Swift operators and nil [duplicate]

Answer 1

Two things happen here (whether we like it or not): First, there is an operator

public func <(lhs: T?, rhs: T?) -> Bool

which compares two optionals if the underlying type is comparable. The behavior is not documented (as far as I know), but it seems that nil aka Optional.None is considered less than all non-nil values Optional.Some(value).

Second, enum Optional has a constructor

/// Construct a non-`nil` instance that stores `some`.
public init(_ some: Wrapped)

Now in

if x < 10 { ... }

the lhs has the type Optional. The only candidate for the < operator is the above-mentioned one comparing two optionals. Therefore the rhs is inferred as an optional as well, so this is equivalent to

if x < Optional.Some(10) { ... }

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Update:

This feature has been removed in Swift 3 (SE-0121 – Remove Optional Comparison Operators) and that code no longer compiles with Xcode 8 (currently beta 6).