[Bug fortran/58001] Make it possible to silence "Extension: Tab character in format" warning

[sgk at troutmask dot apl.washington.edu]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58001

--- Comment #6 from Steve Kargl <sgk at troutmask dot apl.washington.edu> ---
On Tue, Jul 30, 2013 at 12:59:00PM +0000, dominiq at lps dot ens.fr wrote:
> 
> If compiled with -Wno-tabs, it gives
> 
> pr58001.f90:2.1:
> 
>  print 1894
>  1
> Warning: Nonconforming tab character at (1)
> pr58001.f90:3.14:
> 
> 1894  format( '123')
>               1
> Warning: Nonconforming tab character at (1)
> pr58001.f90:3.14:
> 
> 1894  format( '123')
>               1
> Warning: Extension: Tab character in format at (1)
> 
> Is this the expected behavior?

With -Wno-tabs, gfortran should report a warning for the
occurence of every tab used in the context of a character
from the Fortran character set.  Taking your code, which I've
deleted here, and replace all whitespace by tabs, I get the
expected number of warnings.  Historically, I wanted to use
-Wtabs as you expected, but there was too much wailing on 
the gfortran lists, so it was changed to -Wno-tabs with the
meaning that "no tabs are allowed" in the context of the
Fortran character set.  You'll notice that tabs within
a literal character string are not flagged.  The history
of -Wtabs verse -Wno-tabs can be found in the mailinglist
archive.

Now, for the problem at hand, it seems that there is a
bug in io.c.   Here the code in question:


/* Eat up the spaces and return a character.  */

static char
next_char_not_space (bool *error)
{
  char c;
  do
    {
      error_element = c = next_char (NONSTRING);
      if (c == '\t')
    {
      if (gfc_option.allow_std & GFC_STD_GNU)
        gfc_warning ("Extension: Tab character in format at %C");
      else
        {
          gfc_error ("Extension: Tab character in format at %C");
          *error = true;
          return c;
        }
    }
    }
  while (gfc_is_whitespace (c));
  return c;
}

Notice that there is no check for gfc_option.warn_tabs.