#include <array>
#include <utility>

template < class F, size_t... Ns >
constexpr
std::array<decltype(std::declval<F>()(size_t(0))),sizeof...(Ns)>
generate_array( F&& f, std::index_sequence<Ns...> )
{
    return { f(Ns)... };
}

template < size_t N, class F >
constexpr
std::array<decltype(std::declval<F>()(size_t(0))),N>
generate_array( F&& f ) {
    return generate_array( std::forward<F>(f), std::make_index_sequence<N>() );
}

// Usage example
struct A {
    constexpr explicit A(int i) : _i(i) {}

    int _i;
};

// Define functor. Constexpr lambda expressions are C++17 or later.
constexpr A generator( size_t i ) { return A(i); }

extern constexpr std::array<A,10> a = generate_array<10>( generator );