220 9824 article Path: news.gmane.org!not-for-mail From: Richard Smith Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: auto variable templates? Date: Sat, 15 Mar 2014 17:17:24 -0700 Lines: 133 Approved: news@gmane.org Message-ID: References: Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 Content-Type: multipart/alternative; boundary=047d7b5d84ef2b25a604f4ae3a2a X-Trace: ger.gmane.org 1394929039 5412 80.91.229.3 (16 Mar 2014 00:17:19 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Sun, 16 Mar 2014 00:17:19 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBDVNBJG4YAIBBFG3SOMQKGQEFOO3CHY@isocpp.org Sun Mar 16 01:17:27 2014 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-oa0-f69.google.com ([209.85.219.69]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1WOyla-00018A-9Y for gclcip-std-proposals@m.gmane.org; Sun, 16 Mar 2014 01:17:26 +0100 Original-Received: by mail-oa0-f69.google.com with SMTP id i7sf16545657oag.4 for ; Sat, 15 Mar 2014 17:17:25 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20130820; h=x-gm-message-state:mime-version:sender:in-reply-to:references:date :message-id:subject:from:to:x-original-sender :x-original-authentication-results:reply-to:precedence:mailing-list :list-id:list-post:list-help:list-archive:list-subscribe :list-unsubscribe:content-type; bh=o+ou10l35ll78GFMv0MmbFtUuO223H4kJJIRDgtlXvo=; b=dzcdcVY7av6pVwhde4Sq5WhmAxzVhC4kH3Pk/KsXjxrpcVf/qpFA3TmgDitMRTqQVb vb4tDMqFsKPhC6cOOpMzJrCp/Tk/O5bsYs4ZwynRM2R1eQK5k8XcizH8stc3Dl1KKInd O2LJx7Gf+KQW9UbIW5fXk7biPwhETbZIwRNAIx2PbJGOjJ9l//Ucxrpv3e+gxXu065C3 gzbpV0J/1a7idQb8ruMlKvC3/PFCoY1+7+ioEkx4M7uiHH2zuMkTP9WRh7gj1iAEOna2 QTW4j5n5g9qolNeg34z509QZcJzzYL975/WIQNHQBtbQl3yLvKkS7BwtFmt7wJLKFOhZ ZVNA== X-Gm-Message-State: ALoCoQkTQMxacUuPot4u6it3AIV3SfzbyeK32m6aDTN5+0t1VN0DSggCvSOydO5Tfx+SJ+ANrrkd X-Received: by 10.182.24.134 with SMTP id u6mr6253404obf.24.1394929045159; Sat, 15 Mar 2014 17:17:25 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.140.50.81 with SMTP id r75ls332637qga.71.gmail; Sat, 15 Mar 2014 17:17:24 -0700 (PDT) X-Received: by 10.58.238.35 with SMTP id vh3mr12993507vec.16.1394929044443; Sat, 15 Mar 2014 17:17:24 -0700 (PDT) Original-Received: from mail-vc0-x233.google.com (mail-vc0-x233.google.com [2607:f8b0:400c:c03::233]) by mx.google.com with ESMTPS id x7si3033860vel.0.2014.03.15.17.17.24 for (version=TLSv1 cipher=ECDHE-RSA-RC4-SHA bits=128/128); Sat, 15 Mar 2014 17:17:24 -0700 (PDT) Received-SPF: pass (google.com: domain of metafoo@gmail.com designates 2607:f8b0:400c:c03::233 as permitted sender) client-ip=2607:f8b0:400c:c03::233; Original-Received: by mail-vc0-f179.google.com with SMTP id ij19so4270521vcb.24 for ; Sat, 15 Mar 2014 17:17:24 -0700 (PDT) X-Received: by 10.58.154.10 with SMTP id vk10mr13265394veb.18.1394929044234; Sat, 15 Mar 2014 17:17:24 -0700 (PDT) Original-Sender: metafoo@gmail.com Original-Received: by 10.58.218.131 with HTTP; Sat, 15 Mar 2014 17:17:24 -0700 (PDT) In-Reply-To: X-Original-Sender: richard@metafoo.co.uk X-Original-Authentication-Results: mx.google.com; spf=pass (google.com: domain of metafoo@gmail.com designates 2607:f8b0:400c:c03::233 as permitted sender) smtp.mail=metafoo@gmail.com; dkim=pass header.i=@gmail.com Precedence: list Mailing-list: list std-proposals@isocpp.org; contact std-proposals+owners@isocpp.org List-ID: X-Google-Group-Id: 