220 39168 <79029827-71df-4af1-ae50-607e96c5e322@isocpp.org> article Path: news.gmane.org!.POSTED!not-for-mail From: toby.brull@gmail.com Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Concepts requires clause should allow types that constexpr implicitly convert to bool? Date: Sat, 14 Jul 2018 17:31:00 -0700 (PDT) Lines: 97 Approved: news@gmane.org Message-ID: <79029827-71df-4af1-ae50-607e96c5e322@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: multipart/mixed; boundary="----=_Part_134683_1546255666.1531614660505" X-Trace: blaine.gmane.org 1531614537 6329 195.159.176.226 (15 Jul 2018 00:28:57 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Sun, 15 Jul 2018 00:28:57 +0000 (UTC) To: ISO C++ Standard - Future Proposals Original-X-From: std-proposals+bncBCSZVKUG2IJRBRNLVLNAKGQEDNT6YUQ@isocpp.org Sun Jul 15 02:28:53 2018 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-yw0-f200.google.com ([209.85.161.200]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1feUuB-0001YG-Vj for gclcip-std-proposals@m.gmane.org; Sun, 15 Jul 2018 02:28:52 +0200 Original-Received: by mail-yw0-f200.google.com with SMTP id p8-v6sf1151846ywl.14 for ; Sat, 14 Jul 2018 17:31:03 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=isocpp-org.20150623.gappssmtp.com; s=20150623; h=date:from:to:message-id:subject:mime-version:x-original-sender :reply-to:precedence:mailing-list:list-id:list-post:list-help :list-archive:list-subscribe:list-unsubscribe; bh=fWLeiWKD1XysH4A4Sl1rdNLM8GexYBFVu+mY3+toKhs=; b=kRefM4d/7PnO8S5hR6eXZQjwTXJQwhhfwXtijsIr1oU/XMj++PwuYigJCOdRysIKky A/ANp7UBpqC7Uiipb3hobUmJeaSfaBEqA4KpKqdYI2GuqYxpnZxdS/2Eg2W+3XkiXPFq ccJ6LaNI5ZODD35rSo+I0gEtDZAAmd72lI3Wjdie74uU+JCgR/gR6xUbNSJoR8hD/COh XzukHu7EiPdFIJ5o43/UwXBN4pZ0CjtWHEc/NKGCZVZcPCYBClvmtf9LbhBbaT25/tJx UouS2tUbnWiaS/eQ6Cnt2a4H1PwHWjbd8KBGsZRDNzGC+okaR3Yt+rkw+cJS/AyhBZhX 1y8g== DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=date:from:to:message-id:subject:mime-version:x-original-sender :reply-to:precedence:mailing-list:list-id:list-post:list-help :list-archive:list-subscribe:list-unsubscribe; bh=fWLeiWKD1XysH4A4Sl1rdNLM8GexYBFVu+mY3+toKhs=; b=AYcG/aOycyTcgVeh0SVxPsWMY3SRGRbmoaGWa7W9vUGi/i2Xj7ZchyYR2YCm7OHB4c Tqt4OTOhe3iUR7OVI0BGHPibA68d9cDwtFBdRI5JhfsOYQJZwtWoFuo06zKRDdTbNBEJ B2W963FBVwaN22IxheSXTsK1lE8B4MYlXg2tsArZauNZUs3xOwyEBAmNkkrnG9q4uybv PPBAPp6xiH8ztBB5YGw3jFeSqRhgSegiYv0F473Y4dv9Eu/totnOr1kMMvb9wUSidit3 j+7zfslQY6rtB94wPGF8mqeO8B5KZq6nzahRQkH6D26ayRIMdicGtSo/dOilA/q0PqQE qb9Q== 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: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=fWLeiWKD1XysH4A4Sl1rdNLM8GexYBFVu+mY3+toKhs=; b=c6/ahVVHksA1bBMkq4hRvSrrSnSrwkvPYyzEJZtuNMb1UR4w7vJLlVg5kD8FvaAQYd xxcXQO0sGOYOOS36LogYYmLTgk+r79QYbLGveED7vVbmipMp/72EZ7izldSaxu5opWSy FlkxwhDtW4G88cBWZ6DUXosHiSVrp3NEYIC75Rwg3v9c/y17OtW0s9DIyA5eQ/rlw0WW QruZqZPdw1L3x1RWb97pmCKfRj1dbjQMTlyDPqb3Kex+K2AgNo9Q0wSFn6NSUIMMwCBM Wbmz92D0+V0j5D06sRY0ihAQl7d3e05Hgqt0I3Fhlepci9sh5hW/d8Vg/2gxnk/1yiVo jE8g== X-Gm-Message-State: AOUpUlHXYYerYO4YvylvQSMqWOJqhMNDR+FHqPZ7cb52mjYVs9wkN2bg IT8xUB26UQT2HKdlVTYd1p2w7A== X-Google-Smtp-Source: AAOMgpeCgAW/756Da3mlAxUXErk47Xk9pV5n8W0uo7CPvSpKVfzQTDqhYotwsOIbbtU2Z32lhWZ9jA== X-Received: by 2002:a81:ac0a:: with SMTP id k10-v6mr3550451ywh.135.1531614662783; Sat, 14 Jul 2018 17:31:02 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 2002:a25:3881:: with SMTP id f123-v6ls7830287yba.16.gmail; Sat, 14 Jul 2018 17:31:01 -0700 (PDT) X-Received: by 2002:a5b:349:: with SMTP id q9-v6mr1366942ybp.7.1531614661166; Sat, 14 Jul 2018 17:31:01 -0700 (PDT) X-Original-Sender: toby.brull@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:39168 Archived-At: ------=_Part_134683_1546255666.1531614660505 Content-Type: multipart/alternative; boundary="----=_Part_134684_1833482427.1531614660506" ------=_Part_134684_1833482427.1531614660506 Content-Type: text/plain; charset="UTF-8" Apologies, if this is off-topic. It seems that the following doesn't compile in all versions of gcc that I tried (7.2, 8.1, 9.0). It complains that the type of the value in the "requires" clause is not bool, although the value is implicitly constexpr convertible to bool. template constexpr auto is_even (std::integral_constant = {}) { return std::bool_constant<(v % 2 == 0)> {}; } template requires is_even (std::integral_constant {}) // (*) int some_function () { return (v * 3); } int main () { std::cout << some_function () << '\n'; } Workarounds are, of course, to append "()" or to prepend "!!" to the line marked with (*), but might be better without? If this is not a bug in gcc, I hoped someone would be able to make that change easily at this point. -- 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. 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Apologies, if this is off-topic.

=
It seems that the following doesn't compile in all versions of gcc= that I tried (7.2, 8.1, 9.0). It complains that the type of the value in t= he "requires" clause is not bool, although the value is implicitl= y constexpr convertible to bool.


template<typename T, T v>
=
constexpr auto is_even (std::int= egral_constant<T, v> =3D {})
{
=C2= =A0 return std::bool_constant<(v % 2 =3D=3D 0)> {};
= }

template<typename T, T v>
=C2=A0 requires is_even (std::integral_constant<T, v>= ; {})=C2=A0 // (*)
i= nt some_function ()
= {
=C2=A0 return (v *= 3);
}
<= div>
int main ()
{
}

Workarounds are, of course, to append &qu= ot;()" or to prepend "!!" to the line marked with (*), but m= ight be better without? If this is not a bug in gcc, I hoped someone would = be able to make that change easily at this point.

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To view this discussion on the web visit https://groups.google.= com/a/isocpp.org/d/msgid/std-proposals/79029827-71df-4af1-ae50-607e96c5e322= %40isocpp.org.
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