[notes]
This is a regression test for #45 "NullPointerException in ATNConfig.hashCode".
https://github.com/antlr/antlr4/issues/45
The original cause of this issue was an error in the tool's ATN state optimization,
which is now detected early in {@link ATNSerializer} by ensuring that all
serialized transitions point to states which were not removed.

[type]
Parser

[grammar]
grammar T;
start : ID ':' expr;
expr : primary expr? {<Pass()>} | expr '->' ID;
primary : ID;
ID : [a-z]+;

[start]
start

[input]
x:x

