220 34173 <481b4d44-74e0-4cd3-8141-62d35dea8350@isocpp.org> article Path: news.gmane.org!.POSTED!not-for-mail From: "T. C." Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Structured bindings with polymorphic lambdas Date: Mon, 28 Aug 2017 01:38:04 -0700 (PDT) Lines: 242 Approved: news@gmane.org Message-ID: <481b4d44-74e0-4cd3-8141-62d35dea8350@isocpp.org> References: <8497b31b-7c9d-4334-ac22-0809c958cb93@isocpp.org> <15a70d94-e039-4f82-ba01-bde9d3fbacbd@isocpp.org> <6b50933d-291b-4b47-8c0c-82c97d9cb406@isocpp.org> <348b59eb-c288-482e-a7f4-473d8f1fbfea@isocpp.org> <029d244a-cc53-40de-9c2f-59b1bc776c1b@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: multipart/mixed; boundary="----=_Part_6283_1253384204.1503909484358" X-Trace: blaine.gmane.org 1503909492 28287 195.159.176.226 (28 Aug 2017 08:38:12 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Mon, 28 Aug 2017 08:38:12 +0000 (UTC) Cc: rmn100@gmail.com To: ISO C++ Standard - Future Proposals Original-X-From: std-proposals+bncBCQ43G7NQIIRB3NMR7GQKGQEEDGEXDA@isocpp.org Mon Aug 28 10:38:07 2017 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-pf0-f197.google.com ([209.85.192.197]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1dmFYW-0006kC-2V for gclcip-std-proposals@m.gmane.org; Mon, 28 Aug 2017 10:38:00 +0200 Original-Received: by mail-pf0-f197.google.com with SMTP id 85sf31719425pfo.14 for ; Mon, 28 Aug 2017 01:38:07 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=isocpp-org.20150623.gappssmtp.com; s=20150623; h=date:from:to:cc:message-id:in-reply-to:references:subject :mime-version:x-original-sender:reply-to:precedence:mailing-list :list-id:list-post:list-help:list-archive:list-subscribe :list-unsubscribe; bh=qwHIgipwDJOMlPFoMVPqs4FbAqrq67bfXvz/KhVXTb4=; b=XDtmeA/RMyVF6cBdk9byFud5zDERGMDMxy4Opr9WSS673hmYqui9MFDFHm+V/st1yK 6OlFWfjg2sqNfF+DSaBvHv+oyC8D0/haddlSJbb8+/Dt+I0mhd4HLO0SWpQTK5xX7gOS TOPIPQg40u9HtqFVHoYBeV7ZgxN0eVOuYMic7ROpJCsXreFpGCKHS+NcCPSmtdW/mhqH 8UxmsZXxZlB3wL/x236fXhxdgDn8QL5o5ZhPW3ITXuCqBPfYLXmxEU46o9sQWkkPml3n d7EWq8ieVvd4v2nY6mp4DbtsiWpXJgnI/dXkYJ/bqhSSsaMYeL/2I5beWWsGQviGBG1d /Tog== DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=date:from:to:cc:message-id:in-reply-to:references:subject :mime-version:x-original-sender:reply-to:precedence:mailing-list :list-id:list-post:list-help:list-archive:list-subscribe :list-unsubscribe; bh=qwHIgipwDJOMlPFoMVPqs4FbAqrq67bfXvz/KhVXTb4=; b=VA1c32jfpdCHVS3HNcolz7GogUGGFkSGW1c3IO/H0pmgbUz29NVBVLcjJYeUh3qppW sj8gXS24ZA0DiH+pgosZHjBSoFxzMCc0fAvu15dZh5SDg77y/ERG8dmXsBnwTUwlWYVk OdFEI80Q5Smw6SrCJ6QrWtQAkxnpoP5H5VyZhHqTVWXs3VOQOCtqeF0uNwBAAIRkcrzu yeasrcYj2uXzOjh1F6/YL2EhOqQ/z91FXPCRAu4AIftWS6ML7+Ar3p74A9CgKOLuy7gp yjbqaOVHQ5Qj0Mvh5i+8h6D4bwCayGYCALOUuB3Lc/7y5o8L0DIlHkkigZfG2TtSGvZ9 pNtQ== 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:cc: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=qwHIgipwDJOMlPFoMVPqs4FbAqrq67bfXvz/KhVXTb4=; b=iOGQwp3nsAbU6/lJgZNZ3RCSn8s63SZFDUwK+N8u/7ryEzRPGV348avpwyF36v+cJC t04GUxUnQgzr8ioezqQyjBUi1c+GIVMnTyJ9M97ZgBW5povoRxgHmoo/3zFrqWT6GpbM IeWPDKakjeuUpdfblUAZQ5zH8p1Ih02zr9BSTXjRnh9O40cGmca9AIuvfzy4qANCXXbv 0XLZ6TbL7yn3ehz/Fh2gFPSGxnp5Lr/sXMVJLxZxGbOg/PpxcmVZEGLxkz0FKYQTXdl8 jKpOLU2Qw+f7kGSDbBF9DrjzXBrzPqWx9LHijmcSbaHsoF7ypX7IildlIAXs+YbIr9H/ aUaQ== X-Gm-Message-State: AHYfb5jlBYalknk8EimGn9GsdOrl9D8ZPPqDNycKdx+3EVYyMDUKzx2M yn+7bxWXLLPpVfaq X-Received: by 10.98.101.199 with SMTP id z190mr4311205pfb.48.1503909486308; Mon, 28 Aug 2017 01:38:06 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.107.129.98 with SMTP id c95ls3226151iod.32.gmail; Mon, 28 Aug 2017 01:38:05 -0700 (PDT) X-Received: by 10.31.58.216 with SMTP id h207mr32542vka.16.1503909484936; Mon, 28 Aug 2017 01:38:04 -0700 (PDT) In-Reply-To: <029d244a-cc53-40de-9c2f-59b1bc776c1b@isocpp.org> X-Original-Sender: rs2740@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:34173 Archived-At: ------=_Part_6283_1253384204.1503909484358 Content-Type: multipart/alternative; boundary="----=_Part_6284_2019659389.1503909484359" ------=_Part_6284_2019659389.1503909484359 Content-Type: text/plain; charset="UTF-8" You don't have to have multiple translation units to run into ODR problems. Consider (the idea was first pointed out to me by Casey Carter a while ago): struct X { int a; }; template<> struct std::tuple_size; template struct std::tuple_element { using type = int; }; template int get(X) { return 1; } auto [a] = X(); // is_decomposable<1, X>::value == true ; is_decomposable<3, X>::value == false ? template<> struct std::tuple_size : std::integral_constant {}; auto [b, c, d] = X(); // auto [e] = X(); // error now // is_decomposable<1, X>::value == ? Also, to illustrate my question about extent of the check, what should the following code do? (This is basically GB 20's motivating example.) struct Y {}; template<> struct std::tuple_size { const int value = 2; /* note missing "static" */ }; // tuple_element/get omitted static_assert(is_decomposable<2, Y>::value); 1) fire the static assert 2) somehow not fire the static assert 3) cause a hard error 4) something else -- 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/481b4d44-74e0-4cd3-8141-62d35dea8350%40isocpp.org. ------=_Part_6284_2019659389.1503909484359 Content-Type: text/html; charset="UTF-8" Content-Transfer-Encoding: quoted-printable
You don't have to have multiple translation units= to run into ODR problems. Consider (the idea was first pointed out to me b= y Casey Carter a while ago):=C2=A0

