220 9829 <878usaa8hz.fsf@euclid.axiomatics.org> article Path: news.gmane.org!not-for-mail From: Gabriel Dos Reis Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: auto variable templates? Date: Sun, 16 Mar 2014 00:49:12 -0700 Organization: axiomatics.org Lines: 84 Approved: news@gmane.org Message-ID: <878usaa8hz.fsf@euclid.axiomatics.org> References: Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 X-Trace: ger.gmane.org 1394956145 20510 80.91.229.3 (16 Mar 2014 07:49:05 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Sun, 16 Mar 2014 07:49:05 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBCSYNQUB2UHRB6NOSWMQKGQEYMHLPWY@isocpp.org Sun Mar 16 08:49:15 2014 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-pd0-f197.google.com ([209.85.192.197]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1WP5oo-0005XJ-Q4 for gclcip-std-proposals@m.gmane.org; Sun, 16 Mar 2014 08:49:15 +0100 Original-Received: by mail-pd0-f197.google.com with SMTP id fp1sf10337672pdb.0 for ; Sun, 16 Mar 2014 00:49:13 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20130820; h=x-gm-message-state:from:to:subject:in-reply-to:organization :references:sender:date:message-id:lines:mime-version :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=BbjFzI4E088VNTB5uDrZsJvPPTJPjqTfmj5n/WwRb3Q=; b=e7VEnWnziWNDL7i9vro3GWlhDnhQSrpkGgKruFmWj9sNP0l/9WkDVxKabuHDTNknwk nvmBJkMQJGXjsGCVuOmdgz20EaMrnyhVGF67D0/sqSlRi36R55saj5A5bSIhKT1lGfOT bLTPF4+O7kjuGH2q1x3nzsqOWDjyZlPoG6OlWunxFVYGzNuGcYE9BrSOmZ4WcuXCmIJP noTJxVmCLi8vd6NoAfK2P9QTf6V8TfskijMvotFNS00yEvXyX/99b6Xng484K8t9X65v +uwT/A298lyETq/tFdENuqvRBnMiuuPuHsNIqvDRwCmbiG5eUXkkYC61fN18kzV/YXcT ulaw== X-Gm-Message-State: ALoCoQmes9CrjpteRAvjaF/oGrl+Dfi1eKaBblsWUI6JW/LICcvnoxtiuS27OGe2u9tMxsUJ9lOL X-Received: by 10.66.253.9 with SMTP id zw9mr7639503pac.38.1394956153686; Sun, 16 Mar 2014 00:49:13 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.182.190.1 with SMTP id gm1ls551081obc.65.gmail; Sun, 16 Mar 2014 00:49:12 -0700 (PDT) X-Received: by 10.60.246.10 with SMTP id xs10mr15302354oec.18.1394956152850; Sun, 16 Mar 2014 00:49:12 -0700 (PDT) Original-Received: from mail.axiomatics.org ([2600:3c00::f03c:91ff:feae:e0e6]) by mx.google.com with ESMTP id e6si7848738oen.136.2014.03.16.00.49.12 for ; Sun, 16 Mar 2014 00:49:12 -0700 (PDT) Received-SPF: neutral (google.com: 2600:3c00::f03c:91ff:feae:e0e6 is neither permitted nor denied by best guess record for domain of gdr@axiomatics.org) client-ip=2600:3c00::f03c:91ff:feae:e0e6; Original-Received: from euclid.axiomatics.org (localhost [IPv6:::1]) by mail.axiomatics.org (Postfix) with ESMTP id 919C5E70D for ; Sun, 16 Mar 2014 00:49:12 -0700 (PDT) In-Reply-To: (Johannes Schaub's message of "Sun, 16 Mar 2014 01:31:35 +0100") Original-Sender: gdr@axiomatics.org Original-Lines: 76 X-Original-Sender: gdr@axiomatics.org X-Original-Authentication-Results: mx.google.com; spf=neutral (google.com: 2600:3c00::f03c:91ff:feae:e0e6 is neither permitted nor denied by best guess record for domain of gdr@axiomatics.org) smtp.mail=gdr@axiomatics.org 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:9829 Archived-At: Johannes Schaub writes: | 2014-03-16 1:17 GMT+01:00 Richard Smith : | > On Sat, Mar 15, 2014 at 5:13 PM, Johannes Schaub | > 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." | > | | Thanks, I have never interpreted this sentence in that way. It just | sounds to me as a warning to prematurely rejecting compilers. IMO a | much better way to word this would be along the way of | | "A template that is made ill-formed by a rule not in this paragraph | is still well-formed as long as a valid specialization can be | generated for it." | | As it is stated in the Standard, it sounds like the rule talks about | well-formed templates only. The general model of C++ templates, since day 1, is that if decl is a valid declaration, then template decl is also a valid declaration. There are a few exceptions (like you can't have a non-static data member templates or virtual function templats), but that is basically it. Inverting that principle gives a "formal" rule that Richard pointed out in his previous message, although tha rules cover more grounds. -- Gaby -- --- 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. Visit this group at http://groups.google.com/a/isocpp.org/group/std-proposals/. .