Is the behaviour of the compiler undefined, with Undefined Behaviour?

Question

When I answered this question, I wrote:

First, it is important to note that it is not only the behaviour of the user program that is undefined, it is

Answer 1

The C++ standard defines behavior for code, it doesn't define behavior for the compiler. As such, it doesn't really make sense to refer to undefined behavior of the compiler -- it was never well-defined to begin with. The only requirement is that it produces an implementation that conforms to the standard guidelines for the code. How it does this is an implementation detail.