C++17 可选树，错误：不完整类型的无效使用

当我编译包含可选类型的二叉树时：

#include <optional>

class BinaryTree
{
public:
    BinaryTree();
    int value;
    std::optional<BinaryTree> left,right;
};

int main()
{
    return 0;
}

via

g++ -std=c++17 -Wfatal-errors main.cpp 

我面临这个错误

In file included from /usr/include/c++/7/bits/move.h:54:0,
                 from /usr/include/c++/7/bits/stl_pair.h:59,
                 from /usr/include/c++/7/utility:70,
                 from /usr/include/c++/7/optional:36,
                 from main.cpp:1:
/usr/include/c++/7/type_traits: In instantiation of ‘struct std::is_trivially_copy_constructible<BinaryTree>’:
/usr/include/c++/7/optional:103:8:   required from ‘class std::_Optional_base<BinaryTree>’
/usr/include/c++/7/optional:451:11:   required from ‘class std::optional<BinaryTree>’
main.cpp:8:28:   required from here
/usr/include/c++/7/type_traits:1409:12: error: invalid use of incomplete type ‘class BinaryTree’
     struct is_trivially_copy_constructible
            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
compilation terminated due to -Wfatal-errors.

我应该如何修复这个错误？

可选包含参与时类型的完整实例。

在一个类型的实例中存储该类型的两个实例是不可能的；它需要 2

考虑使用指向类型的唯一指针而不是可选指针。

如果您希望能够复制树，那么您必须编写一个值指针。值指针是一个知道如何深度复制自身的指针。