RE: fix #25627 (#25736)

Follow up PR to #25700

@demotomohiro 

This doesn't seem to mirror your suggested approach completely. I still
went with a recursive walk. Could probably add some kind of "clean
types" and "dirty types" cache through this to minimize the recursions,
but that seems like a little much.

tests/objects/t25627.nim

@@ -20,3 +20,51 @@ let dir = DirStruct()
 doAssert dir.root.kind == fsoDir
 doAssert dir.root.dirName == "/"
 doAssert dir.root.files.len == 0
+
+block:
+  type
+    Opt[T] = object
+      when T is ref:
+        val: T
+        x: int
+      else:
+        val: T
+        x: string
+
+    DefaultOpt = ref object
+      files: Opt[DefaultOpt]
+
+    OptDirStruct = object
+      root = DefaultOpt()
+
+  let dir = OptDirStruct()
+  doAssert dir.root.files.x is int
+
+block:
+  type
+    Opt[T] = object
+      when T is ref:
+        x: int
+      else:
+        x: string
+
+    Foo[T] = object
+      x: Opt[T]
+
+    Nested = ref object
+      files: Foo[Nested]
+
+  let nested = Nested()
+  doAssert nested.files.x.x is int
+
+block:
+  type
+    Foo[T] = object
+      x = sizeof(T)
+
+    Sized = ref object
+      files: Foo[Sized]
+
+  let sized = Sized()
+  doAssert sized.files.x == sizeof(Sized)
+