add constraints for variadic json_ref constructors

Fixes #1292

include/nlohmann/detail/json_ref.hpp

@@ -3,6 +3,8 @@
 #include <initializer_list>
 #include <utility>
 
+#include <nlohmann/detail/meta/type_traits.hpp>
+
 namespace nlohmann
 {
 namespace detail
@@ -25,10 +27,12 @@ class json_ref
         : owned_value(init), value_ref(&owned_value), is_rvalue(true)
     {}
 
-    template<class... Args>
+    template <
-    json_ref(Args&& ... args)
+        class... Args,
-        : owned_value(std::forward<Args>(args)...), value_ref(&owned_value), is_rvalue(true)
+        enable_if_t<std::is_constructible<value_type, Args...>::value, int> = 0 >
-    {}
+    json_ref(Args && ... args)
+        : owned_value(std::forward<Args>(args)...), value_ref(&owned_value),
+          is_rvalue(true) {}
 
     // class should be movable only
     json_ref(json_ref&&) = default;

single_include/nlohmann/json.hpp

@@ -10591,6 +10591,9 @@ class serializer
 #include <initializer_list>
 #include <utility>
 
+// #include <nlohmann/detail/meta/type_traits.hpp>
+
+
 namespace nlohmann
 {
 namespace detail
@@ -10613,10 +10616,12 @@ class json_ref
         : owned_value(init), value_ref(&owned_value), is_rvalue(true)
     {}
 
-    template<class... Args>
+    template <
-    json_ref(Args&& ... args)
+        class... Args,
-        : owned_value(std::forward<Args>(args)...), value_ref(&owned_value), is_rvalue(true)
+        enable_if_t<std::is_constructible<value_type, Args...>::value, int> = 0 >
-    {}
+    json_ref(Args && ... args)
+        : owned_value(std::forward<Args>(args)...), value_ref(&owned_value),
+          is_rvalue(true) {}
 
     // class should be movable only
     json_ref(json_ref&&) = default;

test/src/unit-regression.cpp

@@ -33,6 +33,14 @@ SOFTWARE.
 #include <nlohmann/json.hpp>
 using nlohmann::json;
 
+#if (defined(__cplusplus) && __cplusplus >= 201703L) || (defined(_HAS_CXX17) && _HAS_CXX17 == 1) // fix for issue #464
+    #define JSON_HAS_CPP_17
+#endif
+
+#ifdef JSON_HAS_CPP_17
+    #include <variant>
+#endif
+
 #include "fifo_map.hpp"
 
 #include <fstream>
@@ -1649,4 +1657,12 @@ TEST_CASE("regression tests")
 
         CHECK(diffs.size() == 1); // Note the change here, was 2
     }
+
+#ifdef JSON_HAS_CPP_17
+    SECTION("issue #1292 - Serializing std::variant causes stack overflow")
+    {
+        static_assert(
+            not std::is_constructible<json, std::variant<int, float>>::value, "");
+    }
+#endif
 }