Runtime error when using moveInitializeFrom

moveInitializeFrom.swift

let count = 12

// Simple class that prints when it is created and destroyed
class Whiner {
    let _i: Int
    init(_ i: Int) { _i = i; println("Init: \(_i)") }
    deinit { println("Deinit: \(_i)") }
}

var ump: UnsafeMutablePointer<Whiner> = nil

for i in 0..<count {
    // allocate some fresh memory, one bigger than last time
    var newalloc = UnsafeMutablePointer<Whiner>.alloc(i+1)

    // initialize it from the old memory, destroying old values
    // I thought this would de-initialize the memory pointed to
    // by ump, and move the objects to the new memory of newallow
//    newalloc.moveInitializeFrom(ump, count: i)
    
    // after about 7 iterations (presumably the point at which alloc'd-then-freed
    // memory is reallocated) i get the following runtime error:
    //
    // fatal error: UnsafeMutablePointer.moveInitializeFrom non-following
    // overlapping range; use moveInitializeBackwardFrom

    // I would understand this if memory is being re-used in-place,
    // but this is instead freshly allocated memory so why would it matter?

    // calling moveInitializeBackwardFrom eliminates the problem,
    // but should this be necessary?
    newalloc.moveInitializeBackwardFrom(ump, count: i)
    
    // then add one more element on the end
    newalloc.advancedBy(i).initialize(Whiner(i))
    // deallocate the old memory
    ump.dealloc(i)
    // repoint to new memory
    ump = newalloc
}

// just to check the objects are all there correctly
let ubp = UnsafeBufferPointer(start: ump, count: count)
println(",".join(map(ubp, { toString($0._i) } )))


ump.destroy(count)
ump.dealloc(count)