Ensure that serialized transitions only point to states which weren't removed. Add unit test for a current failure case (will be a regression test once fixed).

tool/src/org/antlr/v4/automata/ATNSerializer.java

@@ -133,6 +133,11 @@ public class ATNSerializer {
 			if ( s==null ) continue; // might be optimized away
 			for (int i=0; i<s.getNumberOfTransitions(); i++) {
 				Transition t = s.transition(i);
+
+				if (atn.states.get(t.target.stateNumber) == null) {
+					throw new IllegalStateException("Cannot serialize a transition to a removed state.");
+				}
+
 				int src = s.stateNumber;
 				int trg = t.target.stateNumber;
 				int edgeType = Transition.serializationTypes.get(t.getClass());

tool/test/org/antlr/v4/test/TestParseErrors.java

@@ -28,6 +28,7 @@
 
 package org.antlr.v4.test;
 
+import org.antlr.v4.automata.ATNSerializer;
 import org.junit.Test;
 
 /** test runtime parse errors */
@@ -253,4 +254,27 @@ public class TestParseErrors extends BaseTest {
 		assertEquals(expecting, result);
 	}
 
+	/**
+	 * 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.
+	 */
+	@Test
+	public void testInvalidATNStateRemoval() throws Exception {
+		String grammar =
+			"grammar T;\n" +
+			"start : ID ':' expr;\n" +
+			"expr : primary expr? {} | expr '->' ID;\n" +
+			"primary : ID;\n" +
+			"ID : [a-z]+;\n" +
+			"\n";
+		String result = execParser("T.g", grammar, "TParser", "TLexer", "start", "x:x", true);
+		String expecting = "";
+		assertEquals(expecting, result);
+		assertNull(this.stderrDuringParse);
+	}
+
 }