PR libstdc++/85671 allow copy elision in path concatenation

By performing the /= operation on a named local variable instead of a
temporary the copy made for the return value can be elided.

	PR libstdc++/85671
	* include/bits/fs_path.h (operator/): Permit copy elision.
	* include/experimental/bits/fs_path.h (operator/): Likewise.

From-SVN: r260009

libstdc++-v3/ChangeLog

+2018-05-07  Jonathan Wakely  <jwakely@redhat.com>
+
+	PR libstdc++/85671
+	* include/bits/fs_path.h (operator/): Permit copy elision.
+	* include/experimental/bits/fs_path.h (operator/): Likewise.
+
 2018-05-07  Edward Smith-Rowland  <3dw4rd@verizon.net>
 
 	Moar PR libstdc++/80506

libstdc++-v3/include/bits/fs_path.h

@@ -552,7 +552,11 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
 
   /// Append one path to another
   inline path operator/(const path& __lhs, const path& __rhs)
-  { return path(__lhs) /= __rhs; }
+  {
+    path __result(__lhs);
+    __result /= __rhs;
+    return __result;
+  }
 
   /// Write a path to a stream
   template<typename _CharT, typename _Traits>

libstdc++-v3/include/experimental/bits/fs_path.h

@@ -510,7 +510,11 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
 
   /// Append one path to another
   inline path operator/(const path& __lhs, const path& __rhs)
-  { return path(__lhs) /= __rhs; }
+  {
+    path __result(__lhs);
+    __result /= __rhs;
+    return __result;
+  }
 
   /// Write a path to a stream
   template<typename _CharT, typename _Traits>