From 1eda841d320083d037eb90e1728e7df60328c8b0 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?S=C3=A9bastien=20Loriot?= <sebastien.loriot@cgal.org>
Date: Fri, 3 Sep 2021 11:03:22 +0200
Subject: [PATCH] workaround user taking ref to temporary

If D is a temporary, it's lifetime extension is done
when we go out of the scope of choose_parameter
so the const D& returned is dangling
---
 BGL/include/CGAL/boost/graph/Named_function_parameters.h | 6 ++++++
 BGL/test/BGL/test_cgal_bgl_named_params.cpp              | 5 ++++-
 2 files changed, 10 insertions(+), 1 deletion(-)

diff --git a/BGL/include/CGAL/boost/graph/Named_function_parameters.h b/BGL/include/CGAL/boost/graph/Named_function_parameters.h
index acad6c9b93b..4738b8e4db0 100644
--- a/BGL/include/CGAL/boost/graph/Named_function_parameters.h
+++ b/BGL/include/CGAL/boost/graph/Named_function_parameters.h
@@ -313,6 +313,12 @@ const D& choose_parameter(const internal_np::Param_not_found&, const D& d)
   return d;
 }
 
+template <typename D>
+D choose_parameter(const internal_np::Param_not_found&, D&& d)
+{
+  return d;
+}
+
 template <typename T, typename D>
 T& choose_parameter(T& t, D&)
 {
diff --git a/BGL/test/BGL/test_cgal_bgl_named_params.cpp b/BGL/test/BGL/test_cgal_bgl_named_params.cpp
index ab403c28000..c6af93cfee9 100644
--- a/BGL/test/BGL/test_cgal_bgl_named_params.cpp
+++ b/BGL/test/BGL/test_cgal_bgl_named_params.cpp
@@ -43,10 +43,13 @@ void test_values_and_types(const NamedParameters& np)
 }
 
 template<class NamedParameters>
-void test_no_copyable(const NamedParameters&)
+void test_no_copyable(const NamedParameters& np)
 {
   typedef typename inp::Get_param<typename NamedParameters::base,inp::visitor_t>::type NP_type;
   static_assert( boost::is_same<NP_type,std::reference_wrapper<const B> > ::value );
+
+  const A<4>& a  = params::choose_parameter(params::get_parameter_reference(np, inp::edge_index), A<4>(4));
+  assert(a.v==4);
 }
 
 template <class NamedParameters>