[Bug c/114526] ISO C does not prohibit extensions: fix misconception.

["harald at gigawatt dot nl" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114526

--- Comment #9 from Harald van Dijk <harald at gigawatt dot nl> ---
(In reply to Joseph S. Myers from comment #8)
> "rejects", in the ISO C sense, only applies to errors and pedwarns in GCC;
> not to warnings conditional on -pedantic (of which there are also some, but
> which don't turn into errors with -pedantic).
> 
> If you have cases where something that is only *undefined as a property of a
> particular execution of the program* (as opposed to undefined as a property
> of a translation unit or of the collection of translation units making up a
> program, or violating a Constraint or syntax rule) but that are errors or
> pedwarns, those should be reported as separate bugs.

Bug 83584, which like this one is closed as a duplicate of 11234, is about
exactly that.

  void *f(void) { return (void *)f; }
  int main(void) { return 0; }

This is a strictly conforming program. It violates no syntax rule or
constraint, and exhibits no translation-time undefined behaviour, yet it
triggers a pedwarn, turning into an error with -pedantic-errors. It would have
undefined behaviour i f f were ever called, but it is not called.