fix #15815 (#15817)

lib/pure/json.nim

@@ -1120,7 +1120,10 @@ when defined(nimFixedForwardGeneric):
 
   proc initFromJson[T](dst: var Option[T]; jsonNode: JsonNode; jsonPath: var string) =
     if jsonNode != nil and jsonNode.kind != JNull:
-      dst = some(default(T))
+      when T is ref:
+        dst = some(new(T))
+      else:
+        dst = some(default(T))
       initFromJson(dst.get, jsonNode, jsonPath)
 
   macro assignDistinctImpl[T: distinct](dst: var T;jsonNode: JsonNode; jsonPath: var string) =

tests/stdlib/toptions.nim

@@ -0,0 +1,16 @@
+discard """
+  output: '''{"foo":{"test":"123"}}'''
+"""
+
+import json, options
+
+type
+  Foo = ref object
+    test: string
+  Test = object
+    foo: Option[Foo]
+
+let js = """{"foo": {"test": "123"}}"""
+let parsed = parseJson(js)
+let a = parsed.to(Test)
+echo $(%*a)