Does an expression with undefined behaviour that is never actually executed make a program erroneous?

Question

In many discussions about undefined behavior (UB), the point of view has been put forward that in the mere presence in a program of any construct that has UB in a p

Answer 1

In the general case the best we can say here is that it depends.

One case where the answer is no, happens when dealing with indeterminate values. The latest draft clearly makes it undefined behavior to produce an indeterminate value during an evaluation with some exceptions but the code sample clearly shows how subtle it could be:

[ Example:

int f(bool b) {
  unsigned char c;
  unsigned char d = c; // OK, d has an indeterminate value
  int e = d;           // undefined behavior
  return b ? d : 0;    // undefined behavior if b is true
}

— end example ]

so this line of code:

return b ? d : 0;

is only undefined if b is true. This seems to be the intuitive approach and seems to be how John Regehr sees it as well, if we read It’s Time to Get Serious About Exploiting Undefined Behavior.

In this case the answer is yes, the code is erroneous even though we are not calling the code invoking undefined behavior:

constexpr const char *str = "Hello World" ;      

constexpr char access()
{
    return str[100] ;
}

int main()
{
}

clang chooses to make access erroneous even though it is never invoked (see it live).