Fixes to positions

 - harden addChild to also work if existing children don't have positions
   (this can happen if they came from other compilation units)
 - tweak position of Deriver, so that it always points to the deriving class
   template.

Author: odersky

compiler/src/dotty/tools/dotc/ast/Desugar.scala

@@ -22,7 +22,13 @@ object desugar {
   /** If a Select node carries this attachment, suppress the check
    *  that its type refers to an acessible symbol.
    */
-  val SuppressAccessCheck = new Property.Key[Unit]
+  val SuppressAccessCheck: Property.Key[Unit] = new Property.Key
+
+  /** An attachment for companion modules of classes that have a `derives` clause.
+   *  The position value indicates the start position of the template of the
+   *  deriving class.
+   */
+  val DerivingCompanion: Property.Key[Position] = new Property.Key
 
   /** Info of a variable in a pattern: The named tree and its type */
   private type VarInfo = (NameTree, Tree)
@@ -545,12 +551,17 @@ object desugar {
 
     // The thicket which is the desugared version of the companion object
     //     synthetic object C extends parentTpt derives class-derived { defs }
-    def companionDefs(parentTpt: Tree, defs: List[Tree]) =
-      moduleDef(
+    def companionDefs(parentTpt: Tree, defs: List[Tree]) = {
+      val mdefs = moduleDef(
         ModuleDef(
           className.toTermName, Template(emptyConstructor, parentTpt :: Nil, companionDerived, EmptyValDef, defs))
             .withMods(companionMods | Synthetic))
-      .withPos(cdef.pos).toList
+        .withPos(cdef.pos).toList
+      if (companionDerived.nonEmpty)
+        for (modClsDef @ TypeDef(_, _) <- mdefs)
+          modClsDef.putAttachment(DerivingCompanion, impl.pos.startPos)
+      mdefs
+    }
 
     val companionMembers = defaultGetters ::: eqInstances ::: enumCases
 

compiler/src/dotty/tools/dotc/typer/Namer.scala

@@ -196,7 +196,6 @@ class Namer { typer: Typer =>
   val TypedAhead: Property.Key[tpd.Tree] = new Property.Key
   val ExpandedTree: Property.Key[untpd.Tree] = new Property.Key
   val SymOfTree: Property.Key[Symbol] = new Property.Key
-  val DerivingCompanion: Property.Key[Unit] = new Property.Key
   val Deriver: Property.Key[typer.Deriver] = new Property.Key
 
   /** A partial map from unexpanded member and pattern defs and to their expansions.
@@ -508,7 +507,7 @@ class Namer { typer: Typer =>
     val childStart = child.pos.start
     def insertInto(annots: List[Annotation]): List[Annotation] =
       annots.find(_.symbol == defn.ChildAnnot) match {
-        case Some(Annotation.Child(other)) if childStart <= other.pos.start =>
+        case Some(Annotation.Child(other)) if other.pos.exists && childStart <= other.pos.start =>
           if (child == other)
             annots // can happen if a class has several inaccessible children
           else {
@@ -574,7 +573,7 @@ class Namer { typer: Typer =>
           if (fromTempl.derived.nonEmpty) {
             if (modTempl.derived.nonEmpty)
               ctx.error(em"a class and its companion cannot both have `derives' clauses", mdef.pos)
-            res.putAttachment(DerivingCompanion, ())
+            res.putAttachment(desugar.DerivingCompanion, fromTempl.pos.startPos)
           }
           res
         }
@@ -1012,11 +1011,11 @@ class Namer { typer: Typer =>
       typr.println(i"completing $denot, parents = $parents%, %, parentTypes = $parentTypes%, %")
 
       if (impl.derived.nonEmpty) {
-        val derivingClass =
-          if (original.removeAttachment(DerivingCompanion).isDefined ||
-              original.mods.is(Synthetic)) cls.companionClass.asClass
-          else cls
-        val deriver = new Deriver(derivingClass, impl.pos.startPos)(localCtx)
+        val (derivingClass, derivePos) = original.removeAttachment(desugar.DerivingCompanion) match {
+          case Some(pos) => (cls.companionClass.asClass, pos)
+          case None => (cls, impl.pos.startPos)
+        }
+        val deriver = new Deriver(derivingClass, derivePos)(localCtx)
         deriver.enterDerived(impl.derived)
         original.putAttachment(Deriver, deriver)
       }

compiler/test/dotc/run-from-tasty.blacklist

@@ -10,5 +10,6 @@ typeclass-derivation1.scala
 typeclass-derivation2.scala
 typeclass-derivation2a.scala
 typeclass-derivation3.scala
+derive-generic.scala
 deriving-interesting-prefixes.scala
 

compiler/test/dotc/run-test-pickling.blacklist

@@ -18,4 +18,5 @@ typeclass-derivation1.scala
 typeclass-derivation2.scala
 typeclass-derivation2a.scala
 typeclass-derivation3.scala
+derive-generic.scala
 deriving-interesting-prefixes.scala

tests/neg/deriving.scala

@@ -1,6 +1,6 @@
 import reflect.Generic
 
-sealed trait A derives Generic  // error: cannot take shape, it has anonymous or inaccessible subclasses
+sealed trait A derives Generic // error: cannot take shape, it has anonymous or inaccessible subclasses
 
 object A {
   def f() = {