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From: "ppalka at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/100288] [11/12 Regression] g++-11 internal error and fails
 to precompile a concept
Date: Mon, 03 May 2021 14:01:24 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D100288

--- Comment #5 from Patrick Palka <ppalka at gcc dot gnu.org> ---
(In reply to Frank B. Brokken from comment #4)
> Dear ppalka at gcc dot gnu.org, you wrote:
> >=20
> > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D100288
> >=20
> > Patrick Palka <ppalka at gcc dot gnu.org> changed:
> >=20
> >            What    |Removed                     |Added
> > -----------------------------------------------------------------------=
-----
> >            Keywords|                            |ice-on-invalid-code
> >            See Also|                            |https://gcc.gnu.org/bu=
gzill
> >                    |                            |a/show_bug.cgi?id=3D99=
599
> >=20
> > --- Comment #3 from Patrick Palka <ppalka at gcc dot gnu.org> ---
> > Agreed, the testcase looks invalid much like PR99599.  One workaround f=
or
> > avoiding the constraint recursion here would be to change the signature=
 of
> >=20
> >   template <OstreamInsertable Type>
> >   inline void operator<<(CSVTabIns &&tab, Type const &value)
> >=20
> > to something like
> >=20
> >   template <std::same_as<std::remove_cvref_t<CSVTabIns>> U, OstreamInse=
rtable
> > Type>
> >   inline void operator<<(U &&tab, Type const &value)
> >=20
> > so that the constraint OstreamInsertable<Type> is checked on this overl=
oad only
> > if the first argument to << has the expected type.
> >=20
> > --=20
> > You are receiving this mail because:
> > You reported the bug.
>=20
> Hi Patrick,
>=20
> Thanks for your e-mail.
>=20
> Since (AFAICS) you directly and only sent your e-mail to me I'm wondering
> whether you want me to comment on your e-mail. It's getting kind of late
> here,
> but I could send a more extensive reply tomorrow. Let me know if that's w=
hat
> you want.

Oh sorry, that was just an automated email for the comment I posted to the
bugzilla PR, which gets sent to everyone on the CC list of the PR.  I wasn't
expecting any sort of reply :)

>=20
> I tried your suggestion, and it seems to solve the issue. But at the same
> time
> it puzzles me why=20
>=20
>     template <OstreamInsertable Type>
>     inline void operator<<(CSVTabIns &&tab, Type const &value)
>     {}
>=20
> won't be instantiated for FMT in
>=20
>     inline void operator<<(CSVTabIns &tab, FMT::FMTHline hline)
>     {
>         tab << (*hline)(1);      // insert hline in the next column
>     }
>=20
> when FMT defines FMTHline as  'typedef FMT (*FMTHline)(unsigned)' and=20
>=20
>     std::ostream &operator<<(std::ostream &out, FMT const &fmt)
>=20
> is declared. I would have expected that=20
>=20
>         tab << (*hline)(1);      // insert hline in the next column
>=20
> would cause the compiler to instantiate=20
>=20
>     template <OstreamInsertable Type>
>     inline void operator<<(CSVTabIns &&tab, Type const &value)
>     {}
>=20
> for Type =3D FMT.
>=20


The compiler does try to instantiate that overload for Type =3D FMT there, =
but
when checking the constraint OstreamInsertable<FMT> on this overload it nee=
ds
to resolve the << in

    requires(std::ostream &out, Type value)
    {
        out << value;
    };

for Type =3D FMT, during which it considers that same operator<< overload. =
 And
CWG2369 says we now first check constraints before non-dependent conversion=
s,
but we're in the middle of checking the constraints on this overload, so we
enter a constraint loop.

> Maybe I'm missing something here? And it's also not something that caused=
 the
> compiler's internal error.

Yeah, we shouldn't be seeing an internal error even on an invalid testcase.=
=20
This needs to be fixed.=