Handle missing delta positions when unpickling

A missing delta position signifies that the node has
the same position as its parent. Once that case was
added, we can now enable positions when reading from Tasty.

Author: odersky

src/dotty/tools/dotc/FromTasty.scala

@@ -66,18 +66,17 @@ object FromTasty extends Driver {
               case info: ClassfileLoader =>
                 info.load(clsd) match {
                   case Some(unpickler: DottyUnpickler) =>
-                    val (List(unpickled), source) = unpickler.body(readPositions = false)
+                    val (List(unpickled), source) = unpickler.body(readPositions = true)
                     val unit1 = new CompilationUnit(source)
                     unit1.tpdTree = unpickled
                     unit1
                   case _ =>
-                    cannotUnpickle(s"it does not have a TASTY attribute")
+                    cannotUnpickle(s"its class file ${info.classfile} does not have a TASTY attribute")
                 }
               case info =>
                 cannotUnpickle(s"its info of type ${info.getClass} is not a ClassfileLoader")
             }
           case _ =>
-            println(defn.EmptyPackageClass.info.decls)
             ctx.error(s"class not found: $className")
             unit
         }

src/dotty/tools/dotc/core/pickling/TreeUnpickler.scala

@@ -860,11 +860,12 @@ class TreeUnpickler(reader: TastyReader, tastyName: TastyName.Table) {
     }
   }
 
-  private def setNormalized(tree: Tree, parentPos: Position): Unit = {
-    assert(tree.pos.exists)
-    val absPos = Position(parentPos.start + offsetToInt(tree.pos.start), parentPos.end - tree.pos.end)
-    tree.setPosUnchecked(absPos)
-  }
+  private def setNormalized(tree: Tree, parentPos: Position): Unit =
+    tree.setPosUnchecked(
+      if (tree.pos.exists)
+        Position(parentPos.start + offsetToInt(tree.pos.start), parentPos.end - tree.pos.end)
+      else
+        parentPos)
 
   def normalizePos(x: Any, parentPos: Position)(implicit ctx: Context): Unit =
     traverse(x, parentPos, setNormalized)

src/dotty/tools/dotc/transform/Pickler.scala

@@ -89,7 +89,7 @@ class Pickler extends Phase {
       }
     pickling.println("************* entered toplevel ***********")
     for ((cls, unpickler) <- unpicklers) {
-      val (unpickled, source) = unpickler.body(readPositions = false)
+      val (unpickled, source) = unpickler.body(readPositions = true)
       testSame(i"$unpickled%\n%", beforePickling(cls), cls, source)
     }
   }