fix corner case in unused (#4682)

fixes #4681

lib/compress.js

@@ -5743,7 +5743,10 @@ merge(Compressor.prototype, {
                         if (prop.key instanceof AST_Node) prop.key.walk(tw);
                         if (!prop.value) return;
                         if (is_export || prop instanceof AST_ClassField && prop.static) {
+                            var save_scope = scope;
+                            scope = node;
                             prop.value.walk(tw);
+                            scope = save_scope;
                         } else {
                             initializations.add(def.id, prop.value);
                         }
@@ -5793,6 +5796,12 @@ merge(Compressor.prototype, {
                     assignments.add(def.id, node);
                     return true;
                 }
+            } else if (node instanceof AST_This && scope instanceof AST_DefClass) {
+                var def = scope.name.definition();
+                if (!(def.id in in_use_ids)) {
+                    in_use_ids[def.id] = true;
+                    in_use.push(def);
+                }
             }
             return scan_ref_scoped(node, descend, true);
         });