Empty std::array doesn't have constexpr begin()?

Question

The following code doesn't compile with clang and libc++: https://godbolt.org/z/rDL-_K

#include <array>
int main() {
    static constexpr std::array<int, 0> xs{};
    constexpr auto i = xs.begin();  // non-constexpr function 'begin' cannot be used in a constant expression

    return 0;
}

But if you change the 0 to a 1, it compiles.

Is this a bug in libc++ or is there a good reason for it? How could I work around this if I have generic constexpr code that uses begin/end?

(I've seen this question, but my example intentionally uses static to avoid this problem.)

Answer 1

I believe this is a bug in libc++ which was introduced in: https://reviews.llvm.org/D41223

In particular, the _LIBCPP_CONSTEXPR_AFTER_CXX14 is missing on begin(), end(), etc.

It seems to work in libstdc++: https://godbolt.org/z/xmfdot

The bug has been reported at https://bugs.llvm.org/show_bug.cgi?id=40124 but it sounds like it may not be fixed due to difficulties obtaining a constexpr pointer to an object when one has not been constructed.