Is it possible to test if a constexpr function is evaluated at compile time?

Question

Since the extended versions of constexpr (I think from C++14) you can declare constexpr functions that could be used as "real" cons

Answer 1

I think the canonical way to do that is with static_assert. static_asserts are evaluated at compile time, so they will break the build if their condition is false.

#include 

constexpr int foo(const int s) {
  return s + 4;
}

int main()
{
    std::cout << foo(3) << std::endl;
    const int bar = 3;
    std::cout << foo(bar) << std::endl;
    constexpr int a = 3;
    std::cout << foo(a) << std::endl;

    static_assert(foo(3) == 7, "Literal failed");
    static_assert(foo(bar) == 7, "const int failed");
    static_assert(foo(a) == 7, "constexpr int failed");
    return 0;
}

clang++ -std=c++14 so1.cpp compiles fine for me, showing that everything works as expected.