use integer_pack, make_integer_seq and type_pack_e

We were already using __make_integer_seq, incorporate the __integer_pack
code from the concepts refactor branch and add cases for intel, nvcc and
pgi to be able to use one or more of these intrinsics as well.  It turns
out EDG supports __make_integer_seq and __integer_pack, but only when configured some
certain way.  NVCC supports both, with the clang builtin supported only
when clang is selected as a host compiler and __integer_pack supported
only in version 10+ and only with a g++ >= 8 selected as host compiler.
Both of PGI and Intel seem to do something similar, supporting only
__integer_pack, and only when their targeted compiler and libstdc++ is
8+.  I have yet to try XL, but that's next.

include/camp/defines.hpp

@@ -98,10 +98,39 @@ namespace camp
 #define CAMP_HAVE_OMP_OFFLOAD 1
 #endif
 
-#if defined(__has_builtin)
-#if __has_builtin(__make_integer_seq)
+// This works for clang, nvcc 10 and higher using clang as a host compiler
+#define CAMP_USE_MAKE_INTEGER_SEQ 0
+#define CAMP_USE_TYPE_PACK_ELEMENT 0
+#if defined(_MSC_FULL_VER) && _MSC_FULL_VER >= 191125507
+  // __has_builtin exists but does not always expose this
+#undef CAMP_USE_MAKE_INTEGER_SEQ
 #define CAMP_USE_MAKE_INTEGER_SEQ 1
+#undef CAMP_USE_TYPE_PACK_ELEMENT
+#define CAMP_USE_TYPE_PACK_ELEMENT 1
+#elif defined(__has_builtin)
+#if __has_builtin(__make_integer_seq) && ((!defined(__NVCC__) || __CUDACC_VER_MAJOR >= 10))
+#undef CAMP_USE_MAKE_INTEGER_SEQ
+#define CAMP_USE_MAKE_INTEGER_SEQ 1
+#undef CAMP_USE_TYPE_PACK_ELEMENT
+#define CAMP_USE_TYPE_PACK_ELEMENT 1
+#endif
 #endif
+
+// This works for:
+//   GCC >= 8
+//   intel 19+ in GCC 8 or higher mode
+//   nvcc 10+ in GCC 8 or higher mode
+//   PGI 19+ in GCC 8 or higher mode
+#if __GNUC__ >= 8 && (\
+    /* intel compiler in gcc 8+ mode */ \
+    ((!defined(__INTEL_COMPILER)) || __INTEL_COMPILER >= 1900) \
+    /* nvcc in gcc 8+ mode */ \
+  ||((!defined(__NVCC__)) || __CUDACC_VER_MAJOR >= 10) \
+  ||((!defined(__PGIC__)) || __PGIC__ >= 19) \
+    )
+#define CAMP_USE_INTEGER_PACK 1
+#else
+#define CAMP_USE_INTEGER_PACK 0
 #endif
 
 // libstdc++ from GCC below version 5 lacks the type trait

include/camp/list/at.hpp

@@ -22,6 +22,15 @@ namespace camp
 
 namespace detail
 {
+  template <typename T, idx_t Idx>
+  struct _at;
+
+  #if CAMP_USE_TYPE_PACK_ELEMENT
+  template <idx_t Idx, template <class...> class T, typename... Pack>
+  struct _at<T<Pack...>, Idx> {
+    using type = __type_pack_element<Idx, Pack...>;
+  };
+  #else
   // Lookup from metal::at machinery
   template <idx_t, typename>
   struct entry {
@@ -45,8 +54,6 @@ namespace detail
       : decltype(_lookup_impl<Idx>(declptr<entries<indices, vals>>())) {
   };
 
-  template <typename T, idx_t Idx>
-  struct _at;
   template <template <class...> class T, typename X, typename... Rest>
   struct _at<T<X, Rest...>, 0> {
     using type = X;
@@ -65,6 +72,7 @@ namespace detail
                                   make_idx_seq_t<sizeof...(Rest)>,
                                   Idx>::type;
   };
+  #endif
 }  // namespace detail
 
 // TODO: document

include/camp/number.hpp

@@ -36,11 +36,16 @@ namespace detail
 {
   template <typename T, typename N>
   struct gen_seq;
-#if defined(CAMP_USE_MAKE_INTEGER_SEQ) && !__NVCC__
+#if CAMP_USE_MAKE_INTEGER_SEQ
   template <typename T, T N>
   struct gen_seq<T, integral_constant<T, N>> {
     using type = __make_integer_seq<int_seq, T, N>;
   };
+#elif CAMP_USE_INTEGER_PACK
+  template <typename T, T N>
+  struct gen_seq<T, integral_constant<T, N>> {
+    using type = int_seq<T, __integer_pack(N)...>;
+  };
 #else
   template <typename T, typename S1, typename S2>
   struct concat;