Pair_optional_adaptor no longer inherits from std::optional

first is now non-reference
removed union to avoid default construction (-> T is supposed to be lightweight)

STL_Extension/include/CGAL/Pair_optional_adaptor.h

@@ -17,16 +17,23 @@ namespace CGAL {
 
 // T is supposed to be a handle
 template<typename T>
-class Pair_optional_adaptor : public std::optional<T> {
+class Pair_optional_adaptor {
 public:
-  Pair_optional_adaptor(const std::optional<T>& obj) : std::optional<T>(obj), second(obj.has_value()), first(t_storage.t) {
+  Pair_optional_adaptor(const std::optional<T>& obj) : first(), second(obj.has_value()) {
     if (obj.has_value())
-      t_storage.t = *obj;
-  } //boost::tuples::detail::swallow_assign
+      first = *obj;
+  }
+
+  Pair_optional_adaptor(const std::nullopt_t& obj) : first(), second(false) {}
 
-  Pair_optional_adaptor(const std::nullopt_t& obj) : std::optional<T>(std::nullopt), second(false), first(t_storage.t) {}
+  Pair_optional_adaptor(std::pair<T, bool>& p) : first(p.first), second(p.second) {}
 
-  Pair_optional_adaptor(std::pair<T, bool>& p) : std::optional<T>(p.second ? p.first : std::optional<T>()), first(t_storage.t), second(p.second), t_storage(p.first) {}
+  operator std::optional<T>() {
+    if (second)
+      return std::optional<T>(first);
+    else
+      return std::nullopt;
+  }
 
 #ifndef CGAL_NO_DEPRECATED_CODE
   CGAL_DEPRECATED_MSG("you are using the deprecated API, please update your code")
@@ -60,31 +67,22 @@ class Pair_optional_adaptor : public std::optional<T> {
   }
 
   CGAL_DEPRECATED_MSG("you are using the deprecated API, please update your code")
-    operator std::tuple<T&, std::_Ignore const&>() {
-    return std::tuple<T&, std::_Ignore const&>(first, std::ignore);
+  operator std::tuple<T&, decltype(std::ignore)&>() {
+    return std::tuple<T&, decltype(std::ignore)&>(first, std::ignore);
   }
 
   CGAL_DEPRECATED_MSG("you are using the deprecated API, please update your code")
-    operator std::tuple<std::_Ignore const&, bool&>() {
-    return std::tuple<std::_Ignore const&, bool&>(std::ignore, second);
+  operator std::tuple<decltype(std::ignore)&, bool&>() {
+    return std::tuple<decltype(std::ignore)&, bool&>(std::ignore, second);
   }
 
   CGAL_DEPRECATED_MSG("you are using the deprecated API, please update your code")
-    operator std::tuple<std::_Ignore const&, std::_Ignore const&>() {
-    return std::tuple<std::_Ignore const&, std::_Ignore const&>(std::ignore, std::ignore);
+  operator std::tuple<decltype(std::ignore)&, decltype(std::ignore)&>() {
+    return std::tuple<decltype(std::ignore)&, decltype(std::ignore)&>(std::ignore, std::ignore);
   }
 #endif
-
-  T &first;
+  T first;
   bool second;
-
-private:
-  union T_value {
-    T t;
-    int i;
-    T_value() : i(0) {}
-    T_value(T t) : t(t) {}
-  } t_storage;
 };
 
 } // CGAL

Surface_mesh/include/CGAL/Surface_mesh/Properties.h

@@ -23,6 +23,7 @@
 #include <string>
 #include <typeinfo>
 #include <vector>
+#include <optional>
 
 namespace CGAL {