220 39176 <0f77fb8d-5058-47e9-b1ec-519958786893@isocpp.org> article Path: news.gmane.org!.POSTED!not-for-mail From: Arthur O'Dwyer 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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I would expect requires(x) to follow exactly the same rules as noexcept(x).= =20 If it doesn't, that's a bug in my book. =E2=80=93Arthur On Sunday, July 15, 2018 at 2:38:44 AM UTC-7, Nicolas Lesser wrote: > > Yes, this is intended. See [temp.constr.atomic]p3. > > And I agree that any type implicitly convertible to bool should be allowe= d. > > 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=20 >> tried (7.2, 8.1, 9.0). It complains that the type of the value in the=20 >> "requires" clause is not bool, although the value is implicitly constexp= r=20 >> convertible to bool. >> >> >> template >> constexpr auto is_even (std::integral_constant =3D {}) >> { >> return std::bool_constant<(v % 2 =3D=3D 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 lin= e=20 >> marked with (*), but might be better without? If this is not a bug in gc= c,=20 >> I hoped someone would be able to make that change easily at this point. >> >> --=20 >> You received this message because you are subscribed to the Google Group= s=20 >> "ISO C++ Standard - Future Proposals" group. >> To unsubscribe from this group and stop receiving emails from it, send a= n=20 >> email to std-proposal...@isocpp.org . >> To post to this group, send email to std-pr...@isocpp.org . >> To view this discussion on the web visit=20 >> https://groups.google.com/a/isocpp.org/d/msgid/std-proposals/79029827-71= df-4af1-ae50-607e96c5e322%40isocpp.org=20 >> >> . >> > --=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/0f77fb8d-5058-47e9-b1ec-519958786893%40isocpp.or= g. ------=_Part_159665_1521112249.1531694692616 Content-Type: text/html; charset="UTF-8" Content-Transfer-Encoding: quoted-printable
Another easy workaround would be to change the word `auto`= to `bool`.
I would expect requires(x) to follow exactly the same rules= as noexcept(x). If it doesn't, that's a bug in my book.
=
=E2=80=93Arthur

On Sunday, July 15, 2018 at 2:38:44 A= M UTC-7, Nicolas Lesser wrote:
=
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....@gmail.com> wrote:
Apologies, if this is off-topic.
<= div>
It seems that the following doesn't compile in all v= ersions 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 valu= e is implicitly constexpr convertible 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<typename T, T v>
=C2=A0 requires is_even (std::integral_constant<= ;T, v> {})=C2=A0 // (*)
int some_function ()
{
=C2=A0 ret= urn (v * 3);
}

int main ()
{
=C2=A0 std::cout << some_function<int, 4> () <<= '\n';
}

Workarounds are, of course, to ap= pend "()" or to prepend "!!" to the line marked with (*= ), but might be better without? If this is not a bug in gcc, I hoped someon= e would be able to make that change easily at this point.

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