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From: Jonathan Wakely <jwakely.gcc@gmail.com>
To: Nikolas Klauser <nikolasklauser@berlin.de>
Cc: Jakub Jelinek <jakub@redhat.com>,
	Jason Merrill <jason@redhat.com>,
	gcc@gcc.gnu.org,  Louis Dionne <ldionne.2@gmail.com>,
	Mark de Wever <koraq@xs4all.nl>,
	aaron@aaronballman.com,  "libstdc++" <libstdc++@gcc.gnu.org>
Subject: Re: GCC support for extensions from later standards
Date: Tue, 8 Aug 2023 17:10:02 +0100	[thread overview]
Message-ID: <CAH6eHdSC6_SQdZT3GhwO4GPq3CuHN2M_fSfRu9mKk6YiTVZ+sg@mail.gmail.com> (raw)
In-Reply-To: <CAH6eHdT=wGYzfmiUOB4frcgDs=fom94CSo0wN+LQ6oLwGnupOQ@mail.gmail.com>

On Tue, 8 Aug 2023 at 17:07, Jonathan Wakely wrote:
>
> On Tue, 8 Aug 2023 at 17:04, Nikolas Klauser wrote:
> >
> > Luckily most of these aren’t problems for libc++. We only support the latest GCC. We can only use `if constexpr` in C++11, but that is already a win I think.
>
> Can you use it in C++11 though? The body of a constexpr function must
> be a single return statement, so if-constexpr isn't allowed.

Clang allows it with multiple warnings:

ifc.cc:3:6: warning: constexpr if is a C++17 extension [-Wc++17-extensions]
 if constexpr (sizeof(i) >= 4)
    ^
ifc.cc:3:3: warning: use of this statement in a constexpr function is
a C++14 extension [-Wc++14-extensions]
 if constexpr (sizeof(i) >= 4)
 ^
ifc.cc:8:5: warning: multiple return statements in constexpr function
is a C++14 extension [-Wc++14-extensions]
   return 0;
   ^
ifc.cc:5:5: note: previous return statement is here
   return i << 3;
   ^

But GCC gives a warning for if-constexpr and then an error for the
invalid function body:

ifc.cc: In function 'constexpr int f(int)':
ifc.cc:3:6: warning: 'if constexpr' only available with '-std=c++17'
or '-std=gnu++17' [-Wc++17-extensions]
   3 |   if constexpr (sizeof(i) >= 4)
     |      ^~~~~~~~~
ifc.cc:9:1: error: body of 'constexpr' function 'constexpr int f(int)'
not a return-statement
   9 | }
     | ^

  reply	other threads:[~2023-08-08 16:10 UTC|newest]

Thread overview: 11+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2023-08-02 19:01 Nikolas Klauser
2023-08-06 19:19 ` Jason Merrill
2023-08-06 19:43   ` Jonathan Wakely
2023-08-07 13:04     ` Jonathan Wakely
2023-08-08  8:55       ` Jonathan Wakely
2023-08-08  3:03   ` Nikolas Klauser
2023-08-08  7:33     ` Jakub Jelinek
2023-08-08 16:03       ` Nikolas Klauser
2023-08-08 16:07         ` Jonathan Wakely
2023-08-08 16:10           ` Jonathan Wakely [this message]
2023-08-08 16:33             ` Nikolas Klauser

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