临时const数组不绑定到右值引用

Question

I have the following test program:

#include <iostream>
#include <type_traits>
#include <utility>

template<typename Ty, std::size_t N>
void foo(Ty (&&)[N])
{
    std::cout << "Ty (&&)[" << N << "]\t" << std::is_const<Ty>::value << '\n';
}

template<typename Ty, std::size_t N>
void foo(Ty (&)[N])
{
    std::cout << "Ty (&)[" << N << "]\t" << std::is_const<Ty>::value << '\n';
}

template<typename Ty>
using id = Ty;

int main()
{
    std::cout.setf(std::cout.boolalpha);

    foo(id<int[]>{1, 2, 3, 4, 5});
    foo(id<int const[]>{1, 2, 3, 4, 5}); // <-- HERE.
    int xs[]{1, 2, 3, 4, 5};
    foo(xs);
    int const ys[]{1, 2, 3, 4, 5};
    foo(ys);
    foo(std::move(xs));
    foo(std::move(ys));
}

I would expect that the line marked with an arrow would call the rvalue overload like the non-const call just above it but it doesn't.

Is this just a bug in GCC or is there something in the standard that causes the lvalue overload to be selected?

Answer 1

According to the standard §12.2 [class.temporary] :

Temporaries of class type are created in various contexts: binding a reference to a prvalue (8.5.3), returning a prvalue (6.6.3), a conversion that creates a prvalue (4.1, 5.2.9, 5.2.11, 5.4), throwing an exception (15.1), entering a handler (15.3), and in some initializations (8.5).

So id<int const[]>{1, 2, 3, 4, 5} is a temporary and therefore is an prvalue §3.10 [basic.lval] :

An rvalue (so called, historically, because rvalues could appear on the right-hand side of an assignment expression) is an xvalue, a temporary object (12.2) or subobject thereof, or a value that is not associated with an object.

A prvalue (“pure” rvalue) is an rvalue that is not an xvalue.

Hence shall be selected overloaded function with rvalue reference argument.