Interestingly, in the US, for all its "optimistic" procedures, a controller isn't allowed to issue a conditional ("behind X") line-up-and-wait.

But, if an arriving aircraft is still on the runway, a controller can clear a departing aircraft for takeoff if there will be adequate separation when the departing aircraft starts takeoff roll (i.e., the arriving aircraft will be clear of the runway at that point).

[1] 7110.65 3-9-5 Anticipating Separation