Question

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

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

on wandbox

使用 libc ++ 时，上面的代码会按照预期clang++进行编译和工作。

使用 libstdc ++ 时，上述代码无法使用clang++和g++进行编译，但出现以下错误：

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 ++ 基于继承的std::tuple实现会在调用std::tuple时元组元素从std::get派生时导致歧义。我倾向于认为这是 libstdc ++ 中的实现缺陷 - 是这样的吗？或者标准中是否存在使代码片段格式不正确的内容？

Answer 1

正如T.C.在评论中所提到的，这是由known bug #71096引起的。

使用从`std :: tuple`派生的元素在`std :: tuple`上调用`std :: get` - 形成错误？

1 个答案: