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Empty refinements prevent unwanted widening when assigning singleton-typed values to a val.
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| // 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 |
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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")