Revert copy-constructor modification which causes TVM segfaults.

See dmlc#658

include/dmlc/optional.h

@@ -80,22 +80,17 @@ class optional {
   optional() : is_none(true) {}
   /*! \brief constructs an object that does contain a nullopt value. */
   optional(dmlc::nullopt_t) noexcept : is_none(true) {} // NOLINT(*)
-  /*! \brief copy constructor, if other contains a value, then stored value is
-   * direct-intialized with it. */
-  optional(const optional &other) = default;
-  /*! \brief move constructor: If other contains a value, then stored value is
-   * direct-intialized with it. */
-  optional(optional &&other) noexcept(
-      std::is_nothrow_move_constructible<T>::value
-          &&std::is_nothrow_move_assignable<T>::value) {
-    if (!other.has_value()) {
-      reset();
-    } else if (has_value()) {
-      **this = std::move(*other);
-    } else {
-      new (&val) T(std::move(*other));
-      is_none = false;
+  /*! \brief construct an optional object with another optional object */
+  optional(const optional<T>& other) {
+#pragma GCC diagnostic push
+#if __GNUC__ >= 6
+#pragma GCC diagnostic ignored "-Wmaybe-uninitialized"
+#endif
+    is_none = other.is_none;
+    if (!is_none) {
+      new (&val) T(other.value());
     }
+#pragma GCC diagnostic pop
   }
   /*! \brief constructs an optional object that contains a value, initialized as
    * if direct-initializing */