@Test
	public void testNegativeSeek() {
		CharStream input = createStream("");
		assertThrows(IllegalArgumentException.class, () -> input.seek(-1));
	}
The {@link IntStream} interface does not specify the behavior when the
EOF symbol is consumed, but {@link UnbufferedCharStream} handles this
particular case by throwing an {@link IllegalStateException}.
tool-testsuite/test/org/antlr/v4/test/tool/TestUnbufferedCharStream.java