Literal types

[milessabin]

• Initial implementation — SIP-23 Implementation of literal types scala#5310
• Spec update — included in PR
• Fix issues and respond to feedback from TLS experience — in progress
• SIP update
• Coordination with Dotty

From the initial PR,

• Literals can now appear in type position, designating the corresponding singleton type.
• Widening/narrowing behaviour is consistent with 2.11.x and between 2.12.x with the flag enabled and disabled. The use of final on a val definition to explicitly request an unwidened type to be inferred is preserved. Type parameter inference remains the same unless the inference of a singleton type is explicitly requested via a <: Singleton bound.
• A scala.ValueOf[T] type class and corresponding scala.Predef.valueOf[T] operator has been added yielding the unique value of types with a single inhabitant.
• isInstanceOf/asInstanceOf tests/conversions are implemented via equality/identity tests for singleton types.
• Support for scala.Symbol literal types has been added.

[milessabin]

Fixed in scala/scala#5310 and subsequent bugfix PRs.