Is it well-defined/legal to placement-new multiple times at the same address?

Question

(Note: this question was motivated by trying to come up with preprocessor hackery to generate a no-op allocation to answer this other question:

Macro that accept new

Answer 1

Peforming placement-new several times on the same block of memory is perfectly fine. Moreover, however strange it might sound, you are not even requred to destruct the object that already resides in that memory (if any). The standard explicitly allows that in 3.8/4

4 A program may end the lifetime of any object by reusing the storage which the object occupies or by explicitly calling the destructor for an object of a class type with a non-trivial destructor. For an object of a class type with a non-trivial destructor, the program is not required to call the destructor explicitly before the storage which the object occupies is reused or released;[...]

In other words, it is your responsibility to take into account the consequences of not calling the destructor for some object.

However, calling the destructor on the same object twice as you do in your code is not allowed. Once you created the second object in the same region of memory, you effectively ended the lifetime of the first object (even though you never called its destructor). Now you only need to destruct the second object.