220 22242 <4c6e5856-d2e2-4080-bd03-d44aab07088f@isocpp.org> article Path: news.gmane.org!not-for-mail From: Nicol Bolas Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Return type deduction should be SFINAE friendly Date: Wed, 4 Nov 2015 07:53:46 -0800 (PST) Lines: 131 Approved: news@gmane.org Message-ID: <4c6e5856-d2e2-4080-bd03-d44aab07088f@isocpp.org> References: <8ec93f40-633c-4eaf-84b1-0fbcd4a4f9ca@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 Content-Type: multipart/mixed; boundary="----=_Part_209_1720886613.1446652426168" X-Trace: ger.gmane.org 1446652432 11040 80.91.229.3 (4 Nov 2015 15:53:52 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Wed, 4 Nov 2015 15:53:52 +0000 (UTC) Cc: asutton@uakron.edu To: ISO C++ Standard - Future Proposals Original-X-From: std-proposals+bncBCEKFTV6ZUMBBC6U5CYQKGQE35G7PAA@isocpp.org Wed Nov 04 16:53:52 2015 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-yk0-f198.google.com ([209.85.160.198]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1Zu0Ng-0006Vw-Nf for gclcip-std-proposals@m.gmane.org; Wed, 04 Nov 2015 16:53:48 +0100 Original-Received: by ykft191 with SMTP id t191sf97931465ykf.0 for ; Wed, 04 Nov 2015 07:53:47 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20120113; h=date:from:to:cc:message-id:in-reply-to:references:subject :mime-version:content-type:x-original-sender:reply-to:precedence :mailing-list:list-id:x-spam-checked-in-group:list-post:list-help :list-archive:list-subscribe:list-unsubscribe; bh=fDodKUlOVgPE0cUK4GN7v7Yl2KTaWSAXY8ejYVOgnoc=; b=gK1cbA/gW7f/jU+YZ9zNItGUxneFmz6gN4HNwqdVdEv6unUH7zdi6r12KA+coJcTru Nn1gHADCifa6kDW5PeI5ojSZpR9cL6yvJXSvYEfhYRQexyTrqUhluKreQ7ly1zLaawKF LfcO0IktAi9VIBxxX0tc6tRJAX43QYSIRTCfGL59f/Ydbgr/peKnGuaYuvS3LLNFnVHJ dBijCBMvQ0m7p0a8F9V8kB81UvUrc3p8viF//gYS+buFO4IB2NloZ3/cq/nGs6Wn4Zrl rFjGe6CBbJxMoZaa7J8txy6mZlkVkFmsGoI8A0tiNsrmHqFaIcyXq6rxEBzNIVWVx8xJ XBsw== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20130820; h=x-gm-message-state:date:from:to:cc:message-id:in-reply-to :references:subject:mime-version:content-type:x-original-sender :reply-to:precedence:mailing-list:list-id:x-spam-checked-in-group :list-post:list-help:list-archive:list-subscribe:list-unsubscribe; bh=fDodKUlOVgPE0cUK4GN7v7Yl2KTaWSAXY8ejYVOgnoc=; b=caK4x1WATluLawU2PAuh/Er4aULPMPr6S0vreZ2RAMTB6vm1VJtrSEe8WFwwXXqvKL UrBZ9YtNNzQCA5tuYVe6UpugUKrqUBrsyaV1zalmvMtVoKV6jKSkqFFTXgLZYb/CCQ3f n4UoMzQ7atIU5HBE9ZdRqxdbRgrJj/h9paKpfugbvuxDiHWuzv82feFRlCIkzbSV3dzY ggZuUZMSV0/xuWeM/Z6MMAPXLY8XXBbCw0b5CgyOYdh+cE2PaYBd9D9lYaMDa8OgLSGR aJ5XOAvIZJsArMUTHNHb7s44J3ciqHbIxX3GZlkpTTPp6bEVmmnyRYlQdkU4+/ujlK/L u40Q== X-Gm-Message-State: ALoCoQlKvYL6BdaBwFjeWITtu7fCKFgzgUxM6UNzSvB23RwhIfjDyPDWFDx+VSJ4qq40nMEbT1Hy X-Received: by 10.140.235.193 with SMTP id g184mr1788780qhc.10.1446652427909; Wed, 04 Nov 2015 07:53:47 -0800 (PST) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.50.142.37 with SMTP id rt5ls457867igb.24.gmail; Wed, 04 Nov 2015 07:53:46 -0800 (PST) X-Received: by 10.50.141.138 with SMTP id ro10mr102500igb.5.1446652426947; Wed, 04 Nov 2015 07:53:46 -0800 (PST) In-Reply-To: <8ec93f40-633c-4eaf-84b1-0fbcd4a4f9ca@isocpp.org> X-Original-Sender: jmckesson@gmail.com Precedence: list Mailing-list: list std-proposals@isocpp.org; contact std-proposals+owners@isocpp.org List-ID: X-Spam-Checked-In-Group: std-proposals@isocpp.org 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:22242 Archived-At: ------=_Part_209_1720886613.1446652426168 Content-Type: multipart/alternative; boundary="----=_Part_210_268168931.1446652426168" ------=_Part_210_268168931.1446652426168 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable On Wednesday, November 4, 2015 at 1:12:39 AM UTC-5, Tomasz wrote: > > W dniu =C5=9Broda, 4 listopada 2015 02:06:58 