math/vtk9: fix build with clang and 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 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/
PR: 280893
Approved by: yuri (maintainer)
MFH: 2024Q3