On Wed, 16 Apr 2025 at 15:22, Jonathan Wakely <jwak...@redhat.com> wrote:
>
> On Wed, 16 Apr 2025 at 12:59, Tomasz Kaminski <tkami...@redhat.com> wrote:
> >
> >
> >
> > On Wed, Apr 16, 2025 at 1:32 PM Jonathan Wakely <jwak...@redhat.com> wrote:
> >>
> >> On Wed, 16 Apr 2025 at 12:18, Jonathan Wakely <jwak...@redhat.com> wrote:
> >> >
> >> > This fixes:
> >> > FAIL: 17_intro/headers/c++1998/operator_names.cc -std=gnu++23 (test for
> >> > excess errors)
> >> > FAIL: 17_intro/headers/c++1998/operator_names.cc -std=gnu++26 (test for
> >> > excess errors)
> >> >
> >> > The purpose of 'not defined<format_kind<R>>' is to be ill-formed (as
> >> > required by [format.range.fmtkind]) and to give an error that includes
> >> > the string "not defined<format_kind<R>>". That was intended to tell you
> >> > that format_kind<R> is not defined, just like it says!
> >> >
> >> > But user code can use -fno-operator-names so we can't use 'not' here,
> >> > and "! defined" in the diagnostic doesn't seem as user-friendly. It also
> >> > raises questions about whether it was intended to be the preprocessor
> >> > token 'defined' (it's not) or where 'defined' is defined (it's not).
> >> >
> >> > Replace it with __no_primary_template<format_kind<R>> and a comment,
> >> > which seems almost as good. The diagnostic now looks like:
> >> >
> >> > In file included from fmt.cc:1:
> >> > .../include/c++/15.0.1/format: In instantiation of 'constexpr const auto
> >> > std::format_kind<int>':
> >> > fmt.cc:3:15: required from here
> >> > 3 | auto i = std::format_kind<int>;
> >> > | ^~~~~~~~~~~~~~~~
> >> > .../include/c++/15.0.1/format:5164:31: error: use of
> >> > 'std::format_kind<int>' before deduction of 'auto'
> >> > 5164 | = __no_primary_template(format_kind<_Rg>); // must define
> >> > specialization
> >> > | ^~~~~~~~~~~~~~~~
> >> > .../include/c++/15.0.1/format:5164:30: error: '__no_primary_template'
> >> > was not declared in this scope
> >> > 5164 | = __no_primary_template(format_kind<_Rg>); // must define
> >> > specialization
> >> > | ~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~
> >>
> >> Maybe "must define specialization" isn't really ideal, because the
> >> problem might be that users are trying to use format_kind<const R>
> >> when they should use format_kind<R>, and telling them to define a
> >> specialization for const R is wrong.
> >
> > I do not think that user's are expected to use format_kind directly
> > will be confused by current specialization. I think you could adjust the
> > comment:
> > // must define specialization or _Rg is reference or cv-qualified type
>
> In general what appears in the diagnostic will only be one line, so if
> we're adding a comment that we expect users to see, it needs to be
> quite short.
>
> Although we could force it to span multiple lines like this:
>
> In file included from fmt.cc:1:
> .../include/c++/15.0.1/format: In instantiation of 'constexpr const
> auto std::format_kind<int>':
> fmt.cc:3:15: required from here
> 3 | auto i = std::format_kind<int>;
> | ^~~~~~~~~~~~~~~~
> .../include/c++/15.0.1/format:5165:11: error: use of
> 'std::format_kind<int>' before deduction of 'auto'
> 5165 | format_kind<_Rg> // can specialize this for non-const
> input ranges
> | ^~~~~~~~~~~~~~~~
> .../include/c++/15.0.1/format:5164:16: error:
> '__primary_template_not_defined' was not declared in this scope
> 5164 | = __primary_template_not_defined(
> | ~~~~~~~^
> 5165 | format_kind<_Rg> // can specialize this for non-const
> input ranges
> |
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> 5166 | );
> | ~
This looks reasonable with Clang too:
In file included from fmt.cc:1:
.../include/c++/15.0.1/format:5164:9: error: use of undeclared
identifier '__primary_template_not_defined'
5164 | = __primary_template_not_defined(
| ^
.../include/c++/15.0.1/format:5165:11: note: in instantiation of
variable template specialization 'std::format_kind<int>' requested
here
5165 | format_kind<_Rg> // can specialize this for non-const
input ranges
| ^
fmt.cc:3:15: note: in instantiation of variable template
specialization 'std::format_kind' requested here
3 | auto i = std::format_kind<int>;
| ^
1 error generated.
