PR# 18747 Traversing INTEGER_REF with copy semantics violates precondition

Traversing an INTEGER_REF object with copy semantics by using class
OBJECT_GRAPH_DEPTH_FIRST_TRAVERSABLE violates precondition
object_marked of {OBJECT_GRAPH_MARKER}.unmark.  This is only visible in the
debugger because `traverse' has a rescue clause that ignores any problems.

I'm filing this report because although Manu says you know about problems
with expanded types, he thought this simple case should work.

Compile with attached class and config file (note: must specify
use_application_options="true" on EiffelBase library in config file).
Click on Run button in EiffelStudio.  Debugger breaks on precondition
violation.

Attachments for problem report #18747