Tests to clarify projection and erased paths

Author: odersky

tests/neg/erased-path.scala

@@ -0,0 +1,10 @@
+
+trait Sys { type X }
+
+trait Obj {
+  erased val s: Sys
+  lazy val t: Sys
+
+  type S = s.X  // error: not a legal path, since nonfinal
+  type T = t.X  // error: not a legal path, since nonfinal
+}

tests/neg/erased-singleton.scala

@@ -0,0 +1,7 @@
+trait Sys
+
+trait Obj {
+  erased val s: Sys
+
+  type S = s.type  // error: non final
+}

tests/pos-scala2/type-projection.scala

@@ -0,0 +1,28 @@
+trait Txn[S <: Sys[S]] {
+  def system: S
+
+  def newId(): S#Id
+
+  def newVar[A](id: S#Id, init: A): S#Vr[A]
+}
+
+trait Var[Tx, A] {
+  def apply()(implicit tx: Tx): A
+
+  def update(x: A)(implicit tx: Tx): Unit
+}
+
+trait Sys[S <: Sys[S]] {
+  type Tx <: Txn[S]
+  type Id
+  type Vr[A] <: Var[S#Tx, A]
+}
+
+abstract class UseCase[S <: Sys[S]](id: S#Id) {
+  def mk(x: Int)(implicit tx: S#Tx): S#Vr[Int] = {
+    val vr = tx.newVar[Int](id, x)
+    vr
+  }
+
+  def rd(vr: S#Vr[Int])(implicit tx: S#Tx): Int = vr()
+}