VTK: fix clang compatibility (spack#50117)

This is an implementation of the patches from Dimitry Andric described here to fix compatibility with vtk 9.2+ and clang 19+ derivative compilers including clang and apple-clang (and oneapi, but I have not tested). The patch is safe (afaik) to apply to non-clang compilers.

I have split the original patch so each part can be applied individually.

Patch 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 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.)

This impacts vtk@9.2:9.4.

Patch 2:
There is also a typo that was fixed upstream in vtk 9.4+ that the clang compilers choke on. This is reported in spack#49623.

This impacts vtk@9.2:9.3.

Author: Chrismarsh

var/spack/repos/spack_repo/builtin/packages/vtk/package.py

@@ -260,6 +260,17 @@ class Vtk(CMakePackage):
         when="@9.4:",
     )
 
+    # https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=280893
+    #  incorrect member accesses fixed in 9.4
+    # https://gitlab.kitware.com/vtk/vtk/-/commit/98af50ca33
+    patch("vtk_patch_octree_m_children.patch", when="@9.2:9.3")
+
+    # clang 19+ no long providers std::char_traits<> for char8_t
+    # impacts any clang derivative compiler. But can be patched
+    # regardless of compiler
+    # https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=280893
+    patch("vtk_clang19_size_t.patch", when="@9.2:9.4.2")
+
     # Needed to build VTK with external SEACAS >= 2022-10-14
     @when("@9.4:")
     def patch(self):

var/spack/repos/spack_repo/builtin/packages/vtk/vtk_clang19_size_t.patch

@@ -0,0 +1,18 @@
+--- a/ThirdParty/diy2/vtkdiy2/include/vtkdiy2/fmt/format.h
++++ b/ThirdParty/diy2/vtkdiy2/include/vtkdiy2/fmt/format.h
+@@ -480,6 +480,7 @@
+ }
+ }  // 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 @@
+ }
+ }  // 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.

var/spack/repos/spack_repo/builtin/packages/vtk/vtk_patch_octree_m_children.patch

@@ -0,0 +1,11 @@
+--- a/Utilities/octree/octree/octree_node.txx
++++ b/Utilities/octree/octree/octree_node.txx
+@@ -207,7 +207,7 @@
+   {
+     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.