Attempt at adding unordered_map as a basic_json object type

NOTE: This does NOT compile due to unordered_map needing to have an
instance in the CompatibleObjectType CTOR for the basic_json object, and
basic_json being an incomplete type at that time.  For assistance
request only.

src/json.hpp

@@ -105,6 +105,49 @@ static bool approx(const T a, const T b)
 {
     return not (a > b or a < b);
 }
+
+/*!
+@brief Type Trait to convert a type to a void for SFINAE purposes
+*/
+template<typename T>
+struct void_converter
+{
+    using type = void;
+};
+
+// Type to help determine between a map or unordered-map type for the
+// object_t type alias.  Takes advantage of the fact that a map has a
+// key_comparer member type and unordered_map has a hasher member type
+template<template<typename...> class ObjectType, class Key, class Value,
+    template<typename> class AllocatorType, typename Enable = void>
+struct object_t_helper{};
+// Specialization that works for the 'map' type
+template<template<typename...> class ObjectType, class Key, class Value,
+    template<typename> class AllocatorType>
+struct object_t_helper<ObjectType, Key, Value, AllocatorType,
+    typename void_converter<typename ObjectType<char,char>::key_compare>::type
+    >
+{
+    using type = ObjectType<
+        Key,
+        Value,
+        std::less<Key>,
+        AllocatorType<std::pair<Key,Value>>>;
+};
+// Specialization that works for the 'unordered_map' type
+template<template<typename...> class ObjectType, class Key, class Value,
+    template<typename> class AllocatorType>
+struct object_t_helper<ObjectType, Key, Value, AllocatorType,
+    typename void_converter<typename ObjectType<char,char>::hasher>::type
+    >
+{
+    using type = ObjectType<
+        Key,
+        Value,
+        std::hash<Key>,
+        std::equal_to<Key>,
+        AllocatorType<std::pair<Key,Value>>>;
+};
 }
 
 /*!
@@ -333,11 +376,12 @@ class basic_json
 
     @since version 1.0.0
     */
-    using object_t = ObjectType<StringType,
-          basic_json,
-          std::less<StringType>,
-          AllocatorType<std::pair<const StringType,
-          basic_json>>>;
+    using object_t = typename object_t_helper<
+            ObjectType,
+            StringType,
+            basic_json,
+            AllocatorType
+            >::type;
 
     /*!
     @brief a type for an array

test/unit.cpp

@@ -11314,6 +11314,12 @@ TEST_CASE("regression tests")
         CHECK(s2 == "value");
     }
 
+    SECTION("issue #164 - std::unordered_map cannot be used as ObjectType")
+    {
+        // create JSON class with std::unordered_map instead of a std::map
+        nlohmann::basic_json<std::unordered_map> unordered_json;
+    }
+
     SECTION("character following a surrogate pair is skipped")
     {
         CHECK(json::parse("\"\\ud80c\\udc60abc\"").get<json::string_t>() == u8"\U00013060abc");