✨ added overload for std::less<value_t> #486

MSVC needs this overload to compile code containing a std::map that
uses nlohmann::detail::operator as key.

src/json.hpp

@@ -13025,6 +13025,22 @@ struct hash<nlohmann::json>
         return h(j.dump());
     }
 };
+
+/// specialization for std::less<value_t>
+template <>
+struct less<::nlohmann::detail::value_t>
+{
+    /*!
+    @brief compare two value_t enum values
+    @since version 3.0.0
+    */
+    bool operator()(nlohmann::detail::value_t lhs,
+                    nlohmann::detail::value_t rhs) const noexcept
+    {
+        return nlohmann::detail::operator<(lhs, rhs);
+    }
+};
+
 } // namespace std
 
 /*!

src/json.hpp.re2c

@@ -12059,6 +12059,22 @@ struct hash<nlohmann::json>
         return h(j.dump());
     }
 };
+
+/// specialization for std::less<value_t>
+template <>
+struct less<::nlohmann::detail::value_t>
+{
+    /*!
+    @brief compare two value_t enum values
+    @since version 3.0.0
+    */
+    bool operator()(nlohmann::detail::value_t lhs,
+                    nlohmann::detail::value_t rhs) const noexcept
+    {
+        return nlohmann::detail::operator<(lhs, rhs);
+    }
+};
+
 } // namespace std
 
 /*!

test/src/unit-regression.cpp

@@ -795,4 +795,11 @@ TEST_CASE("regression tests")
         std::string s2 = j2.dump();
         CHECK(s1 == s2);
     }
+
+    SECTION("issue #486 - json::value_t can't be a map's key type in VC++ 2015")
+    {
+        // the code below must compile with MSVC
+        std::map<json::value_t, std::string> jsonTypes ;
+        jsonTypes[json::value_t::array] = "array";
+    }
 }