In C++17 mode, GCC is showing an unhelpful error message when diagnosing an incorrect constexpr function, but only if the size of the array in use exceeds 16. No error messages show up in C++20 or above.

I saw this initially on 14.3.1 (eabi-unknown ARM), but it is the same with GCC 15.2 (x64).

This may be a duplicate of 110401 and 113595 (I'm not sure), but I thought it was interesting that the error message is correct for arrays of size 16 or smaller.

Compiler Explorer link: https://godbolt.org/z/rejbGeP9T

#include <array>
#include <cstddef>
#include <cstdint>

template <std::size_t N>
constexpr std::array<std::byte, N> make_byte_array(
    const std::uint8_t (&a)[N]) {
    // Error on next line of code - uninitialized var in constant context
    std::array<std::byte, N> ret;
    for (std::size_t i = 0; i < N; ++i) ret[i] = std::byte{a[i]};
    return ret;
}

int main() {
    // Correct error message for this line:
    // <source>:9:30: error: uninitialized variable 'ret' in 'constexpr' context
    // constexpr std::array<std::byte, 16> a = make_byte_array({ 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15});
    
    // Once you go past 16, you get a bad and confusing error message:
    // <source>:10:5: error: 'goto' is not a constant expression
    constexpr std::array<std::byte, 17> a = make_byte_array({0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16});

    return 0;
}