ср, 12 мая 2021 г. в 18:38, Antony Polukhin <antoshkka@gmail.com>:
>
> ср, 12 мая 2021 г. в 17:44, Jonathan Wakely <jwakely@redhat.com>:
> >
> > On 12/05/21 12:58 +0300, Antony Polukhin wrote:
> > >ср, 12 мая 2021 г. в 12:18, Jonathan Wakely <jwakely@redhat.com>:
> > ><...>
> > >> Or just leave it undefined, as libc++ seems to do according to your
> > >> comment in PR 89728:
> > >>
> > >> error: implicit instantiation of undefined template 'std::__1::ctype<std::__1::basic_string<char> >'
> > >>
> > >> Was your aim to have a static_assert that gives a more descriptive
> > >> error? We could leave it undefined in C++98 and have the static assert
> > >> for C++11 and up.
> > >
> > >Leaving it undefined would be the best. It would allow SFINAE on ctype
> > >and a compile time error is informative enough.
> > >
> > >However, there may be users who instantiate ctype<ThierChar> in a
> > >shared library without ctype<ThierChar> template specializations in
> > >the main executable. Making the default ctype undefined would break
> > >their compilation:
> > >
> > >#include <locale>
> > >// no ctype<ThierChar> specialization
> > >c = std::tolower(ThierChar{42}, locale_from_shared_library()); // OK
> > >right now in libstdc++, fails on libc++
> >
> > What I meant was leaving the partial specialization undefined, not the
> > primary template, i.e.
> >
> > --- a/libstdc++-v3/include/bits/locale_facets.h
> > +++ b/libstdc++-v3/include/bits/locale_facets.h
> > @@ -1476,6 +1476,9 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> >       };
> >   #endif //_GLIBCXX_USE_WCHAR_T
> >
> > +  template<typename _CharT, typename _Traits, typename _Alloc>
> > +    class ctype<basic_string<_CharT, _Traits, _Alloc> >;
> > +
> >     /// class ctype_byname [22.2.1.2].
> >     template<typename _CharT>
> >       class ctype_byname : public ctype<_CharT>
> >
> > This makes your test fail with errors like this:
> >
> > In file included from /home/jwakely/gcc/12/include/c++/12.0.0/locale:40,
> >                   from loc.C:1:
> > /home/jwakely/gcc/12/include/c++/12.0.0/bits/locale_facets.h: In instantiation of 'bool std::isspace(_CharT, const std::locale&) [with _CharT = std::__cxx11::basic_string<char>]':
> > loc.C:16:15:   required from here
> > /home/jwakely/gcc/12/include/c++/12.0.0/bits/locale_facets.h:2600:47: error: invalid use of incomplete type 'const class std::ctype<std::__cxx11::basic_string<char> >'
> >   2600 |     { return use_facet<ctype<_CharT> >(__loc).is(ctype_base::space, __c); }
> >        |              ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~
> >
> > But it shouldn't affect the uses of ctype<TheirChar>.
> >
> > What do you think?
>
> Good idea. That way the compiler message points directly to the
> misused function.
>
> Patch is in attachment

Replaced {} with () in test to be C++98 compatible


-- 
Best regards,
Antony Polukhin