more SFINAE

src/json.hpp.re2c

@@ -54,6 +54,20 @@ iterators allow a ReversibleContainer to be iterated over in reverse.
 namespace nlohmann
 {
 
+
+// Helper to determine whether there's a key_type for T.
+// http://stackoverflow.com/a/7728728/266378
+template<typename T>
+struct has_key_type
+{
+  private:
+    template<typename C> static char test(typename C::key_type*);
+    template<typename C> static int  test(...);
+  public:
+    enum { value = sizeof(test<T>(0)) == sizeof(char) };
+};
+
+
 /*!
 @brief JSON
 
@@ -809,7 +823,7 @@ class basic_json
     template <class T, typename
               std::enable_if<
                   std::is_constructible<typename T::key_type, typename object_t::key_type>::value and
-                  std::is_constructible<basic_json, typename T::mapped_type>::value, int>::type
+                  std::is_constructible<typename T::mapped_type, basic_json>::value, int>::type
               = 0>
     inline T get() const
     {
@@ -826,7 +840,8 @@ class basic_json
     template <class T, typename
               std::enable_if<
                   not std::is_same<T, string_t>::value and
-                  std::is_constructible<basic_json, typename T::value_type>::value, int>::type
+                  not has_key_type<T>::value and
+                  std::is_constructible<typename T::value_type, basic_json>::value, int>::type
               = 0>
     inline T get() const
     {