Modified test for Antlr4 to demonstrate ParserTree crash

gistfile1.txt

    func calculatorHelper() -> ParseTree {
        let input = "2 + 8 / 2"
        let lexer = VisitorCalcLexer(ANTLRInputStream(input))
        let parser = try! VisitorCalcParser(CommonTokenStream(lexer))
        return try! parser.s()
    }
    
    func calculatorHelper2() -> VisitorCalcParser {
        let input = "2 + 8 / 2"
        let lexer = VisitorCalcLexer(ANTLRInputStream(input))
        let parser = try! VisitorCalcParser(CommonTokenStream(lexer))
        return parser
    }
    ///
    /// This test verifies that the visitor correctly dispatches calls for labeled outer alternatives.
    ///
    func testCalculatorVisitor2() throws {

        
        class Visitor: VisitorCalcBaseVisitor<Int> {
            override func visitS(_ ctx: VisitorCalcParser.SContext) -> Int? {
                return visit(ctx.expr()!)
            }
            
            override func visitNumber(_ ctx: VisitorCalcParser.NumberContext) -> Int? {
                return Int((ctx.INT()?.getText())!)
            }
            
            override func visitMultiply(_ ctx: VisitorCalcParser.MultiplyContext) -> Int? {
                let left = visit(ctx.expr(0)!)!
                let right = visit(ctx.expr(1)!)!
                if ctx.MUL() != nil {
                    return left * right
                }
                else {
                    return left / right
                }
            }
            
            override func visitAdd(_ ctx: VisitorCalcParser.AddContext) -> Int? {
                let left = visit(ctx.expr(0)!)!
                let right = visit(ctx.expr(1)!)!
                if ctx.ADD() != nil {
                    return left + right
                }
                else {
                    return left - right
                }
            }
        }
        
        let kaboom = true
        let visitor = Visitor()
        let expected = 6
        let context: ParseTree
        if kaboom {
            context = calculatorHelper()
            let result = visitor.visit(context)
            XCTAssertEqual(expected, result!)
        }
        else {
            let parser = calculatorHelper2()
            context = try! parser.s()
            let result = visitor.visit(context)
            XCTAssertEqual(expected, result!)
        }
    }