diff --git a/src/json.hpp b/src/json.hpp
index 04c8573f430fe425a7838b18be6a7c95a5fd316f..57fecf991e551cf54b19b29c6c0385ed78e1ceb5 100644
--- a/src/json.hpp
+++ b/src/json.hpp
@@ -112,10 +112,13 @@ template<typename T>
 struct has_mapped_type
 {
   private:
-    template<typename C> static char test(typename C::mapped_type*);
-    template<typename C> static char (&test(...))[2];
+    template <typename U, typename = typename U::mapped_type>
+    static int detect(U&&);
+
+    static void detect(...);
   public:
-    static constexpr bool value = sizeof(test<T>(0)) == 1;
+    static constexpr bool value =
+        std::is_integral<decltype(detect(std::declval<T>()))>::value;
 };
 
 /*!
diff --git a/src/json.hpp.re2c b/src/json.hpp.re2c
index 48224835423fa129871366c8646bde232737eb2b..e26ac7c7b7fa3552fc9543cb00159a5db64ca82c 100644
--- a/src/json.hpp.re2c
+++ b/src/json.hpp.re2c
@@ -112,10 +112,13 @@ template<typename T>
 struct has_mapped_type
 {
   private:
-    template<typename C> static char test(typename C::mapped_type*);
-    template<typename C> static char (&test(...))[2];
+    template <typename U, typename = typename U::mapped_type>
+    static int detect(U&&);
+
+    static void detect(...);
   public:
-    static constexpr bool value = sizeof(test<T>(0)) == 1;
+    static constexpr bool value =
+        std::is_integral<decltype(detect(std::declval<T>()))>::value;
 };
 
 /*!