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

The technique listed works, but since it uses static_assert it is not sfinae friendly. A better way (in theory, you'll see what I mean) to do this is to check whether a function is noexcept. Why? Because, constant expressions are always noexcept, even if the functions are not marked as such. So, consider the following code:

template 
constexpr void test_helper(T&&) {}

#define IS_CONSTEXPR(...) noexcept(test_helper(__VA_ARGS__))

test_helper is constexpr, so it will be a constant expression as long as its argument is. If it's a constant expression, it will be noexcept, but otherwise it won't be (since it isn't marked as such).

So now let's define this:

double bar(double x) { return x; }

constexpr double foo(double x, bool b) {
    if (b) return x; 
    else return bar(x);
}

foo is only noexcept if the x is a constant expression, and b is true; if the boolean is false then we call a non constexpr function, ruining our constexpr-ness. So, let's test this:

double d = 0.0;

constexpr auto x = IS_CONSTEXPR(foo(3.0, true));
constexpr auto y = IS_CONSTEXPR(foo(3.0, false));
constexpr auto z = IS_CONSTEXPR(foo(d, true));

std::cerr << x << y << z;

It compiles, great! This gives us compile time booleans (not compile failures), which can be used for sfinae, for example.

The catch? Well, clang has a multi-year bug, and doesn't handle this correctly. gcc however, does. Live example: http://coliru.stacked-crooked.com/a/e7b037932c358149. It prints "100", as it should.