constexpr std::array out-of-bounds array index access compiles without warnings or errors

[dyumin]

The following code

#include <array>

constexpr std::array Array = {1, 2};

int main()
{
    return Array[3];
}

successfully compiles with --std=c++20 -O3 -Wall -Werror -Wpedantic -Winvalid-constexpr on clang 17.0.1.

godbolt link

jfyi
constexpr int Array[] = {1, 2}; produces compillation error error: array index 3 is past the end of the array (that has type 'const int[2]') [-Werror,-Warray-bounds] as expected.

[dyumin]

consteval function works fine (constexpr function is not though)

https://godbolt.org/z/M5YG1739x

[shafik]

Undefined behavior is ill-formed in a constant expression context but in this case return Array[3] does not force constant evaluation. The following would tough: https://godbolt.org/z/8E75KcGsM

constexpr auto x =  Array[3];

[dyumin]

According to compiler explorer msvc just gives up on constant expression and moves everything into runtime with a warning. Which is not great but still better than clang and gcc which just generate nothing.