399137483710 List-Post: , List-Help: , List-Archive: List-Subscribe: , List-Unsubscribe: , Xref: news.gmane.org gmane.comp.lang.c++.isocpp.proposals:9824 Archived-At: --047d7b5d84ef2b25a604f4ae3a2a Content-Type: text/plain; charset=ISO-8859-1 On Sat, Mar 15, 2014 at 5:13 PM, Johannes Schaub < schaub.johannes@googlemail.com> wrote: > 2014-03-16 0:38 GMT+01:00 Richard Smith : > > On Sat, Mar 15, 2014 at 1:46 PM, Andrew Tomazos > > > wrote: > >> > >> Is the following supposed to be ill-formed according to the current > draft? > >> > >> template constexpr auto X = 42; > >> > >> int main() > >> { > >> static_assert(X == 42, ""); > >> } > >> > >> > >> [dcl.spec.auto]/4 says that auto is allowed when declaring variables in > >> namespace-scope, but doesn't explicitly say in variable templates - > >> consequently this is ill-formed. > > > > > > A specialization of a variable template is a variable, so this is > allowed. > > > > I don't understand this reason. The following is not a specialization > of a variable template, is it? > > template constexpr auto X = 42; > > The Standard says in 7.1.6.4p6, "A program that uses auto or > decltype(auto) in a context not explicitly allowed in this section is > ill-formed." > > The section does not explicitly allow "auto" for variable templates. > Why does it matter that the specialization of that template is a > variable? Because the validity of a template isn't governed by the usual language rules. 14.6/8: "No diagnostic shall be issued for a template for which a valid specialization can be generated." -- --- You received this message because you are subscribed to the Google Groups "ISO C++ Standard - Future Proposals" group. To unsubscribe from this group and stop receiving emails from it, send an email to std-proposals+unsubscribe@isocpp.org. To post to this group, send email to std-proposals@isocpp.org. 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On S= at, Mar 15, 2014 at 5:13 PM, Johannes Schaub <schaub.johanne= s@googlemail.com> wrote:
2014-03-16 0:38 GMT+01:00 Richard Smith <= richard@metafoo.co.uk>:
> On Sat, Mar 15, 2014 at 1:46 PM, Andrew Tomazos <andrewtomazos@gmail.com> > wrote:
>>
>> Is the following supposed to be ill-formed according to the curren= t draft?
>>
>> template<class> constexpr auto X =3D 42;
>>
>> int main()
>> {
>> =A0 =A0 =A0 =A0 static_assert(X<int> =3D=3D 42, ""= );
>> }
>>
>>
>> [dcl.spec.auto]/4 says that auto is allowed when declaring variabl= es in
>> namespace-scope, but doesn't explicitly say in variable templa= tes -
>> consequently this is ill-formed.
>
>
> A specialization of a variable template is a variable, so this is allo= wed.
>

I don't understand this reason. The following is not a specializa= tion
of a variable template, is it?

=A0 =A0template<class> constexpr auto X =3D 42;

The Standard says in 7.1.6.4p6, "A program that uses auto or
decltype(auto) in a context not explicitly allowed in this section is
ill-formed."

The section does not explicitly allow "auto" for variable templat= es.
Why does it matter that the specialization of that template is a
variable?

Because the validity of a templat= e isn't governed by the usual language rules. 14.6/8: "No diagnost= ic shall be issued for a template for which a valid specialization can be g= enerated."

--

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