Created attachment 157552 [details]
Test case.

When an enum method accesses its own implicit to_string() method, it must provide the "this" ref in order to access it.  Calling other methods in its scope does not require "this".  The compiler says:

test.vala:13.16-13.24: error: The name `to_string' does not exist in the context of `Xyzzy.fail'
        return to_string();
               ^^^^^^^^^

However, if an explicit to_string() method is coded in the enum, the problem does not surface.

Test case attached.