isvalue yields error for non-chosen field, while it should return false
## Summary
According to Section 7.1.8.4, EXAMPLE 3, isvalue should yield false if its parameter is a non-chosen (sub)field of a union. Titan however, returns an error because of the referenced field not being the chosen one.
## Steps and/or TTCN-3 code to reproduce
```
type union MyUnion_isv {
integer ch1,
integer ch2
}
template MyUnion_isv mw_myUnion := { ch1 := ? }
type record MyRecord_isv {
MyUnion_isv u optional
}
template MyRecord_isv mw_myRecord := { u := mw_myUnion }
testcase isvalue_fun() runs on ct_empty{
var boolean v_checkResult := isvalue(mw_myUnion.ch2); // yields false
if(not v_checkResult){setverdict(pass);}else{setverdict(fail);}
v_checkResult := isvalue(mw_myRecord.u.ch2); // yields false
if(not v_checkResult){setverdict(pass);}else{setverdict(fail);}
v_checkResult := isvalue(m_myRecord.u.ch2); // yields false
if(not v_checkResult){setverdict(pass);}else{setverdict(fail);}
}
```
## What is the current bug behavior?
Error.
## What is the expected correct behavior?
Pass.
## Relevant logs and/or screenshots
```
../src/operators.ttcn:353.51-53: error: Reference to inactive field `ch2' in a template of union type `@operators.MyUnion_isv'. The active field is `ch1'.
```
## Possible fixes
## Titan version
8.1.0
## Platform details (OS type and version)
Microsoft Windows 10 Enterprise 10.0.19042
/cc @aknappqwt @mmagyari
issue