#include <new>
#include <type_traits>
#include <utility>
 
// constructs a T at the uninitialized memory pointed to by p using
// list-initialization for aggregates and non-list initialization otherwise
template<class T, class... Args>
T* construct(T* p, Args&&... args)
{
    if constexpr (std::is_aggregate_v<T>)
        return ::new (static_cast<void*>(p)) T{std::forward<Args>(args)...};
    else
        return ::new (static_cast<void*>(p)) T(std::forward<Args>(args)...);
}
 
struct A { int x, y; };
struct B { B(int, const char*) {} };
 
int main()
{
    std::aligned_union_t<1, A, B> storage;
    [[maybe_unused]] A* a = construct(reinterpret_cast<A*>(&storage), 1, 2);
    [[maybe_unused]] B* b = construct(reinterpret_cast<B*>(&storage), 1, "hello");
}