Singleton
scala.Singleton
Singleton is used by the compiler as a supertype for singleton types. This includes literal types, as they are also singleton types.
scala> object A { val x = 42 }defined object Ascala> implicitly[A.type <:< Singleton]res12: A.type <:< Singleton = generalized constraintscala> implicitly[A.x.type <:< Singleton]res13: A.x.type <:< Singleton = generalized constraintscala> implicitly[42 <:< Singleton]res14: 42 <:< Singleton = generalized constraintscala> implicitly[Int <:< Singleton]^error: Cannot prove that Int <:< Singleton.Singleton has a special meaning when it appears as an upper bound on a formal type parameter. Normally, type inference in Scala widens singleton types to the underlying non-singleton type. When a type parameter has an explicit upper bound ofSingleton, the compiler infers a singleton type.
scala> def check42[T](x: T)(implicit ev: T =:= 42): T = xcheck42: [T](x: T)(implicit ev: T =:= 42)Tscala> val x1 = check42(42)^error: Cannot prove that Int =:= 42.scala> def singleCheck42[T <: Singleton](x: T)(implicit ev: T =:= 42): T = xsingleCheck42: [T <: Singleton](x: T)(implicit ev: T =:= 42)Tscala> val x2 = singleCheck42(42)x2: Int = 42Attributes
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