结构化绑定的 decltype(auto) 应该是参考吗？

Question

考虑一个例子：

#include <iostream>
#include <type_traits>
#include <tuple>

int main() {
    auto tup = std::make_tuple(1, 2);
    auto [ a, b ] = tup;
    decltype(auto) e = a;
    std::cout << std::boolalpha << std::is_reference_v<decltype(e)> << std::endl;
}

clang (output: false) and gcc (output: true) are disagreeing in this simple case. Having in mind e.g. e 应该是参考还是 gcc 错误？或者代码格式不正确？

Answer 1

标识符本身是引用。来自 [dcl.struct.bind]/3:

Given the type T_i designated by std::tuple_element<i, E>::type, each v_i is a variable of type “reference to T_i” initialized with the initializer, where the reference is an lvalue reference if the initializer is an lvalue and an rvalue reference otherwise; the referenced type is T_i.

即a和b都是int&&.

但是 decltype(auto) 的实际行为方式来自 [dcl.type.auto.deduct]:

If the placeholder is the decltype(auto) type-specifier, T shall be the placeholder alone. The type deduced for T is determined as described in [dcl.type.simple], as though e had been the operand of the decltype.

这个写法真是别扭，不过归根结底：

decltype(auto) e = a;
~~~~~~~~~~~~~~

表示：

decltype( a  ) e = a;
         ~~~~

和decltype(a)的意思是，从[dcl.type.simple]/4.1:

if e is an unparenthesized id-expression naming a structured binding ([dcl.struct.bind]), decltype(e) is the referenced type as given in the specification of the structured binding declaration;

a 的 引用类型 是 int，因此 e 必须是 int。这意味着它不是参考，而且 clang 是正确的。归档 81176.

结构化绑定的 decltype(auto) 应该是参考吗？

Is decltype(auto) for a structured binding supposed to be a reference?

c++

language-lawyer

c++17

structured-bindings