Constant unassigned optional will not be nil by default

Question

My understanding so far : If you define an optional variable without assign any values, the compiler will automatically assign the nil

Answer 1

Yes it's correct

An optional variable doesn't need to be manually initialized. If you read it before having populated it does contain nil.

From Apple docs

If you define an optional variable without providing a default value, the variable is automatically set to nil for you [...]

On the other hand the compiler does force you to manually initialize an Optional constant (let) before you can read it.

Unlike a variable, the value of a constant cannot be changed once it is set. Attempting to do so is reported as an error when your code is compiled [...]

Why?

A constant can be written only once. It doesn't need to happened on the same line it is initialized but it must happened before your read it.

E.g. this code works fine

let num: Int?
num = 1
print(num)

However if the compiler had put a temporary nil value inside num then the constant would have been wrote twice. Which is against the concept of constant.

let num: Int?
print(num) // nil ??? <- this can't work! 
num = 1
print(num) // 1

Another example

This code snippet works fine

func printArea(width: Int?, height:Int?) {
    let area: Int?
    if let width = width, height = height {
        area = width * height
    } else {
        area = nil
    }
    print(area)
}

Again, if the compiler had put a temporary nil value inside area then...

func printArea(width: Int?, height:Int?) {
    let area: Int?
    print(area) // not possible! area is going to change in a moment
     if let width = width, height = height {
        area = width * height
    } else {
        area = nil
    }
    print(area)
}