add comment and remove unnecessary copy-constructor

gtsam/base/std_optional_serialization.h

@@ -48,6 +48,7 @@
  */
 #ifdef __GNUC__
 #if __GNUC__ >= 7 && __cplusplus >= 201703L
+// Based on https://github.com/borglab/gtsam/issues/1738, we define U as a complete type.
 namespace boost { namespace serialization { struct U{}; } }
 namespace std { template<> struct is_trivially_default_constructible<boost::serialization::U> : std::false_type {}; }
 namespace std { template<> struct is_trivially_copy_constructible<boost::serialization::U> : std::false_type {}; }

gtsam/base/tests/testStdOptionalSerialization.cpp

@@ -60,8 +60,6 @@ public:
   TestOptionalStruct() = default;
   TestOptionalStruct(const int& opt)
       : opt(opt) {}
-  // A copy constructor is needed for serialization
-  TestOptionalStruct(const TestOptionalStruct& other) = default;
   bool operator==(const TestOptionalStruct& other) const {
     // check the values are equal
     return *opt == *other.opt;