Demo of constexpr vector usage and erroneous usage in C++20

constexpr_vector.cpp

// https://godbolt.org/z/vsacWP36z
// Note: Only works with x86_64 GCC (trunk) on Godbolt.org --must have a C++ stdlib that supports constexpr vector

#include <cstddef>
#include <vector>

class Storage {
private:
    std::vector<int> foo;
public:
    constexpr Storage() : foo(0) {}
    constexpr Storage(std::size_t size) : foo(size) {}
};

int main() {
    constexpr Storage d; // OK! -std::vector(0) ctor will be used
    // constexpr Storage f(52); ERROR! operator new used in constexpr!
    constexpr Storage g(0); // OK! -std::vector(0) ctor will be used
    Storage h; // OK, empty storage at runtime
    Storage y(96); // OK, allocated storage at runtime
}