使用派生自 std::tuple 的元素在 std::tuple 上调用 std::get - 格式错误？

Question

struct Y { };
struct X : std::tuple<Y> { };

int main()
{
    std::get<0>(std::make_tuple(X{}));
}

on wandbox

当使用 libc++.

时，以上代码编译并按预期与 clang++ 一起工作

当使用 libstdc++ 时，上述代码无法同时使用 clang++ 和 g++ 进行编译，并出现以下错误：

include/c++/7.0.1/tuple:1302:36: 
error: no matching function for call to ‘__get_helper<0>(std::tuple<X>&)’
    { return std::__get_helper<__i>(__t); }
            ~~~~~~~~~~~~~~~~~~~~~~^~~~~

include/c++/7.0.1/tuple:1290:5: 
note: candidate: template<long unsigned int __i, class _Head, class ... _Tail> 
                constexpr _Head& std::__get_helper(std::_Tuple_impl<_Idx, _Head, _Tail ...>&)
    __get_helper(_Tuple_impl<__i, _Head, _Tail...>& __t) noexcept
    ^~~~~~~~~~~~

include/c++/7.0.1/tuple:1290:5: 
note:   template argument deduction/substitution failed:
include/c++/7.0.1/tuple:1302:36: 
note:   ‘std::_Tuple_impl<0, _Head, _Tail ...>’ is an ambiguous base class of ‘std::tuple<X>’
    { return std::__get_helper<__i>(__t); }
            ~~~~~~~~~~~~~~~~~~~~~~^~~~~

include/c++/7.0.1/tuple:1295:5: 
note: candidate: template<long unsigned int __i, class _Head, class ... _Tail> 
                constexpr const _Head& std::__get_helper(const std::_Tuple_impl<_Idx, _Head, _Tail ...>&)
    __get_helper(const _Tuple_impl<__i, _Head, _Tail...>& __t) noexcept
    ^~~~~~~~~~~~

include/c++/7.0.1/tuple:1295:5: 
note:   template argument deduction/substitution failed:
include/c++/7.0.1/tuple:1302:36: 
note:   ‘const std::_Tuple_impl<0, _Head, _Tail ...>’ is an ambiguous base class of ‘std::tuple<X>’
    { return std::__get_helper<__i>(__t); }
            ~~~~~~~~~~~~~~~~~~~~~~^~~~~

似乎 libstdc++ 的 std::tuple 的基于继承的实现在调用 std::get 时从 std::tuple 派生的元组元素导致歧义.我倾向于认为这是 libstdc++ 中的一个实现缺陷 - 是这样吗？还是标准中有什么东西会使代码片段格式错误？

Answer 1

如T.C. in the comments, this is caused by the known bug #71096所述。

使用派生自 std::tuple 的元素在 std::tuple 上调用 std::get - 格式错误？

Calling `std::get` on `std::tuple` with elements deriving from `std::tuple` - ill-formed?

c++

c++11

language-lawyer

libstdc++

stdtuple