What is the no-undefined-behavior way of deserializing an object from a byte array in C++11 (or later)?

To overcome alignment issues, I need to memcpy into a temporary. What type should that temporary be? gcc complains that the following reinterpret_cast will break strict aliasing rules:

template <typename T>
T deserialize(char *ptr) {
    static_assert(std::is_trivially_copyable<T>::value, "must be trivially copyable");
    alignas(T) char raw[sizeof(T)];
    memcpy(raw, ptr, sizeof(T));
    return *reinterpret_cast<T *>(raw);
}

(e.g. when T is "long").

I don't want to define a T, since I don't want to construct a T before overwriting it.

In a union, doesn't writing one member then reading another count as undefined behavior?

template<typename T>
T deserialize(char *ptr) {
    union {
        char arr[sizeof(T)];
        T obj;
    } u;

    memcpy(u.arr, ptr, sizeof(T));   // Write to u.arr
    return u.obj;   // Read from u.obj, even though arr is the active member.
}