Indirect compile-time literal types

macroImpl.scala

class MacroBundle(val c: Context) {
  /* etc */
  
  /** Typecheck singleton types so as to obtain indirectly
   *  available known-at-compile-time values.
   */
  object Const {
    def unapply(tp: Type): Option[Constant] = tp match {
      case ConstantType(c)    => Some(c)
      case SingleType(_, sym) => unapply(c.typecheck(q"$sym").tpe)
      case _                  => None
    }
  }
  def constInt(tp: Type): Tree = tp match {
    case Const(Constant(n: Int)) => q"$n"
    case _                       => q"-1"
  }
  def constImpl[A: c.WeakTypeTag](value: c.Expr[A]): Tree = constInt(weakTypeOf[A])
  def constTypeImpl[A: c.WeakTypeTag] : Tree              = constInt(weakTypeOf[A])
}

test.scala

package psply.tests

import scala.language.experimental.macros

class ConstTests {
  def const[A <: Int with Singleton](value: A): Int = macro psply.trees.PsplyMacros.constImpl[A]
  def constType[A <: Int with Singleton] : Int      = macro psply.trees.PsplyMacros.constTypeImpl[A]

  val one: 1          = 1
  val oneA: one.type  = one
  val oneB: oneA.type = oneA
  val oneC            = oneB // inferred singleton type
  final val oneD      = 1    // inferred singleton type

  val unknown1: Int = 1
  val unknown2      = 1 // non-final means Int is inferred

  @Test
  def constTest(): Unit = {
    assertEqual(const(1))(1)
    assertEqual(const(one))(1)
    assertEqual(const(oneA))(1)
    assertEqual(const(oneB))(1)
    assertEqual(const(oneC))(1)
    assertEqual(const(oneD))(1)
    // doesn't always return 1
    assertUnequal(const(unknown1))(1)
    assertUnequal(const(unknown2))(1)
  }
  @Test
  def constTypeTest(): Unit = {
    assertEqual(constType[1])(1)
    assertEqual(constType[one.type])(1)
    assertEqual(constType[oneA.type])(1)
    assertEqual(constType[oneB.type])(1)
    assertEqual(constType[oneC.type])(1)
    assertEqual(constType[oneD.type])(1)
    // doesn't always return 1
    assertUnequal(constType[unknown1.type])(1)
    assertUnequal(constType[unknown2.type])(1)
  }
}