From 3b4cfcff17540d0942bd0a596998f2d9436b4d3b Mon Sep 17 00:00:00 2001
From: Abdullah Tahiri <abdullah.tahiri.yo@gmail.com>
Date: Tue, 26 Feb 2019 19:30:20 +0100
Subject: [PATCH] GCC 4.8 fails to covert lvalue during RVO optimization

---
 src/Mod/Part/App/GeometryDefaultExtension.cpp | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/src/Mod/Part/App/GeometryDefaultExtension.cpp b/src/Mod/Part/App/GeometryDefaultExtension.cpp
index 20c5ac2bfd..bc0ba91996 100644
--- a/src/Mod/Part/App/GeometryDefaultExtension.cpp
+++ b/src/Mod/Part/App/GeometryDefaultExtension.cpp
@@ -81,9 +81,20 @@ std::unique_ptr<Part::GeometryExtension> GeometryDefaultExtension<T>::copy(void)
     cpy->value = this->value;
     cpy->setName(this->getName());
 
-    return cpy;
+    return std::move(cpy);
+    // Advise from Scott Meyers Effective Modern c++:
+    //
     // Don't std::move(cpy); RVO optimization Item 25, if the compiler fails to elide, would have to move it anyway
     // move constructor is executed if available (it is). Unique_ptr does not have copy constructor.
+    //
+    // That would work perfectly with GCC 7.3.0. However, GCC 4.8.4 misserably fails:
+    //
+    // /home/travis/build/FreeCAD/FreeCAD/src/Mod/Part/App/GeometryDefaultExtension.cpp: In instantiation of
+    // ‘std::unique_ptr<Part::GeometryExtension> Part::GeometryDefaultExtension<T>::copy() const [with T = long int]’:
+    // /home/travis/build/FreeCAD/FreeCAD/src/Mod/Part/App/GeometryDefaultExtension.cpp:164:16:   required from here
+    // /home/travis/build/FreeCAD/FreeCAD/src/Mod/Part/App/GeometryDefaultExtension.cpp:84:12: error: cannot bind //
+    // ‘std::unique_ptr<Part::GeometryDefaultExtension<long int>, std::default_delete<Part::GeometryDefaultExtension<long int> > >’ lvalue
+    // to ‘std::unique_ptr<Part::GeometryDefaultExtension<long int>, std::default_delete<Part::GeometryDefaultExtension<long int> > >&&’
 }
 
 template <typename T>