Test and refactor TreeInfo#defPath

[retronym]

Review by @odersky

[retronym]

I tried to change Positioned#contains, but that isn't so straight forward given that Positioned is not a sealed class, so I can't really just type-case over Modifiers and Tree.

I was going for something like:

    def contains(that: Positioned): Boolean = {
      object accum extends untpd.TreeAccumulator[Boolean] {
        override def apply(x: Boolean, tree: untpd.Tree): Boolean = {
          def modsContains = tree match {
            case md: untpd.MemberDef => md.mods.envelope contains that.pos
            case _ => false
          }
          x || (tree.envelope contains that.pos) || modsContains || foldOver(x, tree)
        }
      }
      (this eq that) || (that mach {
        case tree: Tree => accum(false, this)
        case mods: Modifiers => ... 
      }
    }

  class TreesTest extends DottyTest {
  @Test
  def testContains = checkCompile("frontend", "class A { def bar = { 22 }} ") {
    (tree, context) =>
      implicit val ctx = context
      val aTree = tree.find(tree => tree.symbol.name == typeName("A")).get.asInstanceOf[TypeDef]
      val barTree = tree.find(tree => tree.symbol.name == termName("bar")).get.asInstanceOf[ValDef]
      assertTrue(bar contains bar.mods)
      assertTrue(aTree contains aTree.mods)
      assertTrue(aTree contains bar)
      assertTrue(aTree contains bar.mods)
      assertFalse(barTree contains aTree)
      assertFalse(barTree.mods contains barTree)
  }
}

[retronym]

Can we make these available for untyped trees, too?

[retronym]

Regarding a traversal over trees and modifiers: we've added something similar to scalac recently: scala/scala#3033. Having a canonical traversal over "not quite trees" is pretty useful for the pickler, tree printing.

So maybe it is worth introducing PositionedAccumulator (and sealing Positioned).

[odersky]

The change looks good to me. Regarding contains, we seal Positioned and add a PositionedAccumulator; I see no problem with that. The current solution works, but it's not as extensible as it could be.