What are the types of identifiers introduced by structured bindings in C++17?

Question

To my knowledge, identifiers introduced by structured bindings in C++17 are in fact references to some "hidden" variable. Such that

auto [ a, b ] = std::make_tuple(1, 2);

is kind-of equivalent to

auto e = std::make_tuple(1, 2);
auto& a = std::get<0>(e);
auto& b = std::get<1>(e);

However, if I print out std::is_reference<decltype(a)>::value, I get 0 in the first case 1 in the second. Why is that?

Answer 1

if I print out std::is_reference<decltype(a)>::value, I get 0 in the first case 1 in the second.

Why is that even if we can prove that a and b refer to the elements in the tuple and one can modify those values by means of them?
I'm not a language lawyer, but probably it is due to this bullet of the standard (working draft):

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

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Side note. You should use this form to do so that a and b refer to the elements in the tuple:

auto tup = std::make_tuple(1, 2);
auto & [ a, b ] = tup;

It follows a minimal, working example:

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

int main() {
    auto tup = std::make_tuple(1, 2);
    auto & [ a, b ] = tup;
    a = 0;
    std::cout << a << ", " << std::get<0>(tup) << std::endl;
}

See it on Coliru. On the other side, what you get is a copy of the values using the expression below:

auto [ a, b ] = std::make_tuple(1, 2);

Here is an article that explains it better and is a bit more comprehensible than the standard for humans.