Swift lazy instantiating using self

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For some reason, a lazy property needs an explicit type annotation if its
initial value refers to self. This is mentioned on the swift-evolution mailing list, however I cannot explain why that is
necessary.

With

lazy var first: FirstClass = FirstClass(second: self)
//            ^^^^^^^^^^^^

your code compiles and runs as expected.

Here is another example which demonstrates that the problem occurs
also with structs, i.e. it is unrelated to subclassing:

func foo(x: Int) -> Int { return x + 1 }

struct MyClass {
    let x = 1

    lazy var y = foo(0)            // No compiler error
    lazy var z1 = foo(self.x)      // error: use of unresolved identifier 'self'
    lazy var z2: Int = foo(self.x) // No compiler error
}

The initial value of y does not depend on self and does not need a
type annotation. The initial values of z1/z2 depend on self,
and it compiles only with an explicit type annotation.

Update: This has been fixed in Swift 4/Xcode 9 beta 3,
lazy property initializers can now reference instance members without explicit self, and without explicit type annotation. (Thanks to @hamish for the update.)

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