[Bug 281478] devel/libxsd-frontend: fix build with libc++ 19

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=281478

--- Comment #2 from commit-hook at FreeBSD.org ---
A commit in branch main references this bug:

URL:
https://cgit.FreeBSD.org/ports/commit/?id=c1c4aae636e9e8c5fc146caeb9ad734e20f97115

commit c1c4aae636e9e8c5fc146caeb9ad734e20f97115
Author:     Dimitry Andric <dim at FreeBSD.org>
AuthorDate: 2024-09-13 10:36:00 +0000
Commit:     Dimitry Andric <dim at FreeBSD.org>
CommitDate: 2024-09-28 09:45:51 +0000

    devel/libxsd-frontend: fix build with libc++ 19

    As noted in the libc++ 19 release notes [1], std::char_traits<> is now
    only provided for char, char8_t, char16_t, char32_t and wchar_t, and any
    instantiation for other types will fail.

    This causes www/qt5-webengine to fail to compile with clang 19 and
    libc++ 19, resulting in errors similar to:

      /usr/include/c++/v1/string:820:42: error: implicit instantiation of
undefined template 'std::char_traits<XSDFrontend::Bits::None>'
        820 |   static_assert(is_same<_CharT, typename
traits_type::char_type>::value,
            |                                          ^
     
/wrkdirs/usr/ports/devel/libxsd-frontend/work/libxsd-frontend-2.1.0/libxsd-frontend/xml.hxx:100:23:
note: in instantiation of template class
'std::basic_string<XSDFrontend::Bits::None>' requested here
        100 |       NarrowString r (s);
            |                       ^
      /usr/include/c++/v1/__fwd/string.h:23:29: note: template is declared here
         23 | struct _LIBCPP_TEMPLATE_VIS char_traits;
            |                             ^

    The `Bits::None` construct used by libxsd-frontend is meant to disable
    unwanted conversions, but it no longer works if `std::char_traits>` is
    undefined for this type.

    Fix it by removing the `Bits` namespace and only enabling the converting
    constructors if the first type of `StringTemplate` is `wchar_t`.

    [1] https://libcxx.llvm.org/ReleaseNotes/19.html#deprecations-and-removals

    PR:             281478
    Approved by:    maintainer timeout (2 weeks)
    MFH:            2024Q3

 .../files/patch-libxsd-frontend_types.hxx (new)    | 62 ++++++++++++++++++++++
 1 file changed, 62 insertions(+)

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