Constant reference data member

Let's suppose we have a structure with a constant reference data member.

struct A {
    A() : i{5} {}
    const int& foo() const { return i; }
    const int& i;
};

Do you have any idea why the output for an integer literal 5 is different?

A a{};
std::cout << a.i << std::endl;
std::cout << a.foo() << std::endl;

5
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