Expr *E,
     E = R.get();
   }
 
+  // Move the return value if we can
+  if (E) {
+    auto NRVOCandidate = this->getCopyElisionCandidate(E->getType(), E, CES_AsIfByStdMove);
+    if (NRVOCandidate) {
+      InitializedEntity Entity =
+          InitializedEntity::InitializeResult(Loc, E->getType(), NRVOCandidate);
+      ExprResult MoveResult = this->PerformMoveOrCopyInitialization(
+          Entity, NRVOCandidate, E->getType(), E);
+      if (MoveResult.get())
+        E = MoveResult.get();
+    }
+  }
+
   // FIXME: If the operand is a reference to a variable that's about to go out
   // of scope, we should treat the operand as an xvalue for this overload
   // resolution.
diff --git a/clang/test/SemaCXX/coroutine-rvo.cpp b/clang/test/SemaCXX/coroutine-rvo.cpp
new file mode 100644
index 0000000..8521b85
--- /dev/null
+++ b/clang/test/SemaCXX/coroutine-rvo.cpp
@@ -0,0 +1,69 @@
+// RUN: %clang_cc1 -triple x86_64-apple-darwin9 %s -stdlib=libc++ -std=c++1z -fcoroutines-ts -fsyntax-only
+
+namespace std::experimental {
+template <class Promise = void> struct coroutine_handle {
+  coroutine_handle() = default;
+  static coroutine_handle from_address(void *) noexcept;
+};
+
+template <> struct coroutine_handle<void> {
+  static coroutine_handle from_address(void *) noexcept;
+  coroutine_handle() = default;
+  template <class PromiseType>
+  coroutine_handle(coroutine_handle<PromiseType>) noexcept;
+};
+
+template <class... Args>
+struct void_t_imp {
+  using type = void;
+};
+template <class... Args>
+using void_t = typename void_t_imp<Args...>::type;
+
+template <class T, class = void>
+struct traits_sfinae_base {};
+
+template <class T>
+struct traits_sfinae_base<T, void_t<typename T::promise_type>> {
+  using promise_type = typename T::promise_type;
+};
+
+template <class Ret, class... Args>
+struct coroutine_traits : public traits_sfinae_base<Ret> {};
+}
+
+struct suspend_never {
+  bool await_ready() noexcept;
+  void await_suspend(std::experimental::coroutine_handle<>) noexcept;
+  void await_resume() noexcept;
+};
+
+struct MoveOnly {
+  MoveOnly() {};
+  MoveOnly(const MoveOnly&) = delete;
+  MoveOnly(MoveOnly&&) noexcept {};
+  ~MoveOnly() {};
+};
+
+template <typename T>
+struct task {
+  struct promise_type {
+    auto initial_suspend() { return suspend_never{}; }
+    auto final_suspend() { return suspend_never{}; }
+    auto get_return_object() { return task{}; }
+    static void unhandled_exception() {}
+    void return_value(T&& value) {}
+  };
+};
+
+task<MoveOnly> f() {
+  MoveOnly value;
+  co_return value;
+}
+
+int main() {
+  f();
+  return 0;
+}
+
+// expected-no-diagnostics
-- 
2.7.4