@kurtschelfthout This is basically just the code from my comment here. Upon further thought and experimentation, I think it might actually be valid, if not terribly elegant—but perhaps there is something that I still do not understand and that the tests do not cover. It essentially hinges on whether what I am doing with Shrink.getValue inside of Gen.promote (after applying runner) is valid in all scenarios.

Fixes #684.¹

• Add support for Async<'Testable> and Task<'Testable> as first-class testables.

  That is, Async<'Testable> and Task<'Testable> are now valid testables for any valid instantiation of 'Testable, just as Lazy<'Testable>, Gen<'Testable>, or 'Generatable -> 'Testable already were.

  [<Property>]
  let ``Unit - used to pass, still passes`` () = ()
  
  [<Property>]
  let ``Task<unit> - used to pass, still passes`` () = task { return () }
  
  [<Property>]
  let ``Task - used to pass, still passes`` () = task { return () } :> Task
  
  [<Property>]
  let ``Async<unit> - used to pass, still passes`` () = async { return () }
  
  [<Property>]
  let ``Property - used to fail, still fails`` b = (b && not b) |> Prop.label "b && not b"
  
  [<Property>]
  let ``Task<Property> - used to pass, now fails`` b = task { return (b && not b) |> Prop.label "b && not b" }
  
  [<Property>]
  let ``Async<Property> - used to pass, now fails`` b = async { return (b && not b) |> Prop.label "b && not b" }
  
  [<Property>]
  let ``Task<Lazy<bool>> - used to pass, now fails`` b = task { return lazy b }
  
  // Etc.