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In-Reply-To: <87imgsi3o3.fsf@oldenburg2.str.redhat.com>
From: Jonathan Wakely <jwakely.gcc@gmail.com>
Date: Tue, 19 May 2020 12:31:57 +0100
Message-ID: <CAH6eHdRNpiJLgbH=B1=TAgL-ya6rA0_WYRx42h6o4dO0PUsUGg@mail.gmail.com>
Subject: Re: Invalid use of 'void'
To: Florian Weimer <fweimer@redhat.com>
Cc: Jonathan Wakely via Gcc-help <gcc-help@gcc.gnu.org>, 
 =?UTF-8?B?0JjQs9C+0YDRjCDQk9C+0YDQu9C+0LI=?= <gorlov.igorek@yandex.ru>
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On Tue, 19 May 2020 at 12:07, Florian Weimer <fweimer@redhat.com> wrote:
>
> * Jonathan Wakely via Gcc-help:
>
> > And G++ does allow arithmetic involving a void* and an integer, e.g.
> > p1 - 32 or p2 + 64, it doesn't allow subtracting one void* from the
> > other. So the extension is only partially supported for C++. We should
> > probably document that.
>
> And it's a regression, GCC 2.7.2.3 can compile it, but GCC 2.95.2
> cannot.  (Of course, I'm only half-serious here.)
>
> The problem I see with such extensions in C++ is that they could alter
> SFINAE outcomes.  But maybe that's less important these days because
> people don't write custom type traits anymore.

Most such extensions are disabled in SFINAE contexts:

template<typename T>
auto f(T* p) -> decltype(p+1) { return p + 1; }

int main()
{
  void* p = nullptr;
  p = p + 1;  // warning
  f(p); // error
}