Use own version of getSingleElement() since we don't care about negat…

…ive values

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

@@ -404,7 +404,7 @@ public IntervalSet getSetFromCharSetLiteral(GrammarAST charSetAST) {
 									   g.fileName, charSetAST.getToken());
 						return new IntervalSet();
 					case INTERVAL_SET:
-						int codePoint = escapeParseResult.intervalSet.getSingleElement();
+						int codePoint = getSingleElement(escapeParseResult.intervalSet);
 						boolean containsMultipleCodePoints = (codePoint == -1);
 						if (inRange) {
 							if (containsMultipleCodePoints) {
@@ -489,6 +489,21 @@ public IntervalSet getSetFromCharSetLiteral(GrammarAST charSetAST) {
 		return set;
 	}
 
+	private static int getSingleElement(IntervalSet set) {
+		// We don't use IntervalSet.getSingleElement() because it can't handle sets containing 0.
+		// See https://github.com/antlr/antlr4/issues/1703 .
+		if (set.size() != 1) {
+			return -1;
+		} else {
+			Interval interval = set.getIntervals().get(0);
+			if (interval.length() == 1) {
+				return interval.a;
+			} else {
+				return -1;
+			}
+		}
+	}
+
 	protected void checkSetCollision(GrammarAST ast, IntervalSet set, int el) {
 		if (set.contains(el)) {
 			g.tool.errMgr.grammarError(ErrorType.CHARACTERS_COLLISION_IN_SET, g.fileName, ast.getToken(),