Re: [PATCH] PR libstdc++/89728 diagnose some missuses of [locale.convenience] functions

[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?