I have just compiled your grammar using ANTLRWorks 1.5 rc1
and JDK 1.7.0.11
.
All compiled well, and mentioned code line looks like:
public final ASTDemoParser.parse_return parse() throws RecognitionException {
It could be arbitrary bug manifestation. Try to recompile it again, and may be you have try to use JDK 1.7
.