Beta 2 behaviour matched the docs at that time but there was plenty of pushback about the behaviour in radars, Apple dev forums[0] and on the wider web[1].

Chris Lattner announced[0] the planned change on the forum 19th June.

[0] https://devforums.apple.com/message/989944#989944

[1] http://blog.human-friendly.com/swift-arrays-too-swift-and-fl...