[RFC] __extension__ and warnings

[Zack Weinberg]

Neil Booth <neil@daikokuya.co.uk> writes:

> Getting things 100% right with __extension__ is hard, and would uglify
> the parsers further.  I don't think the pain is worth the gain, and I
> don't want to go there.  At present we turn of pedantic, warn_traditional,
> warn_pointer_arith and flag_iso in the C front end, and pedantic in the
> C++ front end.
>
> I propose a simpler solution: simply do not emit any pedwarns whilst inside
> the code affected by __extension__.  This is easy to implement inside
> pedwarn() itself.
>
> With this, I can clear up some confused and confusing warning logic, and fix
> its interaction with cpplib's number interpreter.  Thoughts?

I'm definitely in favor of cleaning this stuff up, but I have to point
out that your proposed solution would change the effect of
__extension__ quite a bit.  It is supposed to be the case that all
warnings under if (pedantic) are pedwarn() calls, but it is not the
case that all pedwarn() calls are under if (pedantic); most of the
warnings affected by warn_traditional and warn_pointer_arith are _not_
pedwarn(), nor should they be; and I don't know what-all flag_iso
does.

The spec for pedwarn and pedantic, as I understand it, which may be a
bit out of date, is:

 - If and only if the diagnostic in question is mandated by the
   relevant standard (C89, C99, C++98, F77, etc) use pedwarn()
   (unless you are using error() instead).

 - If, despite this, we do not think the warning should be on by
   default, put the pedwarn call under if (pedantic) or possibly
   if (pedantic||Wall).

The spec for __extension__ is, it asserts that the following
expression makes deliberate use of GNU extensions under proper
#ifdeffage, so don't bug the programmer about it.  Some, but not all,
GNU extensions draw mandatory diagnostics; others are properly
complained about by -Wtraditional and the like.  So the two properties
are somewhat orthogonal.

I'm now imagining turning the DT_* enumeration into a set of bitflags,
so that you could write e.g.

  diagnostic (DT_EXTENSION | DT_MANDATED | DT_WARNING, "...")

to achieve the effect of 'pedwarn but shut up under __extension__.'

zw