fix feedback

Author: simbo1905

json-java21-jtd/src/main/java/json/java21/jtd/Jtd.java

@@ -157,10 +157,15 @@ void validatePropertiesSchema(Frame frame, JtdSchema.PropertiesSchema propsSchem
     }
 
     // Check for additional properties if not allowed
-    // RFC 8927 §2.2.8: In discriminator context, variant schemas ignore additionalProperties enforcement
-    if (!propsSchema.additionalProperties() && frame.discriminatorKey() == null) {
+    // RFC 8927 §2.2.8: Only the discriminator field is exempt from additionalProperties enforcement
+    if (!propsSchema.additionalProperties()) {
+      String discriminatorKey = frame.discriminatorKey();
       for (String key : obj.members().keySet()) {
         if (!propsSchema.properties().containsKey(key) && !propsSchema.optionalProperties().containsKey(key)) {
+          // Only exempt the discriminator field itself, not all additional properties
+          if (discriminatorKey != null && key.equals(discriminatorKey)) {
+            continue; // Skip the discriminator field - it's exempt
+          }
           JsonValue value = obj.members().get(key);
           // Additional property not allowed - create error with the value's offset
           String error = Jtd.Error.ADDITIONAL_PROPERTY_NOT_ALLOWED.message(key);

json-java21-jtd/src/test/java/json/java21/jtd/TestRfc8927.java

@@ -206,26 +206,29 @@ public void testMultipleErrorCollection() throws Exception {
   }
 
   /// Test discriminator tag exemption
-  /// RFC 8927 Section 3.3.8: Discriminator property is exempt from additional properties validation
+  /// RFC 8927 §2.2.8: Only the discriminator field itself is exempt from additionalProperties enforcement
   @Test
   public void testDiscriminatorTagExemption() throws Exception {
     JsonValue schema = Json.parse("{\"discriminator\": \"type\", \"mapping\": {\"person\": {\"properties\": {\"name\": {\"type\": \"string\"}}}}}");
 
     // Valid data with discriminator and no additional properties
     JsonValue validData1 = Json.parse("{\"type\": \"person\", \"name\": \"John\"}");
 
-    // Data with discriminator and additional properties (discriminator should be exempt)
-    JsonValue validData2 = Json.parse("{\"type\": \"person\", \"name\": \"John\", \"extra\": \"allowed\"}");
+    // Data with discriminator and additional properties (only discriminator field should be exempt)
+    JsonValue invalidData2 = Json.parse("{\"type\": \"person\", \"name\": \"John\", \"extra\": \"not_allowed\"}");
 
     Jtd validator = new Jtd();
 
-    // Both should be valid - discriminator is exempt from additional properties check
+    // First should be valid - no additional properties
     Jtd.Result result1 = validator.validate(schema, validData1);
-    Jtd.Result result2 = validator.validate(schema, validData2);
-    
     assertThat(result1.isValid()).isTrue();
-    assertThat(result2.isValid()).isTrue();
-    LOG.fine(() -> "Discriminator tag exemption test - data1: " + validData1 + ", data2: " + validData2);
+    
+    // Second should be invalid - extra field is not exempt, only discriminator field is
+    Jtd.Result result2 = validator.validate(schema, invalidData2);
+    assertThat(result2.isValid()).isFalse();
+    assertThat(result2.errors()).anySatisfy(error -> assertThat(error).contains("extra"));
+    
+    LOG.fine(() -> "Discriminator tag exemption test - valid: " + validData1 + ", invalid: " + invalidData2);
   }
 
   /// Counter-test: Discriminator with invalid mapping