Can code that will never be executed invoke undefined behavior?

Question

The code that invokes undefined behavior (in this example, division by zero) will never get executed, is the program still undefined behavior?

int main(void)         


        

Answer 1

I'd go with the last paragraph of this answer: https://stackoverflow.com/a/18384176/694576

... UB is a runtime issue, not a compiletime issue ...

So, no, there is no UB invoked.

Answer 2

This article discusses this question in section 2.6:

int main(void){
      guard();
      5 / 0;
}

The authors consider that the program is defined when guard() does not terminate. They also find themselves distinguishing notions of “statically undefined” and “dynamically undefined”, e.g.:

The intention behind the standard¹¹ appears to be that, in general, situations are made statically undefined if it is not easy to generate code for them. Only when code can be generated, then the situation can be undefined dynamically.

11) Private correspondence with committee member.

I would recommend looking at the entire article. Taken together, it paints a consistent picture.

The fact that the authors of the article had to discuss the question with a committee member confirms that the standard is currently fuzzy on the answer to your question.

Answer 3

On the subject of undefined behaviour it is often hard to separate the formal aspects from the practical ones. This is the definition of undefined behaviour in the 1989 standard (I don't have a more recent version at hand, but I don't expect this to have changed substantially):

1 undefined behavior
  behavior, upon use of a nonportable or erroneous program construct or of
  erroneous data, for which this International Standard imposes no requirements
2 NOTE Possible undefined behavior ranges from ignoring the situation completely
  with unpredictable results, to behaving during translation or program execution
  in a documented manner characteristic of the environment (with or without the
  issuance of a diagnostic message), to terminating a translation or
  execution (with the issuance of a diagnostic message).

From a formal point of view I'd say your program does invoke undefined behaviour, which means that the standard places no requirement whatsoever on what it will do when run, just because it contains division by zero.

On the other hand, from a practical point of view I'd be surprised to find a compiler that didn't behave as you intuitively expect.