The following code compiles in GCC, CLANG, and MSVC. It initializes the int ir to refer to the non-existent a[1] which is one beyond the end of a.

consteval int foo()
{
    int a[1]{ 42 };
    int& ir = a[1];
    return *(&ir-1);
}

int main()
{
    constexpr int ci=foo();
    return ci;
}

This compiles in spite of the core constant expression evaluation seeming to contain UB. See this which contains “A reference shall be initialized to refer to a valid object or function.”

A more recent n4950 draft removes this language.

Is this no longer UB?