https://gcc.gnu.org/bugzilla/show_bug.cgi?id=123577
Bug ID: 123577
Summary: C++17: Unhelpful constexpr error message when array
size exceeds 16 bytes
Product: gcc
Version: 15.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: ajcozzol at us dot ibm.com
Target Milestone: ---
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;
}
