SI-5508 Fix crasher with private[this] in nested traits

Currently, accessors for private local trait fields are added
very late in the game when the `Mixin` tree transformer treats the
trait. By contrast, fields with weaker access have accessors created
eagerly in `Namers`.

    // Mixin#addLateInterfaceMembers
    val getter = member.getter(clazz)
    if (getter == NoSymbol) addMember(clazz, newGetter(member))

`addMember` mutates the type of the interface to add the getter.
(This seems like a pretty poor design: usually if a phase changes
types, it should do in an `InfoTransformer`.)

However, if an inner class or anonymous function of the trait
has been flattened to a spot where it precedes the trait in the
enclosing packages info, this code hasn't had a chance to run,
and the lookup of the getter crashes as mixins `postTransform`
runs over a selection of the not-yet-materialized getter.

    // Mixin#postTransform
	case Select(qual, name) if sym.owner.isImplClass && !isStaticOnly(sym) =>
	  val iface  = toInterface(sym.owner.tpe).typeSymbol
	  val ifaceGetter = sym getter iface

This commit ensures that `Flatten` lifts inner classes to
a position *after* the enclosing class in the stats of the
enclosing package.

Bonus fix: SI-7012 (the followup ticket to SI-6231 / SI-2897)

Author: retronym

src/compiler/scala/tools/nsc/transform/Flatten.scala

@@ -100,23 +100,36 @@ abstract class Flatten extends InfoTransform {
     /** Buffers for lifted out classes */
     private val liftedDefs = perRunCaches.newMap[Symbol, ListBuffer[Tree]]()
 
-    override def transform(tree: Tree): Tree = {
+    override def transform(tree: Tree): Tree = postTransform {
       tree match {
         case PackageDef(_, _) =>
           liftedDefs(tree.symbol.moduleClass) = new ListBuffer
+          super.transform(tree)
         case Template(_, _, _) if tree.symbol.isDefinedInPackage =>
           liftedDefs(tree.symbol.owner) = new ListBuffer
+          super.transform(tree)
+        case ClassDef(_, _, _, _) if tree.symbol.isNestedClass =>
+          // SI-5508 Ordering important. In `object O { trait A { trait B } }`, we want `B` to appear after `A` in
+          //         the sequence of lifted trees in the enclosing package. Why does this matter? Currently, mixin
+          //         needs to transform `A` first to a chance to create accessors for private[this] trait fields
+          //         *before* it transforms inner classes that refer to them. This also fixes SI-6231.
+          //
+          //         Alternative solutions
+          //            - create the private[this] accessors eagerly in Namer (but would this cover private[this] fields
+          //              added later phases in compilation?)
+          //            - move the accessor creation to the Mixin info transformer
+          val liftedBuffer = liftedDefs(tree.symbol.enclosingTopLevelClass.owner)
+          val index = liftedBuffer.length
+          liftedBuffer.insert(index, super.transform(tree))
+          EmptyTree
         case _ =>
+          super.transform(tree)
       }
-      postTransform(super.transform(tree))
     }
 
     private def postTransform(tree: Tree): Tree = {
       val sym = tree.symbol
       val tree1 = tree match {
-        case ClassDef(_, _, _, _) if sym.isNestedClass =>
-          liftedDefs(sym.enclosingTopLevelClass.owner) += tree
-          EmptyTree
         case Select(qual, name) if sym.isStaticModule && !sym.isTopLevel =>
           exitingFlatten {
             atPos(tree.pos) {
@@ -134,7 +147,10 @@ abstract class Flatten extends InfoTransform {
     /** Transform statements and add lifted definitions to them. */
     override def transformStats(stats: List[Tree], exprOwner: Symbol): List[Tree] = {
       val stats1 = super.transformStats(stats, exprOwner)
-      if (currentOwner.isPackageClass) stats1 ::: liftedDefs(currentOwner).toList
+      if (currentOwner.isPackageClass) {
+        val lifted = liftedDefs(currentOwner).toList
+        stats1 ::: lifted
+      }
       else stats1
     }
   }

test/files/neg/t6231.check

@@ -0,0 +1,6 @@
+object Test {
+  trait NestedTrait { // must be nested and a trait
+    private val _st : Int = 0 // crashes if changed to private[this]
+    val escape = { () => _st }
+  }
+}

test/files/neg/t6231.flags

@@ -0,0 +1,4 @@
+trait TopTrait { // must be nested and a trait
+  private[this] val _st : Int = 0 // crashes if TopTrait is not top level
+  val escape = { () => _st }
+}

test/files/pos/t5508-min-okay.scala

@@ -0,0 +1,6 @@
+object Test {
+  trait NestedTrait { // must be nested and a trait
+    private[this] val _st : Int = 0 // must be private[this]
+    val escape = { () => _st }
+  }
+}

test/files/pos/t5508-min-okay2.scala

@@ -0,0 +1,83 @@
+package TestTestters
+
+trait Test1 {
+  private[this] var _st : Int = 0
+  def close : PartialFunction[Any,Any] = {
+    case x : Int =>
+      _st = identity(_st)
+  }
+}
+
+object Base1 {
+  trait Test2 {
+    private[this] var _st : Int = 0
+    def close : PartialFunction[Any,Any] = {
+      case x : Int =>
+  _st = identity(_st)
+    }
+  }
+}
+
+class Test3 {
+  private[this] var _st : Int = 0
+  def close : PartialFunction[Any,Any] = {
+    case x : Int =>
+      _st = 1
+  }
+}
+
+object Base2 {
+  class Test4 {
+    private[this] var _st : Int = 0
+    def close : PartialFunction[Any,Any] = {
+      case x : Int =>
+  _st = 1
+    }
+  }
+}
+
+class Base3 {
+  trait Test5 {
+    private[this] var _st : Int = 0
+    def close : PartialFunction[Any,Any] = {
+      case x : Int =>
+  _st = 1
+    }
+  }
+}
+
+object Base4 {
+  trait Test6 {
+    private[this] var _st : Int = 0
+    def close : PartialFunction[Any,Any] = {
+      case x : Int => ()
+    }
+  }
+}
+
+object Base5 {
+  trait Test7 {
+    private[this] var _st : Int = 0
+    def close = () => {
+      _st = 1
+    }
+  }
+}
+
+object Base6 {
+  class Test8 {
+    private[this] var _st : Int = 0
+    def close = () => {
+      _st = 1
+    }
+  }
+}
+
+object Base7 {
+  trait Test9 {
+    var st : Int = 0
+    def close = () => {
+      st = 1
+    }
+  }
+}

test/files/pos/t5508-min.scala

@@ -0,0 +1,8 @@
+class Test {
+  def f1(t: String) = {
+    trait T {
+      def xs = Nil map (_ => t)
+    }
+    ()
+  }
+}