@Test
	public void testIllegalNonSetLabel() throws Exception {
		String grammar =
			"grammar T;\n" +
			"ss : op=('=' | '+=' | expr) EOF;\n" +
			"expr : '=' '=';\n" +
			"";

		String expected =
			"error(" + ErrorType.LABEL_BLOCK_NOT_A_SET.code + "): T.g4:2:5: label op assigned to a block which is not a set\n";

		testErrors(new String[] { grammar, expected }, false);
	}
Regression test for #25 "Don't allow labels on not token set subrules".
https://github.com/antlr/antlr4/issues/25
tool-testsuite/test/org/antlr/v4/test/tool/TestBasicSemanticErrors.java