add constraints for variadic json_ref constructors

Fixes #1292

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
 }