[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

1 files changed, 13 insertions, 0 deletions

diff --git a/clang/lib/Sema/SemaCoroutine.cpp b/clang/lib/Sema/SemaCoroutine.cpp
index 826cd48a8a5..9c9d266d3d8 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.