Swift : If variable a is non-optional then why variable b is an optional?

Question

I am trying to understand Implicitly unwrapped optional in swift, I am confused regarding second variable as in below image.

Why variable b is an optional ?

Answer 1

Variable b is an optional because variable a 'might' return nil and in such case b will also be nil.

Edit: You have assigned an optional variable to another variable which you have not defined a specific type for. Xcode is saving you by creating the variable b as an optional. Consider this example:

var a: Int! = 6
a = nil
let b: Int = a
print(b)

Similar to yours, variable a here is an optional of type Int and b is a nonoptional of type Int too. Here I specifically define b to be a non optional integer unlike your b variable. Now, if i try to print variable who is assigned value of a which was at that point nil. This is dangerous and simply results in a fatal error at the assignment statement. The crash is simply understood because the compiler finds a nil value in a the optional type and tries to assign it to a non-optional.

Only optionals can hold nil value therefore when you assigned an optional(nillable) to another variable b, it is automatically treated as an optional because the value on which it relies is nillable and hence variable b is also nillable.