UTC+1 u=C5=BCytkownik Nicol = Bolas=20 > napisa=C5=82: > One reason we don't do that, why we explicitly limit SFINAE to=20 >> parameters/return types, is so that we don't get false positives. If it= =20 >> gets past the signature, then any errors that happen ought to be real,= =20 >> genuine errors that a user has to face. That is, someone did something= =20 >> *wrong*. >> > And I am requesting and SFINAE for return type for function using return= =20 > type deduction. > That didn't address my point. Just because a function uses return type=20 deduction doesn't mean it can't get false positives.=20 Not everyone's functions that use return type deduction are tiny. If you give someone a type and they try to default construct it and that=20 >> fails, then either your template function did something wrong or the use= r=20 >> passed an inappropriate type. Either way, it should hard error out. We= =20 >> (will) have concepts to check things like that in the future, so having = a=20 >> hard error is the right thing. The declaration should not simply disappe= ar. >> > > Have you checked Motivation 3 regarding the concepts. Lack of SFINAE for= =20 > return type deduction essentially breaks concept for generic lambda uses,= =20 > And I do not think that users will constrain every lambda they will write= ..=20 > I don't see the idea behind Motivation 3. It seems like an admission that= =20 proper use of concepts will effectively solve the problem. Then you just=20 declare that people won't always use them. My response to that is... tough. If we give people a tool that fixes a=20 problem, and they refuse to use that tool to fix that exact problem, I=20 refuse to care that they still have the problem. --=20 ---=20 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 e= mail 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-proposa= ls/. ------=_Part_210_268168931.1446652426168 Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable On Wednesday, November 4, 2015 at 1:12:39 AM UTC-5, Tomasz wrote:
W dniu =C5=9Broda, 4 listopada 2015= 02:06:58 UTC+1 u=C5=BCytkownik Nicol Bolas napisa=C5=82:
One reason we d= on't do that, why we explicitly limit SFINAE to parameters/return types= , is so that we don't get false positives. If it gets past the signatur= e, then any errors that happen ought to be real, genuine errors that a user= has to face. That is, someone did something wrong.
And I am requesting and SFINAE for return type for function usin= g return type deduction.

That didn'= ;t address my point. Just because a function uses return type deduction doe= sn't mean it can't get false positives.

Not everyone's = functions that use return type deduction are tiny.

=
If you give someone a type and = they try to default construct it and that fails, then either your template = function did something wrong or the user passed an inappropriate type. Eith= er way, it should hard error out. We (will) have concepts to check things l= ike that in the future, so having a hard error is the right thing. The decl= aration should not simply disappear.

Have yo= u checked Motivation 3 regarding the concepts. Lack of SFINAE for return ty= pe deduction essentially breaks concept for generic lambda uses, And I do n= ot think that users will constrain every lambda they will write.
=

I don't see the idea behind Motivation 3. I= t seems like an admission that proper use of concepts will effectively solv= e the problem. Then you just declare that people won't always use them.=

My response to that is... tough. If we give people a tool that fixe= s a problem, and they refuse to use that tool to fix that exact problem, I = refuse to care that they still have the problem.

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