[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 #18 from Harald van Dijk <harald at gigawatt dot nl> ---
(In reply to Kaz Kylheku from comment #17)
> The standrad does not define the conversion at the *type* level.
> ...
> The program is strictly conforming because it has no problem with type.

The DRs I referenced include ones where type errors have explicitly been stated
not to render behaviour undefined.

DR 132 (C90):

  /* No headers included */
  int checkup()
  {
  /* Case 1 */
  if (0)
  printf("Printing.\n");
  /* Case 2 */
  return 2 || 1 / 0;
  } 

  Response: "The Response to Defect Report #109 addresses this issue. The
translation unit must be successfully translated."

This, despite the fact that it implicitly declares as int(*)(), which is
incompatible with the type it is meant to be declared as.

The distinction you see between type errors and non-type errors is not one that
I believe is supported by previous DR responses.

> We wouldn't say that
> 
>   void f(void) { "abc" / "def"; }
> 
> is strictly conforming because f is not called in the program. There is a
> type problem. Now in this case there is a constraint violation: it requires
> a diagnostic.

My position is that it is *only* because this violates a constraint that this
cannot be part of a strictly conforming program, even if never called, as far
as standard C is concerned. That is why implementations are allowed to reject
it without program flow analysis.

> Anyway, this is all moot because this bugzilla is about GNU C, which has the
> extension. The behavior is locally defined.

Sure, I'm happy to put that aside if it becomes irrelevant to the bug.

> We would like NOT to have a diagnostic under -Wpedantic, so we are on the
> same page.
> 
> Whether your program is strictly conforming or not, we would like not to
> have it diagnosed under the -Wpedantic umbrella, and even if it is changed
> to a program which calls f.
> 
> There is nothing wrong with the diagnostic, but it should be uncoupled from
> -Wpedantic and available under its own option.   Possibly, an umbrella
> option could exist for this kind of "super pedantic" errors, like
> -Wconforming-extensions (warn about the use of GNU extensions that are
> conforming, and thus require no diagnostic by ISO C).

Agreed that having the warning is useful. If 'gcc -std=c99 -pedantic-errors'
emits a warning for this, regardless of whether it is enabled by default, that
is fine, and that does not prevent it from being a valid implementation of the
'c99' utility.