Automatic Unit inference has been deprecated in scala 2.11, and the reason behind this is that it can lead to confusing behavior, especially for people learning the language.
Here’s an example
class Foo[T](value: T)
val x = new Foo
This should not compile, right? You are calling the constructor with no arguments, where one is required. Surprisingly, until scala 2.10.4 this compiles just fine, with no errors or warnings.
And that’s because the compiler inferred a Unit argument, so it actually replaced your code with
val x = new Foo[Unit](()) // Foo[Unit]
As the newly introduced warning message says, this is unlikely to be what you want.
Another famous example is this
scala> List(1,2,3).toSet()
// res1: Boolean = false
calling toSet() should be a compile-time error, since toSet does not take arguments, but the compiler desperately tries to make it compile, ultimately interpreting the code as
scala> List(1,2,3).toSet.apply(())
which means: test whether () belongs to the set. Since it’s not the case, you get a false!
So, starting from scala 2.11, you have to be explicit if you want to pass () (aka Unit) as an argument. That’s why you have to write:
Right(())
instead of
Right()
examples taken from Simplifying Scala — The Past, Present and Future by Simon Ochsenreither.