220 39170 article Path: news.gmane.org!.POSTED!not-for-mail From: Nicolas Lesser Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Concepts requires clause should allow types that constexpr implicitly convert to bool? 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[209.85.220.41]) by mx.google.com with SMTPS id d89-v6sor17401334oic.136.2018.07.15.02.38.42 for (Google Transport Security); Sun, 15 Jul 2018 02:38:42 -0700 (PDT) Received-SPF: pass (google.com: domain of blitzrakete@gmail.com designates 209.85.220.41 as permitted sender) client-ip=209.85.220.41; X-Received: by 2002:aca:5155:: with SMTP id f82-v6mr14374204oib.272.1531647522202; Sun, 15 Jul 2018 02:38:42 -0700 (PDT) In-Reply-To: <79029827-71df-4af1-ae50-607e96c5e322@isocpp.org> X-Original-Sender: blitzrakete@gmail.com X-Original-Authentication-Results: mx.google.com; dkim=pass header.i=@gmail.com header.s=20161025 header.b=laM9tUlm; spf=pass (google.com: domain of blitzrakete@gmail.com designates 209.85.220.41 as permitted sender) smtp.mailfrom=blitzrakete@gmail.com; dmarc=pass (p=NONE sp=QUARANTINE dis=NONE) header.from=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:39170 Archived-At: --0000000000007b6afa0571067af8 Content-Type: text/plain; charset="UTF-8" Yes, this is intended. See [temp.constr.atomic]p3. And I agree that any type implicitly convertible to bool should be allowed. On Sun, Jul 15, 2018, 2:31 AM wrote: > 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. > 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/79029827-71df-4af1-ae50-607e96c5e322%40isocpp.org > > . > -- 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/CALmDwq07Aispgz_-pTGix%3D1vGpF1QupHUFgzw9D0bggGQL5_oQ%40mail.gmail.com. --0000000000007b6afa0571067af8 Content-Type: text/html; charset="UTF-8" Content-Transfer-Encoding: quoted-printable
Yes, this is intended. See [temp.constr.atomic]p3.

And I agree that any type= implicitly convertible to bool should be allowed.
<= br>
On Sun, Jul 15,= 2018, 2:31 AM <toby.brull@gmai= l.com> wrote:
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 "requi= res" clause is not bool, although the value is implicitly constexpr co= nvertible to bool.


template<typename T, T v>
constexpr auto is_even (std::integral_constant<T,= v> =3D {})
{
=C2=A0 return std::bool= _constant<(v % 2 =3D=3D 0)> {};
}
=
template<typ= ename T, T v>
=C2= =A0 requires is_even (std::integral_constant<T, v> {})=C2=A0 // (*)
int some_function ()<= /font>
{
<= font face=3D"courier new, monospace">=C2=A0 return (v * 3);
}

int main ()
{=
=C2=A0 std::cout &l= t;< some_function<int, 4> () << '\n';
}
Workarounds are, of course, to append "()" or to pre= pend "!!" to the line marked with (*), but might be better withou= t? 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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