220 33215 article Path: news.gmane.org!.POSTED!not-for-mail From: Nicole Mazzuca Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Idea for a proposal Date: Sat, 15 Jul 2017 13:03:07 -0700 Lines: 294 Approved: news@gmane.org Message-ID: References: <915caa4a-875c-2540-6941-fbe019aa2b47@gmail.com> <15d714d7-4454-4022-87f0-808a60619f05@isocpp.org> <65ad7b18-0aa5-49a8-bd19-1887cc8b7649@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: multipart/alternative; boundary="001a114274ba8a8a25055460a7de" X-Trace: blaine.gmane.org 1500148996 20754 195.159.176.226 (15 Jul 2017 20:03:16 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Sat, 15 Jul 2017 20:03:16 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBD4ITEVL7ABBB7HJVHFQKGQEZPEHO7Q@isocpp.org Sat Jul 15 22:03:12 2017 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-yb0-f198.google.com ([209.85.213.198]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1dWTHN-0004mX-4L for gclcip-std-proposals@m.gmane.org; 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[2607:f8b0:4001:c0b::234]) by mx.google.com with ESMTPS id a6si5767147ita.54.2017.07.15.13.03.08 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Sat, 15 Jul 2017 13:03:08 -0700 (PDT) Received-SPF: pass (google.com: domain of npmazzuca@gmail.com designates 2607:f8b0:4001:c0b::234 as permitted sender) client-ip=2607:f8b0:4001:c0b::234; Original-Received: by mail-it0-x234.google.com with SMTP id h199so4460685ith.1 for ; Sat, 15 Jul 2017 13:03:08 -0700 (PDT) X-Received: by 10.36.7.202 with SMTP id f193mr183815itf.80.1500148988019; Sat, 15 Jul 2017 13:03:08 -0700 (PDT) Original-Received: by 10.107.52.16 with HTTP; Sat, 15 Jul 2017 13:03:07 -0700 (PDT) Original-Received: by 10.107.52.16 with HTTP; Sat, 15 Jul 2017 13:03:07 -0700 (PDT) In-Reply-To: <65ad7b18-0aa5-49a8-bd19-1887cc8b7649@isocpp.org> X-Original-Sender: npmazzuca@gmail.com X-Original-Authentication-Results: mx.google.com; dkim=pass header.i=@gmail.com header.b=OMG2jRBX; spf=pass (google.com: domain of npmazzuca@gmail.com designates 2607:f8b0:4001:c0b::234 as permitted sender) smtp.mailfrom=npmazzuca@gmail.com; dmarc=pass (p=NONE sp=NONE dis=NONE) header.from=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:33215 Archived-At: --001a114274ba8a8a25055460a7de Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable Hmm, that makes the current semantics even more strange to me. It's at least, imo, not very clear. However, if those are the correct semantics, I guess this is resolved. On Jul 12, 2017 14:36, "Barry Revzin" wrote: > What it means right now is: > > Concept foo(); > > becomes > > template requires Concept T foo(); > > I don't think that's true? The wording in N4674 is A constrained-type-specifier C1 within the declared return type of an > abbreviated function template declaration does not designate a placeholde= r > if its introduced constraint-expression (10.1.7.4.2) is determined to be > equivalent, using the rules in 17.6.5.1 for comparing expressions, to the > introduced constraint-expression for a constrained-type-specifier C2 in t= he > parameter-declaration-clause of that function declaration. Instead, C1 is > replaced by the template parameter invented for C2 (11.3.5). Since Concept isn't used anywhere else, then it still counts as a placeholder. So: Concept foo(Concept ); // <=3D=3D> template requires Concept // T foo(T ); Concept foo(); // <=3D=3D> auto foo(); // static_assert(Concept); There's also examples like: template concept bool C =3D true; template concept bool D =3D false; C z1 =3D 0; // OK: z1 has type int D z2 =3D 0; // error: constraints not satisfied C cf1() { return 0.0; }; // OK: cf1 returns double D cf2() { return 0.0; }; // error: constraints not satisfied auto cf3() -> C; // OK: cf3=E2=80=99s return type will be deduced when it i= s defined --=20 You received this message because you are subscribed to a topic in the Google Groups "ISO C++ Standard - Future Proposals" group. To unsubscribe from this topic, visit https://groups.google.com/a/ isocpp.org/d/topic/std-proposals/lzIEK6_-J84/unsubscribe. To unsubscribe from this group and all its topics, 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/65ad7b18-0aa5-49a8- bd19-1887cc8b7649%40isocpp.org .. --=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. To view this discussion on the web visit https://groups.google.com/a/isocpp= ..org/d/msgid/std-proposals/CAEwx-OsVLrYQkGxLBmwoGvvCSRYPofihR6Xa9R1P-F3aEa1= ucw%40mail.gmail.com. --001a114274ba8a8a25055460a7de Content-Type: text/html; charset="UTF-8" Content-Transfer-Encoding: quoted-printable
Hmm, that makes the current semantics even more strange t= o me. It's at least, imo, not very clear. However, if those are the cor= rect semantics, I guess this is resolved.
<= br>
On Jul 12, 2017 14:36, "Barry Revzin&quo= t; <barry.revzin@gmail.com= > wrote:

What it means right now is:

Concept foo();

becomes

template<typename T> requires Concept<= T> T foo();=


I= don't think that's true? The wording in N4674 is=C2=A0
<= br>
A constrained-ty= pe-specifier C1 within the declared return type of an abbreviated function = template declaration does not designate a placeholder if its introduced constraint-e= xpression (10.1.7.4.2) is determined to be equivalent, using the rules in 17.6.5.1 for comparing e= xpressions, to the introduced constraint-expression for a constrained-type-specifier C2 in the parameter-= declaration-clause of that function declaration. Instead, C1 is replaced by the template param= eter invented for C2 (11.3.5).

Since Concept isn't used anyw= here else, then it still counts as a placeholder. So:

<= div>
Concept<= span style=3D"color:#000" class=3D"m_6689280322255370458styled-by-prettify"= > foo(Concept ); // <=3D=3D> template <class T> requires Con= cept<T>
=C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0= =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0// =C2=A0 =C2=A0 =C2=A0T foo(T= );

Concept foo(); =C2= =A0 =C2=A0 =C2=A0 =C2=A0 // <=3D=3D> auto foo(); =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2= =A0 =C2=A0// =C2=A0 =C2=A0 =C2=A0static_assert(Concept<dec= ltype(foo())>);

There's also exa= mples like:

template <typename<= /span> T> concept bool C =3D true;
<= span style=3D"color:#008" class=3D"m_6689280322255370458styled-by-prettify"= >template <typename T= > concept bool D =3D false;
C z1 =3D 0; // OK: z1 has type int<= span style=3D"color:#000" class=3D"m_6689280322255370458styled-by-prettify"= >
D z2 =3D 0; // e= rror: constraints not satisfied
C cf1() { return 0.0; }; // OK: cf1 returns double
D cf2() { return 0.0; }; // error: constraints not s= atisfied
auto cf3() -> C; // OK: cf3=E2=80=99s retur= n type will be deduced when it is defined


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