Fix the RapidJSON problem with std::size_t by explicitly casting to…

… uint64.

benchmark/utils/general.hpp

@@ -108,8 +108,10 @@ std::ostream &operator<<(std::ostream &os, const rapidjson::Value &value)
 
 // helper for setting rapidjson object members
 template <typename T, typename NameType, typename Allocator>
-void add_or_set_member(rapidjson::Value &object, NameType &&name, T &&value,
-                       Allocator &&allocator)
+gko::xstd::enable_if_t<
+    !std::is_same<typename std::decay<T>::type, gko::size_type>::value, void>
+add_or_set_member(rapidjson::Value &object, NameType &&name, T &&value,
+                  Allocator &&allocator)
 {
     if (object.HasMember(name)) {
         object[name] = std::forward<T>(value);
@@ -120,6 +122,30 @@ void add_or_set_member(rapidjson::Value &object, NameType &&name, T &&value,
 }
 
 
+/**
+   @internal This is required to fix some MacOS problems (and possibly other
+   compilers). There is no explicit RapidJSON constructor for `std::size_t` so a
+   conversion to a known constructor is required to solve any ambiguity. See the
+   last comments of https://github.com/ginkgo-project/ginkgo/issues/270.
+ */
+template <typename T, typename NameType, typename Allocator>
+gko::xstd::enable_if_t<
+    std::is_same<typename std::decay<T>::type, gko::size_type>::value, void>
+add_or_set_member(rapidjson::Value &object, NameType &&name, T &&value,
+                  Allocator &&allocator)
+{
+    if (object.HasMember(name)) {
+        object[name] =
+            std::forward<std::uint64_t>(static_cast<std::uint64_t>(value));
+    } else {
+        auto n = rapidjson::Value(name, allocator);
+        object.AddMember(
+            n, std::forward<std::uint64_t>(static_cast<std::uint64_t>(value)),
+            allocator);
+    }
+}
+
+
 // helper for splitting a delimiter-separated list into vector of strings
 std::vector<std::string> split(const std::string &s, char delimiter = ',')
 {