[antlr-interest] Optional Tree path not followed

[Anakreon]

Hello everyone.

I have a problem with a TreeParser rule.
The antlr code for the rule is:
sub_call
  :
<tsub> #(SUB_CALL sub_name:expression <sub_call>
            (argListValues)?
      <sub_call_end>)
;
where the tokens <name> is some action code.

and the argListValues definition is:
argListValues
  :
  !#(ARGLIST_VALUES <arg_val> (arg:expression <arg_val_end>)+)
  ;

The problem is that when a argListValues could be matched from the sub_call
rule, it isn't.

I checked the code produced when the sub_call defines the match of rule argListValues
as mandatory.

Here is the code:
			case SUB_CALL:
			{
				AST __t27 = _t;
				AST tmp18_AST = null;
				AST tmp18_AST_in = null;
				tmp18_AST = astFactory.create((AST)_t);
				tmp18_AST_in = (AST)_t;
				ASTPair __currentAST27 = currentAST.copy();
				currentAST.root = currentAST.child;
				currentAST.child = null;
				match(_t,SUB_CALL);
				_t = _t.getFirstChild();
				sub_name = _t==ASTNULL ? null : (AST)_t;
				expression(_t);
				_t = _retTree;
				sub_name_AST = (AST)returnAST;
				
					lastStackSize = argStack.size();
					boolean isArray = sub_name_AST != null
						&& sub_name_AST.getType() == IDENTIFIER
						&& getIdentifierType(sub_name_AST.getText()) == ARRAY;
					//this added by me in the generated code
					log.debug(Boolean.toString(_t == null));
				argListValues(_t);
				_t = _retTree;
				statement_AST = (AST)currentAST.root;
				
					if (isArray) {
						statement_AST = translateArray(sub_name_AST, lastStackSize);
					} else {
						statement_AST = handleFunctionSubEnd(sub_name_AST, lastStackSize); 
					}
					if (statement_AST != null) {
						statement_AST = (AST)astFactory.make( (new ASTArray(2)).add(astFactory.create(EXPR,"EXPR")).add(statement_AST));
					}
				
				currentAST.root = statement_AST;
				currentAST.child = statement_AST!=null &&statement_AST.getFirstChild()!=null ?
					statement_AST.getFirstChild() : statement_AST;
				currentAST.advanceChildToEnd();
				currentAST = __currentAST27;
				_t = __t27;
				_t = _t.getNextSibling();
				break;
			}
  
The diff when the argListValues rule is optional is:
.................................
				{
				if (_t==null) _t=ASTNULL;
				switch ( _t.getType()) {
				case ARGLIST_VALUES:
				{
					argListValues(_t);
					_t = _retTree;
					break;
				}
				case 3:
				{
					break;
				}
				default:
				{
					throw new NoViableAltException(_t);
				}
				}
				}
............................

The interesting thing is that in both cases the statement
log.debug(Boolean.toString(_t == null));
prints true. When the match of argListValues is mandatory and
and the tree analyzed can be matched by argListValues _t is null
but argListValues does not fail. It seems to me that _t is allways null
no matter if argListValues can be matched or not. 

antlr version: 1.7.4

What should I do to investigate if this happens because of action code or
it is a bug in the code generated from antlr?

Anakreon