继承std::optional<T>的operator=在T为原生类型时失效问题问询

我在包装std::optional<T>以添加自定义实用函数时，遇到一个问题：当T为原生类型时，将T类型值赋值给自定义可选类型变量会触发编译错误。原本预期通过using std::optional<T>::operator =;继承的赋值运算符能处理T类型的赋值，但在T是原生类型的场景下（对应代码中的BUG宏），GCC和LLVM都无法匹配到正确的继承运算符。显式提供并转发operator=(T const&)和operator=(T&&)（对应UNBUG宏）可修复该问题。非原生类型时继承的赋值运算符能正常工作，仅原生类型出现此问题。

测试代码 #

#include <optional>

template <typename T>
class my_optional : public std::optional<T>
    {
        public:
            // 尝试继承所有成员
            using std::optional<T>::optional;
            using std::optional<T>::operator =;
            my_optional             (                           ) =default;
            my_optional             (my_optional      const& opt) =default;
            my_optional             (my_optional          && opt) =default;
            my_optional& operator = (my_optional      const& opt) =default;
            my_optional& operator = (my_optional          && opt) =default;
            my_optional             (std::optional<T> const& opt) : std::optional<T>(          opt ) {};
            my_optional             (std::optional<T>     && opt) : std::optional<T>(std::move(opt)) {};
            my_optional& operator = (std::optional<T> const& opt) { std::optional<T>::operator = (          opt ); return *this; };
            my_optional& operator = (std::optional<T>     && opt) { std::optional<T>::operator = (std::move(opt)); return *this; };

#ifdef UNBUG
            my_optional& operator = (T const& var) { std::optional<T>::operator = (          var ); return *this; };
            my_optional& operator = (T     && var) { std::optional<T>::operator = (std::move(var)); return *this; };
#endif
            
            // my_optional(int, int)    {} // 测试用的随机构造函数
    };


#ifdef BUG
    using my_alias = int;
#else
    struct my_struct {};
    using my_alias = my_struct;
#endif


void my_func()
    {
        my_optional<my_alias> an_opt;
        
        my_alias a_var;
        an_opt = a_var;
    }

编译情况 #

• clang++ -std=c++26 -c -o /dev/null -x c++ inherit-optional-bug-3.cpp：编译成功
• clang++ -std=c++26 -c -o /dev/null -x c++ inherit-optional-bug-3.cpp -DBUG -DUNBUG：编译成功
• clang++ -std=c++26 -c -o /dev/null -x c++ inherit-optional-bug-3.cpp -DBUG：编译失败，报错如下：

inherit-optional-bug-3.cpp:42:10: error: use of overloaded operator '=' is ambiguous (with operand types 'my_optional<my_alias>' (aka 'my_optional<int>') and 'my_alias' (aka 'int'))
   42 |                 an_opt = a_var;
      |                 ~~~~~~ ^ ~~~~~
inherit-optional-bug-3.cpp:14:17: note: candidate function
   14 |                         my_optional& operator = (my_optional          && opt) =default;
      |                                      ^
inherit-optional-bug-3.cpp:18:17: note: candidate function
   18 |                         my_optional& operator = (std::optional<T>     && opt) { std::optional<T>::operator = (std::move(opt)); return *this; };
      |                          

疑问 #

此现象是符合C++标准预期？还是标准库缺陷？或是实现问题？亦或是GCC和LLVM均存在bug？

内容的提问来源于stack exchange，提问作者xaxazak