[Bug 280893] math/vtk9: fix build with clang and libc++ 19

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=280893

            Bug ID: 280893
           Summary: math/vtk9: fix build with clang and libc++ 19
           Product: Ports & Packages
           Version: Latest
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Some People
          Priority: ---
         Component: Individual Port(s)
          Assignee: yuri@freebsd.org
          Reporter: dim@FreeBSD.org
          Assignee: yuri@freebsd.org
             Flags: maintainer-feedback?(yuri@freebsd.org)

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 math/vtk9 to fail to compile with clang 19 and libc++ 19,
resulting in errors similar to:

   
/wrkdirs/usr/ports/math/vtk9/work/VTK-9.2.6/ThirdParty/diy2/vtkdiy2/include/vtkdiy2/fmt/core.h:272:30:
error: implicit instantiation of undefined template
'std::char_traits<fmt::char8_t>'
      272 |       : data_(s), size_(std::char_traits<Char>::length(s)) {}
          |                              ^
   
/wrkdirs/usr/ports/math/vtk9/work/VTK-9.2.6/ThirdParty/diy2/vtkdiy2/include/vtkdiy2/fmt/format.h:487:9:
note: in instantiation of member function
'fmt::basic_string_view<fmt::char8_t>::basic_string_view' requested here
      487 |       : basic_string_view<char8_t>(reinterpret_cast<const
char8_t*>(s)) {}
          |         ^
    /usr/include/c++/v1/__string/char_traits.h:45:8: note: template is declared
here
       45 | struct char_traits;
          |        ^

This is actually a problem in the vendored version of Victor Zverovich's
fmt library [2]. More recent versions of this library have completely
rewritten this part of the header, so work around the issue by making
the basic_string_view<char8_t> part conditional on char8_t being
actually available. (This is only in C++20 and later.)

Clang 19 now also diagnoses incorrect member accesses, which causes
errors similar to:

   
/wrkdirs/usr/ports/math/vtk9/work/VTK-9.2.6/Utilities/octree/octree/octree_node.txx:210:16:
error: no member named '_M_chilren' in 'octree_node<T_, d_, A_>'; did you mean
'm_children'?
      210 |   return this->_M_chilren[child];
          |                ^~~~~~~~~~
          |                m_children
   
/wrkdirs/usr/ports/math/vtk9/work/VTK-9.2.6/Utilities/octree/octree/octree_node.h:37:23:
note: 'm_children' declared here
       37 |   octree_node_pointer m_children;
          |                       ^

This was apparently a typo, and upstream vtk fixed this in
<https://gitlab.kitware.com/vtk/vtk/-/commit/98af50ca33>. Apply that as
a small patch.

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

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