From ba955e3300a0db0842475f08281b30e2c5617e31 Mon Sep 17 00:00:00 2001
From: Alexandru Farcasanu <alexandru@flexion.ai>
Date: Fri, 28 Aug 2026 11:51:26 +0200
Subject: [PATCH] Reject externally tagged variants with more than one field

Reading an rfl::Variant<rfl::Field<...>, ...> from an object with
several keys silently kept the last recognized tag, and an unknown key
followed by a valid tag overwrote the error. Serde, which this format
models, rejects such objects. The reader now returns an error whenever
a second field is encountered, mirroring the existing 'found none'
error.
---
 include/rfl/parsing/FieldVariantReader.hpp    |  6 +++
 .../test_field_variant_multiple_fields.cpp    | 42 +++++++++++++++++++
 2 files changed, 48 insertions(+)
 create mode 100644 tests/json/test_field_variant_multiple_fields.cpp

diff --git a/include/rfl/parsing/FieldVariantReader.hpp b/include/rfl/parsing/FieldVariantReader.hpp
index ec045e5aa..b7454b1a7 100644
--- a/include/rfl/parsing/FieldVariantReader.hpp
+++ b/include/rfl/parsing/FieldVariantReader.hpp
@@ -43,6 +43,12 @@ class FieldVariantReader {
    */
   void read(const std::string_view& _disc_value,
             const InputVarType& _var) const noexcept {
+    if (*field_variant_) {
+      *field_variant_ = error(
+          "Could not parse rfl::Variant: Expected the object to have "
+          "exactly one field, but found more than one.");
+      return;
+    }
     try_matching_fields(
         _disc_value, _var,
         std::make_integer_sequence<int, sizeof...(FieldTypes)>());
diff --git a/tests/json/test_field_variant_multiple_fields.cpp b/tests/json/test_field_variant_multiple_fields.cpp
new file mode 100644
index 000000000..db6ee5e66
--- /dev/null
+++ b/tests/json/test_field_variant_multiple_fields.cpp
@@ -0,0 +1,42 @@
+#include <gtest/gtest.h>
+
+#include <rfl.hpp>
+#include <rfl/json.hpp>
+#include <string>
+
+namespace test_field_variant_multiple_fields {
+
+struct Circle {
+  double radius;
+};
+
+struct Rectangle {
+  double height;
+  double width;
+};
+
+using Shapes =
+    rfl::Variant<rfl::Field<"circle", Circle>, rfl::Field<"rectangle", Rectangle>>;
+
+TEST(json, test_field_variant_multiple_fields) {
+  const std::string faulty_string =
+      R"({"circle":{"radius":2.0},"rectangle":{"height":10.0,"width":5.0}})";
+
+  const auto result = rfl::json::read<Shapes>(faulty_string);
+
+  EXPECT_TRUE(!result.has_value() && true);
+  EXPECT_EQ(result.error().what(),
+            "Could not parse rfl::Variant: Expected the object to have "
+            "exactly one field, but found more than one.");
+}
+
+TEST(json, test_field_variant_unknown_then_known_field) {
+  const std::string faulty_string =
+      R"({"triangle":{"base":3.0},"circle":{"radius":2.0}})";
+
+  const auto result = rfl::json::read<Shapes>(faulty_string);
+
+  EXPECT_TRUE(!result.has_value() && true);
+}
+
+}  // namespace test_field_variant_multiple_fields