Fix duplicate discriminator field in object variants

src/jsony.nim

@@ -477,7 +477,18 @@ proc parseHook*[T: object|ref object](s: string, i: var int, v: var T) =
           new(v)
         break
     i = saveI
-  parseObjectInner(s, i, v)
+  when v.isObjectVariant:
+    # Object variant, check for duplicate discriminator field.
+    # -d:release has different behavior.
+    let discBefore = v.discriminatorField
+    try:
+      parseObjectInner(s, i, v)
+    except FieldDefect:
+      error("Duplicate discriminator field.", i)
+    if v.discriminatorField != discBefore:
+      error("Duplicate discriminator field.", i)
+  else:
+    parseObjectInner(s, i, v)
   eatChar(s, i, '}')
 
 proc parseHook*[T](s: string, i: var int, v: var Option[T]) =

tests/test_objects.nim

@@ -282,6 +282,29 @@ block:
   doAssert c.kind == nkFloat
   doAssert d.kind == nkInt
 
+
+block:
+  # Duplicate discriminator with a conflicting value raises JsonError.
+  var raised = false
+  try:
+    discard """{"kind":"nkFloat","floatVal":3.14,"kind":"nkInt"}""".fromJson(
+      ValueNode)
+  except JsonError:
+    raised = true
+  doAssert raised,
+    "expected JsonError for conflicting duplicate discriminator"
+
+block:
+  # Same test for ref object variant.
+  var raised = false
+  try:
+    discard """{"kind":"nkFloat","floatVal":3.14,"kind":"nkInt"}""".fromJson(
+      RefNode)
+  except JsonError:
+    raised = true
+  doAssert raised,
+    "expected JsonError for conflicting duplicate discriminator"
+
 # test https://forum.nim-lang.org/t/7619
 
 import jsony