fixes #4177

compiler/seminst.nim

@@ -43,9 +43,11 @@ proc rawHandleSelf(c: PContext; owner: PSym) =
       if arg.name.id == c.selfName.id:
         c.p.selfSym = arg
         arg.flags.incl sfIsSelf
-        let t = c.p.selfSym.typ.skipTypes(abstractPtrs)
-        if t.kind == tyObject:
+        var t = c.p.selfSym.typ.skipTypes(abstractPtrs)
+        while t.kind == tyObject:
           addObjFieldsToLocalScope(c, t.n)
+          if t.sons[0] == nil: break
+          t = t.sons[0].skipTypes(abstractPtrs)
 
 proc pushProcCon*(c: PContext; owner: PSym) =
   rawPushProcCon(c, owner)

tests/usingstmt/tthis.nim

@@ -0,0 +1,15 @@
+
+# bug #4177
+
+type
+  Parent = object of RootObj
+    parentField: int
+  Child = object of Parent
+    childField: int
+
+{.this: self.}
+proc sumFields(self: Child): int =
+  result = parentField + childField # Error: undeclared identifier: 'parentField'
+
+proc sumFieldsWorks(self: Child): int =
+  result = self.parentField + childField