220 29793 <1ecc764b-3dce-49ff-86b0-4d1505138fd0@isocpp.org> article Path: news.gmane.org!.POSTED!not-for-mail From: Nicol Bolas Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Extending the scope of generalized lambda captures to include the lambda's trailing return type Date: Thu, 15 Dec 2016 09:10:56 -0800 (PST) Lines: 120 Approved: news@gmane.org Message-ID: <1ecc764b-3dce-49ff-86b0-4d1505138fd0@isocpp.org> References: Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: multipart/mixed; boundary="----=_Part_393_1191957542.1481821856841" X-Trace: blaine.gmane.org 1481821867 21557 195.159.176.226 (15 Dec 2016 17:11:07 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Thu, 15 Dec 2016 17:11:07 +0000 (UTC) To: ISO C++ Standard - Future Proposals Original-X-From: std-proposals+bncBCEKFTV6ZUMBBIM5ZPBAKGQEQUW4AGA@isocpp.org Thu Dec 15 18:10:58 2016 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-pg0-f72.google.com ([74.125.83.72]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1cHZYT-0003jV-Tv for gclcip-std-proposals@m.gmane.org; Thu, 15 Dec 2016 18:10:54 +0100 Original-Received: by mail-pg0-f72.google.com with SMTP id f188sf119391864pgc.1 for ; Thu, 15 Dec 2016 09:10:58 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=isocpp-org.20150623.gappssmtp.com; s=20150623; h=date:from:to:message-id:in-reply-to:references:subject:mime-version :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=/Td3jHvr1EvIO0E2UrAIZwXf40uMopyyoLgzBDBA9Hk=; b=uI0PTujoAHt8B1I8gPcSGAN1hL5UgUVmo9g4UpSGZnJTBZ4lgVbfF6VCLrVAouNQ2w LWQk6zsv2QT4eVuRaAl4N7PQ+kJzNADogbuDway0pMzRRqfRIOLbPeVWBEefJ5Vbcve8 Y7qKfWCHMsH84oeCL6QFyZKLo3kJeGtCdWvVzjd1mIcawp0rOzMvNk8MoQbAszTwp69f NLz6KTQYVkph156u9Tp0V7sQCxonAh1BFAtSlgrVmY0FacpeiviyjOP+dRxsDUb8wQQq Su5rwyLgNWHMKkyG92ERKyzSH6/GkSDhyhrcka4kYpKG5Lzo07cyuKVpetbIkPGDTbgX j2Xg== DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=date:from:to:message-id:in-reply-to:references:subject:mime-version :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=/Td3jHvr1EvIO0E2UrAIZwXf40uMopyyoLgzBDBA9Hk=; b=BFnzhvPafrxO8iZjPnm4KUh8TE8NbWuluLjqAGDK97S4X/tH1AmlcdfeFqfbqUiJip gihXi1dM7HpND57y054mLbVqqWLsX8yvGlB7jSlUUijQFJaEk0itlSSOdWKInMWlgNCI MTpPehaP/6GyCQqfVAjzEI9EHg5t+W6w5xrTs6zOdO6y/xfYA8SasOZcsauE61uTWvUD +2aP4PS6ARRteiPD0HxigHiqdSamaIe9yZJ2t2TQ9f9CGtKdZPJV1b4zlkdmsH9aR1yN MKBiMDE9S6ubK+7DeGJu58k1PxKbEiNDq2f5Op/MpTfQOPNH7O1pmZW/vk09LlypwFZO DZAQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:message-id:in-reply-to:references :subject:mime-version: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=/Td3jHvr1EvIO0E2UrAIZwXf40uMopyyoLgzBDBA9Hk=; b=ZI3mWtKxYTsEVMxnIxNguPyqo+jnJLehxU8mf856r8F9b8l8AxXUPa0L1gYXNmvHgE jyN4mBo0578601D4LBgalMRsR3PUkaOpn1VT1H4zy/AMfSW+4M9bsAEWP/9Xtw0IAGWx fyL8MlbGETf6HBkRzr3q8t73xt4nNs16dYuwjIeulUx8cUHPYlxIwrnjAdtwIAUNrxwf 7ExttzU0W8AapvyFEA/RAz4sDig8ZEU1tgmOsz/iHc73hKDSb3X4DGfobA3ICRE+j/qT 2+mxOJ8CNDaBnCMb0+SiZgjqLQVUjmNYMuCIyRDV3QkiKpzRjbjvrdVlLX/jFDC1LItN TfEw== X-Gm-Message-State: AKaTC03NDpK5aVIvV+NtdcxB5n+RwK8zfYHognX+FpDkFohPJSuNvQ23Yw0ujrz1yNVWBw== X-Received: by 10.99.114.74 with SMTP id c10mr1424155pgn.17.1481821858071; Thu, 15 Dec 2016 09:10:58 -0800 (PST) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.157.26.102 with SMTP id u35ls6759311otu.4.gmail; Thu, 15 Dec 2016 09:10:57 -0800 (PST) X-Received: by 10.157.56.132 with SMTP id p4mr236082otc.20.1481821857295; Thu, 15 Dec 2016 09:10:57 -0800 (PST) In-Reply-To: X-Original-Sender: jmckesson@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:29793 Archived-At: ------=_Part_393_1191957542.1481821856841 Content-Type: multipart/alternative; boundary="----=_Part_394_1631465146.1481821856841" ------=_Part_394_1631465146.1481821856841 Content-Type: text/plain; charset=UTF-8 On Thursday, December 15, 2016 at 12:00:35 PM UTC-5, Vittorio Romeo wrote: > > I would like to write a proposal, but I just realized that it would be a > breaking change: > http://melpon.org/wandbox/permlink/g9eAszk1SoGPA1Lf > > > struct Foo { }; > > int main() > { > Foo x; > auto l = [x = 10]() -> decltype(x) { return 10; }; > } > > If the proposal is accepted, this code will behave differently! > This still feels very counter-intuitive though... I'm inclined to think > it's worth bringing this to the attention of the committee anyway. > > Wait a minute. This *wouldn't* have been a breaking change until C++14. Back then, if you declared a capture, then there *must* have been a variable outside the lambda with that exact name and type. So in that case, `[x]() -> decltype(x) {...}` would be fine, since `lambda::x` and `x` have the same type. It was C++14's generalized lambda capture that made it *possible* for `lambda::x` and `x` to have different types. So the fact that it represents a breaking change may not be that important. I agree that you should try to move forward on it. If the committee wants to reject it due to being a breaking change, so be it. But you should note that it is only breaking when using generalized capture. -- 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. To view this discussion on the web visit https://groups.google.com/a/isocpp.org/d/msgid/std-proposals/1ecc764b-3dce-49ff-86b0-4d1505138fd0%40isocpp.org. ------=_Part_394_1631465146.1481821856841 Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable


On Thursday, December 15, 2016 at 12:00:35 PM UTC-= 5, Vittorio Romeo wrote:

I would like to write a proposal, but I just realized that= it would be a breaking change:
http://melpon.org/wandbox/permlink/= g9eAszk1SoGPA1Lf


struct Foo { };

int main()
{
=C2=A0 =C2=A0 Foo x;
<= span style=3D"font-family:Consolas,monospace">=C2=A0 =C2=A0 auto l =3D [x = =3D 10]() -> decltype(x) { return 10; };
} =C2=A0

If =C2=A0the proposal is accepted, this code will behave differently= !
This still feels very counter-intuitive though...=C2=A0I'm incline= d to think it's worth=C2=A0bringing this to the attention of the commit= tee anyway.



Wait a minute. This would= n't have been a breaking change until C++14. Back then, if you decl= ared a capture, then there must have been a variable outside the lam= bda with that exact name and type. So in that case, `[x]() -> decltype(x= ) {...}` would be fine, since `lambda::x` and `x` have the same type.
It was C++14's generalized lambda capture that made it possible for `lambda::x` and `x` to have different types. So the fact that it repr= esents a breaking change may not be that important.

I agree that you= should try to move forward on it. If the committee wants to reject it due = to being a breaking change, so be it. But you should note that it is only b= reaking when using generalized capture.

--
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To unsubscribe from this group and stop receiving emails from it, send an e= mail to std-proposa= ls+unsubscribe@isocpp.org.
To post to this group, send email to std-proposals@isocpp.org.
To view this discussion on the web visit https://groups.google.= com/a/isocpp.org/d/msgid/std-proposals/1ecc764b-3dce-49ff-86b0-4d1505138fd0= %40isocpp.org.
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