struct X { int a; };

template<> struct std::tuple_size<X>;=
templa= te<size_t I> struct std::tuple_element<I, X&g= t; = { using type =3D int; = };<= span style=3D"color: #000;" class=3D"styled-by-prettify">
template<size_t I> int get(X) { return 1; }


auto [a] =3D X(); // is_decomposable<1, X&g= t;::value =3D=3D true ; is_decomposable<3, X>::value =3D=3D false ?


= template<> struct std::tuple_size<X> : std<= /span>::integral_constant<<= span style=3D"color: #000;" class=3D"styled-by-prettify">size_t, 3> {};
au= to = [b, c, d] <= /span>=3D<= span style=3D"color: #000;" class=3D"styled-by-prettify"> X();

// auto [e] =3D X(); // error now
<= span style=3D"color: #800;" class=3D"styled-by-prettify">// is_decomposable= <1, X>::value =3D=3D ?


Als= o, to illustrate my question about extent of the check, what should the fol= lowing code do? (This is basically GB 20's motivating example.)
struct Y {};template<= /span><> struct std::tuple_size<Y> { const<= span style=3D"color: #000;" class=3D"styled-by-prettify"> int value =3D 2; /* = note missing "static" */ };
// tuple_element/get omitted

static_assert(is_decomposable<2, Y>::value);

1) fire the static assert
2) somehow not= fire the static assert
3) cause a hard error
4) someth= ing else

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