OK

// Playground - noun: a place where people can play

if true {
    var a1:String? = nil
    a1 = "sss"
    if let a2 = a1 {
        println("xxx")
    }
    else{
        println("yyy")
    }
    var b:String = a1!
}

NG

// Playground - noun: a place where people can play

if true {
    var a1:String? = nil
    // a1 = "sss" // !!!
    if let a2 = a1 { // (!) Playground execution aborted: Execution was interrupted, reason: EXC_BAD_INSTRUCTION (code=EXC_I386_INVOP, subcode=0x0).
        println("xxx")
    }
    else{
        println("yyy")
    }
    var b:String = a1!
}

OK

// Playground - noun: a place where people can play

if true {
    var a1:String? = nil
    // a1 = "sss" // !!!
    if let a2 = a1 {
        println("xxx")
    }
    else{
        println("yyy")
    }
    // var b:String = a1! // !!!
}

なぜなのか