INTERNAL ERROR: Unexpected AST node during analyze in Systems Tester

Answer

The internal error "Unexpexted AST node" can occur due to inconsistent/unsupported constructs in the ASN.1 / TTCN-3 source files.
Some of the scenarios in which this error is seen are :

1. Usage of inline enumerated definitions.
   
   Example:
   
MyRecord ::= SEQUENCE{
field1 INTEGER,
field2 BIT STRING,
field3 ENUMERATED {f0,f1,f3}
}

   
   In case this it is the type of construct that's causing the INTERNAL ERROR, solution is to modify the inline ENUMERATED definition in the ASN.1 code to something like:
   
MyEnumerated ::= ENUMERATED {f0,f1,f3}

MyRecord ::= SEQUENCE{
field1 INTEGER,
field2 BIT STRING,
field3 MyEnumerated
}

   
   
2. TestCase uses a component type in statements where there should be a component reference.
   
   
3. Initialization of a variable, whose type is based on one of the fields of a SEQUENCE/record.
   
   Example:
   
MyRecord ::= SEQUENCE{
field1 INTEGER,
field2 BIT STRING,
field3 ENUMERATED {f0,f1,f3}
}

   
   If there's the type MyEnumerated based on the type of one of the fields of the above ASN.1 SEQUENCE like:
   
   
type MyRecord.field3 MyEnumerated;

   
   ... and we then declare a variable of this type using:
   
   
var MyEnumerated v_enumerated;

   
   ... and initialize this variable using the statement:
   
   v_enumerated := f1; // The internal error is reported on the initialize statement here.. .