Implement P0135R1, Guaranteed copy elision.

	* cvt.c (ocp_convert): Don't re-copy a TARGET_EXPR in C++17.

From-SVN: r240820

gcc/cp/ChangeLog

 2016-10-05  Jason Merrill  <jason@redhat.com>
 
+	Implement P0135R1, Guaranteed copy elision.
+	* cvt.c (ocp_convert): Don't re-copy a TARGET_EXPR in C++17.
+
 	PR c++/54293
 	* call.c (reference_binding): Fix binding to member of temporary.
 

gcc/cp/cvt.c

@@ -693,8 +693,11 @@ ocp_convert (tree type, tree expr, int convtype, int flags,
   if (error_operand_p (e))
     return error_mark_node;
 
-  if (MAYBE_CLASS_TYPE_P (type) && (convtype & CONV_FORCE_TEMP))
-    /* We need a new temporary; don't take this shortcut.  */;
+  if (MAYBE_CLASS_TYPE_P (type) && (convtype & CONV_FORCE_TEMP)
+      && !(cxx_dialect >= cxx1z
+	   && TREE_CODE (e) == TARGET_EXPR))
+    /* We need a new temporary; don't take this shortcut.  But in C++17, don't
+       force a temporary if we already have one.  */;
   else if (same_type_ignoring_top_level_qualifiers_p (type, TREE_TYPE (e)))
     {
       if (same_type_p (type, TREE_TYPE (e)))

gcc/testsuite/g++.dg/cpp1z/elide1.C

+// { dg-options -std=c++1z }
+
+struct A
+{
+  A();
+  A(const A&) = delete;
+};
+
+bool b;
+A a = A();
+A a1 = b ? A() : A();
+A a2 = (42, A());
+
+A f();
+A a3 = f();
+A a4 = b ? A() : f();