[unittest] Test returning reference of std::pair

CHANGELOG.md

@@ -8,6 +8,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 
 ### Added
 
+- Add test returning reference of std::pair ([#503](https://github.com/stack-of-tasks/eigenpy/pull/503))
 - Add more general visitor `GenericMapPythonVisitor` for map types test `boost::unordered_map<std::string, int>` ([#504](https://github.com/stack-of-tasks/eigenpy/pull/504))
 - Support for non-[default-contructible](https://en.cppreference.com/w/cpp/named_req/DefaultConstructible) types in map types ([#504](https://github.com/stack-of-tasks/eigenpy/pull/504))
 - Add type_info helpers ([#502](https://github.com/stack-of-tasks/eigenpy/pull/502))

unittest/python/test_std_pair.py

-from std_pair import copy, std_pair_to_tuple
+from std_pair import copy, passthrough, std_pair_to_tuple
 
 t = (1, 2.0)
 assert std_pair_to_tuple(t) == t
 assert copy(t) == t
+assert passthrough(t) == t

unittest/std_pair.cpp

@@ -3,7 +3,6 @@
 
 #include <eigenpy/eigenpy.hpp>
 #include <eigenpy/std-pair.hpp>
-#include <iostream>
 
 namespace bp = boost::python;
 
@@ -17,6 +16,11 @@ std::pair<T1, T2> copy(const std::pair<T1, T2>& pair) {
   return pair;
 }
 
+template <typename T1, typename T2>
+const std::pair<T1, T2>& passthrough(const std::pair<T1, T2>& pair) {
+  return pair;
+}
+
 BOOST_PYTHON_MODULE(std_pair) {
   eigenpy::enableEigenPy();
 
@@ -25,4 +29,6 @@ BOOST_PYTHON_MODULE(std_pair) {
 
   bp::def("std_pair_to_tuple", std_pair_to_tuple<int, double>);
   bp::def("copy", copy<int, double>);
+  bp::def("passthrough", passthrough<int, double>,
+          bp::return_value_policy<bp::copy_const_reference>());
 }