Here's an example (in a modifed glib-2.0.vapi) adding a toggle() method that modifies a local variable. This takes no sense, so the compiler shouldnt permit this. 

In non-simpletypes 'this' is a pointer and shouldnt be modified.

This bug has been discussed in mailing list with the thread subjected as:

   "[Vala] modify simpletype with methods in vapi"

[SimpleType]
[CCode (cname = "gboolean", cheader_filename = "glib.h", type_id = "G_TYPE_BOOLEAN", marshaller_type_name = "BOOLEAN", get     _value_function = "g_value_get_boolean", set_value_function = "g_value_set_boolean", default_value = "FALSE", type_signatu     re = "b")]
[BooleanType]
 public struct bool {
   public bool toggle() {
     this = !this;
     return this;
   }
}