Pointer arithmetic within array using a pointer to objects created placement-new

I ran to an over-specialized container somewhere in production code. It was C++98 upgraded to C++11, but I wonder how many UBs (or implementation-defined behaviors) are in it. Basically, without any useful interfaces the idea boils down to this:

template<class T>
class Box {
public:
    using DataType = T;

    Box() : sz(def_size), data_ptr(allocate(def_size)) {}

    T* data() { return reinterpret_cast<T*>(data_ptr); }

protected:
     using BlockType = alignas(alignof(T)) char[sizeof(T)];

     BlockType * allocate(size_t sz) {
        BlockType* ptr = new BlockType[sz];
        for(auto p = ptr, ptrE = ptr + sz; ptr != ptrE; ++ptr ) {
            auto* obj = new(p) T;
            // do some initialization on obj
        } 
        return  ptr;
    }
private:
    size_t     sz;
    BlockType *data_ptr;
};

One I can think of: compiler is not required to implement that kind of over-alignment. A cross-compiler used for project did.

But did the C++ standard define the use of pointer arithmetics on the value returned by data() as a pointer to an element of an array? Pointers to blocks aren't necessary to be pointers to instances of T?