c++ - Tuple isn't being constructed in order?

Question

The following program:

#include <iostream>
#include <tuple>

struct A {
    A() { std::cout << "A constructor
"; }
};

struct B {
    B() { std::cout << "B constructor
"; }
};

int main() {
    std::tuple<A, B> t;
}

gives different outputs on different compilers:

# libstdc++
B constructor
A constructor
# libc++
A constructor
B constructor

This seem weird... I figured the standard would have guaranteed the tuple elements be constructed in-order, e.g., A, B, ..., Y, Z?

Answer 1

std::tuple construction order is currently unspecified.

A proposal for a concrete decision on its order has been submitted to the committee but until then the order should not be relied on.