Enumerated type subtype check

Summary

According to the standard (6.2.13.2, https://www.etsi.org/deliver/etsi_es/201800_201899/20187301/04.13.01_60/es_20187301v041301p.pdf#page=80): "In case of enumerated types, the template list subtyping shall contain only values of the parent type."

Steps and/or TTCN-3 code to reproduce

type enumerated MyEnum { e_first, e_second, e_third, e_fourth, e_fifth };
type MyEnum EnumSub4 ( EnumSub1, e_fourth); 
 // causes an error as type references are not allowed in the template list 
 // of enumerated types 

What is the current bug behavior?

It is allowed/not checked by the compiler.

What is the expected correct behavior?

The compiler should provide an error message about this.

Titan version

6.6.1

/cc @aknappqwt