>=gcc6.1 and >=c++11 is_constant_evaluated() function implementation with manual memory setting

is_constant_evaluated.cpp

#include <iostream>
#include <type_traits>

namespace {
    unsigned char v{};
    constexpr bool is_constant_evaluated(const unsigned char* e = &v) {
        return e - 0x6017d2; // memory dependent constant value, check output first line
    }
}

constexpr int test() {
    return is_constant_evaluated() ? 45 : ((std::cout << "[noconstexpr call] "), 20);
}

int main() {
    std::cout << (const void*) &v << std::endl; // set is_constant_evaluated constant number to this memory address

    constexpr bool cexpr = is_constant_evaluated();
    std::cout << cexpr << " " << is_constant_evaluated() << std::endl; // '1 0'
    
    constexpr int t = test();
    
    std::cout << t << " and " << test() << std::endl; // '[noconstexpr call] 45 and 20' or '45 and [noconstexpr call] 20'
}

MSVC simple:

constexpr bool is_constant_evaluated() noexcept {
    struct M {
        int a;
    };
    struct N {
        int a;
    };
    struct C {};
    struct D1 : C, M {};
    struct D2 : C, N {};

    return static_cast<int D1::*>(&M::a) != 
        static_cast<int C::*>(static_cast<int D2::*>(&N::a));
}