[cpp] Fix natural types build issues when used with GIDL

This CL includes fixes to breakages found when building FIDL files used
by GIDL

MaxOrdinal currently fails if the number of members is zero -- the index
becomes -1 and std::get fails. This CL fixes the issue by wrapping the
entire body by an "if constexpr".

Clone of arrays currently fails and this CL includes a working
implementation of clone.

Change-Id: I8672fbf2be72871399e37d364df4458a11e8a7ca
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/636582
Fuchsia-Auto-Submit: Benjamin Prosnitz <bprosnitz@google.com>
Reviewed-by: Ian McKellar <ianloic@google.com>
Commit-Queue: Benjamin Prosnitz <bprosnitz@google.com>

src/lib/fidl/cpp/include/lib/fidl/cpp/internal/natural_types.h

@@ -158,15 +158,15 @@ struct NaturalTableCodingTraits {
   // Returns the largest ordinal of a present table member.
   template <size_t I = std::tuple_size_v<decltype(T::kMembers)> - 1>
   static size_t MaxOrdinal(T* value) {
-    auto T::Storage_::*member_ptr = std::get<I>(T::kMembers).member_ptr;
-    const auto& member = value->storage_.*member_ptr;
-    if (member.has_value()) {
-      return I + 1;
-    }
-    if constexpr (I > 0) {
-      return MaxOrdinal<I - 1>(value);
-    } else {
+    if constexpr (I == -1) {
       return 0;
+    } else {
+      auto T::Storage_::*member_ptr = std::get<I>(T::kMembers).member_ptr;
+      const auto& member = value->storage_.*member_ptr;
+      if (member.has_value()) {
+        return I + 1;
+      }
+      return MaxOrdinal<I - 1>(value);
     }
   }
 
@@ -278,6 +278,18 @@ struct NaturalCloneHelper<std::vector<T>> {
   }
 };
 
+template <typename T, size_t N, std::size_t... Indexes>
+std::array<T, N> ArrayCloneHelper(const std::array<T, N>& value, std::index_sequence<Indexes...>) {
+  return std::array<T, N>{NaturalCloneHelper<T>::Clone(std::get<Indexes>(value))...};
+}
+
+template <typename T, size_t N>
+struct NaturalCloneHelper<std::array<T, N>> {
+  static std::array<T, N> Clone(const std::array<T, N>& value) {
+    return ArrayCloneHelper(value, std::make_index_sequence<N>());
+  }
+};
+
 template <typename T>
 T NaturalClone(const T& value) {
   return NaturalCloneHelper<T>::Clone(value);