From 01176efbdf71da67a6e9868e7b5a538d32425920 Mon Sep 17 00:00:00 2001
From: Kenneth Moreland <kmorel@sandia.gov>
Date: Mon, 29 Jul 2019 12:57:21 -0600
Subject: [PATCH] Provide implementation of aligned_union

Although it is mostly C++11 compliant, GCC 4.8 does not have an
implementation of std::aligned_union. We cannot drop this compiler, so
provide our own implementation in that case.
---
 vtkm/internal/Variant.h | 41 ++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 40 insertions(+), 1 deletion(-)

diff --git a/vtkm/internal/Variant.h b/vtkm/internal/Variant.h
index a73e20d27..67813863e 100644
--- a/vtkm/internal/Variant.h
+++ b/vtkm/internal/Variant.h
@@ -14,6 +14,45 @@
 
 #include <vtkm/ListTag.h>
 
+
+// It would make sense to put this in its own header file, but it is hard to imagine needing
+// aligned_union anywhere else.
+#if (defined(VTKM_GCC) && (__GNUC__ == 4)) || (defined(VTKM_ICC) && (__INTEL_COMPILER < 1800))
+#include <algorithm>
+namespace vtkmstd
+{
+template <std::size_t... Xs>
+struct max_size;
+template <std::size_t X>
+struct max_size<X>
+{
+  static constexpr std::size_t value = X;
+};
+template <std::size_t X0, std::size_t... Xs>
+struct max_size<X0, Xs...>
+{
+  static constexpr std::size_t other_value = max_size<Xs...>::value;
+  static constexpr std::size_t value = (other_value > X0) ? other_value : X0;
+};
+// This implementation comes from https://en.cppreference.com/w/cpp/types/aligned_union
+template <std::size_t Len, class... Types>
+struct aligned_union
+{
+  static constexpr std::size_t alignment_value = vtkmstd::max_size<alignof(Types)...>::value;
+
+  struct type
+  {
+    alignas(alignment_value) char _s[vtkmstd::max_size<Len, sizeof(Types)...>::value];
+  };
+};
+} // namespace vtkmstd
+#else
+namespace vtkmstd
+{
+using std::aligned_union;
+} // namespace vtkmstd
+#endif
+
 namespace vtkm
 {
 namespace internal
@@ -51,7 +90,7 @@ class Variant
   {
   };
 
-  typename std::aligned_union<0, Ts...>::type Storage;
+  typename vtkmstd::aligned_union<0, Ts...>::type Storage;
 
   VTKM_EXEC_CONT void* GetPointer() { return reinterpret_cast<void*>(&this->Storage); }
   VTKM_EXEC_CONT const void* GetPointer() const