to recreate add a Tests\expression_04.cs:
$ cat Tests/expression_04.cs
parser using rule -- expression:
line 1:25 missing RPAREN at '.'
Parsed 1 of 1 files. (100%)
parsing expression() finishes early on the "correctResponseIndex++" part. The C# spec looks like it would handle this just fine. I haven't found a fix for this, cs.g departs quite a bit from the C# spec I have in this area.