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Scalameta APIs for parsing and their baseline implementation

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package scala.meta
package quasiquotes

import scala.meta.common._

import scala.annotation.implicitNotFound
import scala.reflect.ClassTag

@implicitNotFound(msg = "don't know how to unlift ${I} into ${O}")
trait Unlift[I, O] extends Convert[I, Option[O]] {
  def unapply(x: I): Option[O] = apply(x)
}

object Unlift {
  def apply[I, O](pf: PartialFunction[I, O]): Unlift[I, O] = new Unlift[I, O] {
    def apply(x: I): Option[O] = pf.lift(x)
  }

  implicit def unliftBool[I >: Lit]: Unlift[I, Boolean] = Unlift { case Lit(x: Boolean) => x }
  implicit def unliftByte[I >: Lit]: Unlift[I, Byte] = Unlift { case Lit(x: Byte) => x }
  implicit def unliftShort[I >: Lit]: Unlift[I, Short] = Unlift { case Lit(x: Short) => x }
  implicit def unliftInt[I >: Lit]: Unlift[I, Int] = Unlift { case Lit(x: Int) => x }
  implicit def unliftLong[I >: Lit]: Unlift[I, Long] = Unlift { case Lit(x: Long) => x }
  implicit def unliftFloat[I >: Lit]: Unlift[I, Float] = Unlift { case Lit(x: Float) => x }
  implicit def unliftDouble[I >: Lit]: Unlift[I, Double] = Unlift { case Lit(x: Double) => x }
  implicit def unliftChar[I >: Lit]: Unlift[I, Char] = Unlift { case Lit(x: Char) => x }
  implicit def unliftString[I >: Lit]: Unlift[I, String] = Unlift { case Lit(x: String) => x }
  implicit def unliftSymbol[I >: Lit]: Unlift[I, Symbol] = Unlift { case Lit(x: Symbol) => x }
  implicit def unliftNull[I >: Lit]: Unlift[I, Null] = Unlift { case Lit(null) => null }
  implicit def unliftUnit[I >: Lit]: Unlift[I, Unit] = Unlift { case Lit(()) => () }

  implicit def unliftIdentity[I, O <: I: ClassTag]: Unlift[I, O] = Unlift { case x: O => x }

  implicit def unliftListToList[I, O: ClassTag](implicit
      unlift: Unlift[I, O]
  ): Unlift[List[I], List[O]] = Unlift { case x: List[I] => x.flatMap(unlift.apply) }

  implicit def unliftListViaImplicit[I <: Tree: ClassTag, O <: Tree](implicit
      conv: I => List[O]
  ): Unlift[I, List[O]] = Unlift { case x: I => conv(x) }

}