c++ - On what architectures is calculating invalid pointers unsafe?

Question

int* a = new int[5] - 1;

This line by itself invokes undefined behavior according to the C++ standard because a is an invalid pointer and not one-past-the-end. At the same time this is a zero overhead way of making a 1-based array (first element is a[1]) which I need for a project of mine.

I'm wondering if this is something that I need to avoid or if the C++ standard is just being conservative to support some bizarre architectures that my code is never going to run on anyway. So the question is, on what architectures will this be a problem? Are any of those widespread?

Edit: To see that the line above does indeed invoke undefined behavior, take a look at this question.

Edit: Dennis Zickefoose points out that compilers are allowed to do anything when undefined behavior is invoked, so both the compiler and the CPU have to offer guarantees beyond the C++ standard for code like this to work. I'm expanding the question to whether any modern C++ compilers have this issue.

Answer 1

Either way, a well-defined, zero-overhead way of creating a one-based array is the following:

int* a = new int[6];

There, problem solved. ;-) (But interesting question still.)