Empty refinements prevent unwanted widening when assigning singleton-typed values to a val.

gistfile1.scala

// Used in shapeless here: https://github.com/milessabin/shapeless/blob/master/core/src/main/scala/shapeless/syntax/singletons.scala#L42

scala> def narrow[T <: AnyRef](t: T): t.type = t
narrow: [T <: AnyRef](t: T)t.type

scala> val s1 = narrow("foo")  // Widened
s1: String = foo

scala> def narrow[T <: AnyRef](t: T): t.type {} = t  // Note empty refinement
narrow: [T <: AnyRef](t: T)t.type

scala> val s2 = narrow("foo")  // Not widened
s2: String("foo") = foo

Brilliant.

@nremond, could you please explain the difference between s1 and s2? I'm not following.

Thanks

s1 is a String that can 'hold' any string value (type is String), while s2 can only hold the string value "foo", because type is String("foo")