Construct an std::array at compile time in C++17 using the preprocessor

I am developing a C++17 framework that has optional third-party dependencies and I'd like to construct an std::array that contains the names of the available dependencies.

The CMake file sets preprocessor definitions such as HAS_DEPENDENCY1, HAS_DEPENDENCY2, ..., so writing such a function with an std::vector is quite easy:

using literal = const char*;

std::vector<literal> available_dependencies() {
   std::vector<literal> dependencies{};
#ifdef HAS_DEPENDENCY1
   dependencies.emplace_back("dependency 1");
#endif
#ifdef HAS_DEPENDENCY2
   dependencies.emplace_back("dependency 2");
#endif
   return dependencies;
}

I'd like to have a compile-time, constexpr equivalent of that. I tried this:

constexpr static std::array available_dependencies{
#ifdef HAS_DEPENDENCY1
    "dependency 1",
#endif
#ifdef HAS_DEPENDENCY2
    "dependency 2"
#endif
};

but there are two issues:

1. if no dependency is available (which can happen), the compiler cannot deduce the template arguments of std::array (the array is empty).
2. if dependency 1 is available but dependency 2 isn't, the code doesn't compile because of the trailing comma.

I managed to address the first issue with something like:

template <typename... Literals>
constexpr std::array<literal, sizeof...(Literals)> make_literal_array(Literals&&... literals) {
   return {literals...};
}

but this doesn't support trailing commas. I tried with an std::initializer_list (they allow trailing commas), but couldn't get it to work.

Perhaps you'll be more inspired than me?

Here is another proposal that uses the fact that initializer lists can cope with trailing commas. Since it is uneasy to directly use std::array in your context, why not defining a std::initializer_list instead (setting the type but not the number of items)

constexpr static std::initializer_list<const char*> available_dependencies_list = {
#ifdef HAS_DEPENDENCY1
    "dependency 1",
#endif
#ifdef HAS_DEPENDENCY2
   "dependency 2",
#endif
};

You can iterate it like in for-range loop

for (auto x : available_dependencies_list) {
    std::cout << x << "\n"; 
}

If you want to have a std::array, you can have one with the following

template<int N,typename T>
constexpr auto as_array (std::initializer_list<T> l)  {
    std::array<T,N> res {};
    std::size_t i=0;
    for (auto x : l)  { res[i++] = x; }
    return res;
}

which can be used e.g.

constexpr auto available_dependencies = as_array <available_dependencies_list.size()> (available_dependencies_list);

The ugly fact here is that one has to provide the size as a template parameter (but in c++17 we can't do the same for the list itself), so one has some kind of redundancy. We can still (shamelessly) use a macro such as

#define AS_ARRAY(l)  as_array<l.size()>(l);
constexpr auto available_dependencies = AS_ARRAY(available_dependencies_list);

Demo

---

Update

According to DavidG's comment, there was a useless std::make_index_sequence<N>() in the answer, so I updated the answer above which makes the code simpler.

Moreover, I have observed an issue with clang when using const char* (no issue with g++ and msvc though), e.g.

error: static assertion expression is not an integral constant expression
static_assert (available_dependencies[0] == "dependency 1");

Using std::string_view instead of const char* for the template parameter of the std::initializer_list makes it work for the 3 compilers

Demo