Compiler error on explicit types in for-comprehension

[ksilin]

Not an issue with your code, but a slight change leads to a compiler error which I cannot understand or fix. Perhaps you can shed some light on it.

adding types inside the for comprehension fails to compile:

      val summed = for {
        resultOne: Int <- one
        resultTwo: Int <- two
      } yield {
        resultOne + resultTwo
      }

The compiler error is: value filter is not a member of com.example.monadT.OptionTransformer[scala.concurrent.Future,Int] [error] resultOne: Int <- one

while this compiles and runs:

      val summed = for {
        resultOne <- one
        resultTwo <- two
      } yield {
        resultOne + resultTwo
      }

How is filter involved here? How would I fix it and still keep explicit types?