optional: use singleton instead of inline variable for c++14

Author: fabinsch

include/proxsuite/helpers/optional.hpp

@@ -21,10 +21,23 @@ using optional = std::optional<T>;
 using nullopt_t = std::nullopt_t;
 inline constexpr nullopt_t nullopt = std::nullopt;
 #else
+namespace detail {
+// Source boost: https://www.boost.org/doc/libs/1_74_0/boost/none.hpp
+// the trick here is to make instance defined once as a global but in a header
+// file
+template<typename T>
+struct nullopt_instance
+{
+  static const T instance;
+};
+template<typename T>
+const T nullopt_instance<T>::instance =
+  T(tl::nullopt); // global, but because 'tis a template, no cpp file required
+} // namespace detail
 template<class T>
 using optional = tl::optional<T>;
 using nullopt_t = tl::nullopt_t;
-inline constexpr nullopt_t nullopt = tl::nullopt;
+constexpr nullopt_t nullopt = detail::nullopt_instance<tl::nullopt_t>::instance;
 #endif
 } // namespace proxsuite