Difficulties getting a constexpr property from a constexpr array

I’m having this issue where I can’t seem to, at compile time, check if all elements in an std::array are equal. It seems so simple and I’m not new to C++ by any means, but I can’t figure it out! (I would just use <algorithm> but sadly those aren’t marked constexpr in C++17, and I’m stuck with C++17 because CUDA.)

Here’s an example (that doesn’t compile).

#include <array>

int main()
{
    constexpr std::array<int, 3> a {0, 0, 0};

    constexpr bool equal = [=](){
        for (int i = 1; i < 3; i++)
        {   
            if constexpr (a[0] != a[i])
                return false;
        }
        return true;
    }();
}

Why does a[0] != a[i] not qualify as constexpr? (This is the error GCC and Clang give me.) How do I get the result I need?

>Solution :

Since your i is not a compile-time constant, you cannot use if constexpr. A simple if is enough which still can check your array at compile-time.

#include <array>

int main()
{
    constexpr std::array<int, 3> a {0, 0, 0};

    constexpr bool equal = [=](){
        for (int i = 1; i < 3; i++)
        {   
            if (a[0] != a[i])
          //^^
                return false;
        }
        return true;
    }();
}