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Reflection Library for the Scala Programming Language
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/*
 * Scala (https://www.scala-lang.org)
 *
 * Copyright EPFL and Lightbend, Inc.
 *
 * Licensed under Apache License 2.0
 * (http://www.apache.org/licenses/LICENSE-2.0).
 *
 * See the NOTICE file distributed with this work for
 * additional information regarding copyright ownership.
 */

package scala
package reflect
package internal


/** The name of this trait defines the eventual intent better than
 *  it does the initial contents.
 */
trait ExistentialsAndSkolems {
  self: SymbolTable =>

  /** Map a list of type parameter symbols to skolemized symbols, which
   *  can be deskolemized to the original type parameter. (A skolem is a
   *  representation of a bound variable when viewed inside its scope.)
   *  !!!Adriaan: this does not work for hk types.
   *
   *  Skolems will be created at level 0, rather than the current value
   *  of `skolemizationLevel`. (See scala/bug#7782)
   */
  def deriveFreshSkolems(tparams: List[Symbol]): List[Symbol] = {
    class Deskolemizer extends LazyType {
      override val typeParams = tparams
      val typeSkolems  = typeParams map (_.newTypeSkolem setInfo this)
      override def complete(sym: Symbol) {
        // The info of a skolem is the skolemized info of the
        // actual type parameter of the skolem
        sym setInfo sym.deSkolemize.info.substSym(typeParams, typeSkolems)
      }
    }

    val saved = skolemizationLevel
    skolemizationLevel = 0
    try new Deskolemizer().typeSkolems
    finally skolemizationLevel = saved
  }

  def isRawParameter(sym: Symbol) = // is it a type parameter leaked by a raw type?
    sym.isTypeParameter && sym.owner.isJavaDefined

  /** If we map a set of hidden symbols to their existential bounds, we
   *  have a problem: the bounds may themselves contain references to the
   *  hidden symbols.  So this recursively calls existentialBound until
   *  the typeSymbol is not amongst the symbols being hidden.
   */
  private def existentialBoundsExcludingHidden(hidden: List[Symbol]): Map[Symbol, Type] = {
    def safeBound(t: Type): Type =
      if (hidden contains t.typeSymbol) safeBound(t.typeSymbol.existentialBound.upperBound) else t

    def hiBound(s: Symbol): Type = safeBound(s.existentialBound.upperBound) match {
      case tp @ RefinedType(parents, decls) =>
        val parents1 = parents mapConserve safeBound
        if (parents eq parents1) tp
        else copyRefinedType(tp, parents1, decls)
      case tp => tp
    }

    // Hanging onto lower bound in case anything interesting
    // happens with it.
    mapFrom(hidden)(s => s.existentialBound match {
      case TypeBounds(lo, hi) => TypeBounds(lo, hiBound(s))
      case _                  => hiBound(s)
    })
  }

  /** Given a set `rawSyms` of term- and type-symbols, and a type
   *  `tp`, produce a set of fresh type parameters and a type so that
   *  it can be abstracted to an existential type. Every type symbol
   *  `T` in `rawSyms` is mapped to a clone. Every term symbol `x` of
   *  type `T` in `rawSyms` is given an associated type symbol of the
   *  following form:
   *
   *    type x.type <: T with Singleton
   *
   *  The name of the type parameter is `x.type`, to produce nice
   *  diagnostics. The Singleton parent ensures that the type
   *  parameter is still seen as a stable type. Type symbols in
   *  rawSyms are fully replaced by the new symbols. Term symbols are
   *  also replaced, except for term symbols of an Ident tree, where
   *  only the type of the Ident is changed.
   */
  final def existentialTransform[T](rawSyms: List[Symbol], tp: Type, rawOwner: Symbol = NoSymbol)(creator: (List[Symbol], Type) => T): T = {
    val allBounds = existentialBoundsExcludingHidden(rawSyms)
    val typeParams: List[Symbol] = rawSyms map { sym =>
      val name = sym.name match {
        case x: TypeName  => x
        case x            => tpnme.singletonName(x)
      }
      def rawOwner0  = rawOwner orElse abort(s"no owner provided for existential transform over raw parameter: $sym")
      val bound      = allBounds(sym)
      val sowner     = if (isRawParameter(sym)) rawOwner0 else sym.owner
      val quantified = sowner.newExistential(name, sym.pos)

      quantified setInfo bound.cloneInfo(quantified)
    }
    // Higher-kinded existentials are not yet supported, but this is
    // tpeHK for when they are: "if a type constructor is expected/allowed,
    // tpeHK must be called instead of tpe."
    val typeParamTypes = typeParams map (_.tpeHK)
    def doSubst(info: Type) = info.subst(rawSyms, typeParamTypes)

    typeParams foreach (_ modifyInfo doSubst)
    creator(typeParams, doSubst(tp))
  }

  /**
   * Compute an existential type from hidden symbols `hidden` and type `tp`.
   * @param hidden   The symbols that will be existentially abstracted
   * @param tp       The original type
   * @param rawOwner The owner for Java raw types.
   */
  final def packSymbols(hidden: List[Symbol], tp: Type, rawOwner: Symbol = NoSymbol): Type =
    if (hidden.isEmpty) tp
    else existentialTransform(hidden, tp, rawOwner)(existentialAbstraction)
}