Let's fix `if let` syntax

[ensan-hcl (ensan)]

About if value != nil, it is really problematic. Currently this code works.

var value: Int? = 42
// some process
if value != nil {
    // some process     
    // finally make value nil
    value = nil
}

If Swift introduces if value != nil that implicitly unwraps value, it starts causing error, because value inside if is no longer Int? but Int. It's source breaking change actually.

(EDIT)
About this smart cast, type guard, or 'inference', we did further discussion. If you have any objection about this, please check the link below.

[Pitch] Enum Case Inferencing

I was reading the discussion on Let’s fix if let syntax. It's a good discussion, but it overlooks how weird Swift is to encourage us to create variables that replace existing variables but with a different type (optional type replaced with non-optional type). let x = x is a weird pattern, but we are all just accustomed to it. Meanwhile, the proposal to just use if x != nil { } is not getting enough attention. There was an objection saying "I find it really weird that merely performing a compari…