Mock a method without arguments but with implicit parameters

You didn't look at the right example, I guess. Look at example 4 for implicit parameters:

class Codec()

trait Memcached {
  def get(key: String)(implicit codec: Codec): Option[Int]
}

val memcachedMock = mock[Memcached]

implicit val codec = new Codec
(memcachedMock.get(_ : String)(_ : Codec)).expects("some_key", *).returning(Some(123))

In your case, of course, the non-implicit params are null, so you want:

(m.getResult()(_: ExecutionContext)).expects(*) returning "..."

Just to complete the cases. If someone tries to do the same with more params the snippet is the following:

trait MemcachedV2 {
   def get(key: String, value: String)(implicit codec: Codec): Option[Int]
}

val memcachedMock2 = mock[MemcachedV2]

(memcachedMock2.get(_, _)(_))
   .expects("some_key","another value", *)
   .returning(Some(123))