Object emitter to cast shadow only

[DNorman]

You can do this with lights by using two lamps one with negative light that does not cast shadows.

Make a light only cast shadow in Cycles

You can read the solution in that book, too: “If your software does not support either of the techniques just described, there’s another way to create an identical effect. This works in any program that supports negative lights. Start with two copies of a spotlight in the same place. Give the first light a positive intensity of 1 and set it to cast shadows. Give the second light a –1 intensity but do not set it to cast shadows. These two lights work as a pair; the first light adds illuminat…

I have not been able do do it with a “geometrical” emitter object, you can set the emitter object to cast negative light like this

Edit: I can not find a way to set an emission object to not cast shadows in cycles. So I think you will have to go with using lights.