var a = a や if let b = b {} のような紛らわしい構文も使えるswiftのシャドウイング ref: http://qiita.com/giigame/items/bdd7ab74ec8321b73b97

file0.swift

func foo(i: Int) {
  var i = i
}

file1.swift

let i = 3
do {
  var i = i   // Error: Variable used within its own initial value
}

file2.swift

let b: Int? = 3
if let b = b {
    print(b)   // 3
}
print(b)   // Optional(3)