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

(cherry picked from commit ddd57b6891e71d29dc4a471978df9c27fa8cd0a4)

2 files changed, 29 insertions, 0 deletions

diff --git a/math/vtk9/files/patch-ThirdParty_diy2_vtkdiy2_include_vtkdiy2_fmt_format.h b/math/vtk9/files/patch-ThirdParty_diy2_vtkdiy2_include_vtkdiy2_fmt_format.h
new file mode 100644
index 000000000000..eb9c13efbe2f
--- /dev/null
+++ b/math/vtk9/files/patch-ThirdParty_diy2_vtkdiy2_include_vtkdiy2_fmt_format.h
@@ -0,0 +1,18 @@
+--- ThirdParty/diy2/vtkdiy2/include/vtkdiy2/fmt/format.h.orig	2023-02-15 04:03:53 UTC
++++ ThirdParty/diy2/vtkdiy2/include/vtkdiy2/fmt/format.h
+@@ -480,6 +480,7 @@ void buffer<T>::append(const U* begin, const U* end) {
+ }
+ }  // namespace internal
+ 
++#ifdef __cpp_char8_t
+ // A UTF-8 string view.
+ class u8string_view : public basic_string_view<char8_t> {
+  public:
+@@ -497,6 +498,7 @@ inline u8string_view operator"" _u(const char* s, std:
+ }
+ }  // namespace literals
+ #endif
++#endif // __cpp_char8_t
+ 
+ // The number of characters to store in the basic_memory_buffer object itself
+ // to avoid dynamic memory allocation.
diff --git a/math/vtk9/files/patch-Utilities_octree_octree_octree__node.txx b/math/vtk9/files/patch-Utilities_octree_octree_octree__node.txx
new file mode 100644
index 000000000000..de91955442bb
--- /dev/null
+++ b/math/vtk9/files/patch-Utilities_octree_octree_octree__node.txx
@@ -0,0 +1,11 @@
+--- Utilities/octree/octree/octree_node.txx.orig	2023-02-15 04:03:53 UTC
++++ Utilities/octree/octree/octree_node.txx
+@@ -207,7 +207,7 @@ const octree_node<T_, d_, A_>& octree_node<T_, d_, A_>
+   {
+     throw std::domain_error("Attempt to access children of an octree leaf node.");
+   }
+-  return this->_M_chilren[child];
++  return this->m_children[child];
+ }
+ 
+ /**\brief Return a reference to a child node.