Fixes #5856. Code based on @loloiccl's PR (#5879).

tests/stdlib/tjsonmacro.nim

@@ -246,4 +246,17 @@ when isMainModule:
     var b = Bird(age: 3, height: 1.734, name: "bardo", colors: [red, blue])
     let jnode = %b
     let data = jnode.to(Bird)
-    doAssert data == b
+    doAssert data == b
+
+  block:
+    type
+      MsgBase = ref object of RootObj
+        name*: string
+
+      MsgChallenge = ref object of MsgBase
+        challenge*: string
+
+    let data = %*{"name": "foo", "challenge": "bar"}
+    let msg = data.to(MsgChallenge)
+    doAssert msg.name == "foo"
+    doAssert msg.challenge == "bar"