Is incrementing a pointer to a 0-sized dynamic array undefined?

Pointers to elements of arrays are allowed to point to a valid element, or one past the end. If you increment a pointer in a way that goes more than one past the end, the behavior is undefined.

For your 0-sized array, p is already pointing one past the end, so incrementing it is not allowed.

See C++17 8.7/4 regarding the + operator (++ has the same restrictions):

f the expression P points to element x[i] of an array object x with n elements, the expressions P + J and J + P (where J has the value j) point to the (possibly-hypothetical) element x[i+j] if 0≤i+j≤n; otherwise, the behavior is undefined.