Fix issue #1805

include/nlohmann/detail/conversions/to_json.hpp

@@ -302,8 +302,8 @@ void to_json(BasicJsonType& j, const T(&arr)[N])
     external_constructor<value_t::array>::construct(j, arr);
 }
 
-template<typename BasicJsonType, typename... Args>
-void to_json(BasicJsonType& j, const std::pair<Args...>& p)
+template < typename BasicJsonType, typename T1, typename T2, enable_if_t < std::is_constructible<BasicJsonType, T1>::value&& std::is_constructible<BasicJsonType, T2>::value, int > = 0 >
+void to_json(BasicJsonType& j, const std::pair<T1, T2>& p)
 {
     j = { p.first, p.second };
 }

single_include/nlohmann/json.hpp

@@ -3733,8 +3733,8 @@ void to_json(BasicJsonType& j, const T(&arr)[N])
     external_constructor<value_t::array>::construct(j, arr);
 }
 
-template<typename BasicJsonType, typename... Args>
-void to_json(BasicJsonType& j, const std::pair<Args...>& p)
+template < typename BasicJsonType, typename T1, typename T2, enable_if_t < std::is_constructible<BasicJsonType, T1>::value&& std::is_constructible<BasicJsonType, T2>::value, int > = 0 >
+void to_json(BasicJsonType& j, const std::pair<T1, T2>& p)
 {
     j = { p.first, p.second };
 }

test/src/unit-regression.cpp

@@ -159,6 +159,16 @@ bool operator==(Data const& lhs, Data const& rhs)
 
 using float_json = nlohmann::basic_json<std::map, std::vector, std::string, bool, std::int64_t, std::uint64_t, float>;
 
+/////////////////////////////////////////////////////////////////////
+// for #1805
+/////////////////////////////////////////////////////////////////////
+
+struct NotSerializableData
+{
+    int mydata;
+    float myfloat;
+};
+
 
 TEST_CASE("regression tests")
 {
@@ -1820,6 +1830,12 @@ TEST_CASE("regression tests")
         CHECK(j.contains(jptr1));
         CHECK(j.contains(jptr2));
     }
+    SECTION("issue #1805 - A pair<T1, T2> is json constructible only if T1 and T2 are json constructible")
+    {
+        static_assert(!std::is_constructible<json, std::pair<std::string, NotSerializableData>>::value, "");
+        static_assert(!std::is_constructible<json, std::pair<NotSerializableData, std::string>>::value, "");
+        static_assert(std::is_constructible<json, std::pair<int, std::string>>::value, "");
+    }
 }
 
 #if not defined(JSON_NOEXCEPTION)