Harden outer proxy computation of inlined code

It turns out that we simply cannot do reliable outer path
computation that fills in the right hand sides of this-proxies
from the types of these proxies. As-seen-from logic can mangle
the types of proxies enough to scramble the necessary information.

What we now do instead is simply count: We record the number
of outer accesses to an outer this in inlineable code, and do the same number
of outer accesses when computing the proxy.

Author: odersky

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

@@ -342,7 +342,7 @@ object tpd extends Trees.Instance[Type] with TypedTreeInfo {
   private def followOuterLinks(t: Tree)(implicit ctx: Context) = t match {
     case t: This if ctx.erasedTypes && !(t.symbol == ctx.owner.enclosingClass || t.symbol.isStaticOwner) =>
       // after erasure outer paths should be respected
-      new ExplicitOuter.OuterOps(ctx).path(t.tpe.widen.classSymbol)
+      new ExplicitOuter.OuterOps(ctx).path(toCls = t.tpe.widen.classSymbol)
     case t =>
       t
   }

compiler/src/dotty/tools/dotc/core/NameOps.scala

@@ -271,6 +271,13 @@ object NameOps {
       else -1
     }
 
+
+    /** The number of hops specified in an outer-select name */
+    def outerSelectHops: Int = {
+      require(isOuterSelect)
+      name.dropRight(nme.OUTER_SELECT.length).toString.toInt
+    }
+
     /** The name of the generic runtime operation corresponding to an array operation */
     def genericArrayOp: TermName = name match {
       case nme.apply => nme.array_apply

compiler/src/dotty/tools/dotc/transform/Erasure.scala

@@ -415,7 +415,7 @@ object Erasure extends TypeTestsCasts{
       if (tree.symbol == ctx.owner.lexicallyEnclosingClass || tree.symbol.isStaticOwner) promote(tree)
       else {
         ctx.log(i"computing outer path from ${ctx.owner.ownersIterator.toList}%, % to ${tree.symbol}, encl class = ${ctx.owner.enclosingClass}")
-        outer.path(tree.symbol)
+        outer.path(toCls = tree.symbol)
       }
 
     private def runtimeCallWithProtoArgs(name: Name, pt: Type, args: Tree*)(implicit ctx: Context): Tree = {

compiler/src/dotty/tools/dotc/transform/ExplicitOuter.scala

@@ -60,7 +60,8 @@ class ExplicitOuter extends MiniPhaseTransform with InfoTransformer { thisTransf
   /** Convert a selection of the form `qual.C_<OUTER>` to an outer path from `qual` to `C` */
   override def transformSelect(tree: Select)(implicit ctx: Context, info: TransformerInfo) =
     if (tree.name.isOuterSelect)
-      outer.path(tree.tpe.widen.classSymbol, tree.qualifier, outOfContext = true).ensureConforms(tree.tpe)
+      outer.path(start = tree.qualifier, count = tree.name.outerSelectHops)
+        .ensureConforms(tree.tpe)
     else tree
 
   /** First, add outer accessors if a class does not have them yet and it references an outer this.
@@ -354,31 +355,32 @@ object ExplicitOuter {
       } else Nil
     }
 
-    /** The path of outer accessors that references `toCls.this` starting from
-     *  node `start`, which defaults to the context owner's this node.
-     *  @param  outOfContext  When true, we take the `path` in code that has been inlined
-     *                        from somewhere else. In that case, we need to stop not
-     *                        just when `toCls` is reached exactly, but also in any superclass
-     *                        of `treeCls`. This compensates the `asSeenFrom` logic
-     *                        used to compute this-proxies in Inliner.
+    /** A path of outer accessors starting from node `start`. `start` defaults to the
+     *  context owner's this node. There are two alternative conditions that determine
+     *  where the path ends:
+     *
+     *   - if the initial `count` parameter is non-negative: where the number of
+     *     outer accessors reaches count.
+     *   - if the initial `count` parameter is negative: where the class symbol of
+     *     the type of the reached tree matches `toCls`.
      */
-    def path(toCls: Symbol,
-             start: Tree = This(ctx.owner.lexicallyEnclosingClass.asClass),
-             outOfContext: Boolean = false): Tree = try {
-      def loop(tree: Tree): Tree = {
+    def path(start: Tree = This(ctx.owner.lexicallyEnclosingClass.asClass),
+             toCls: Symbol = NoSymbol,
+             count: Int = -1): Tree = try {
+      def loop(tree: Tree, count: Int): Tree = {
         val treeCls = tree.tpe.widen.classSymbol
         val outerAccessorCtx = ctx.withPhaseNoLater(ctx.lambdaLiftPhase) // lambdalift mangles local class names, which means we cannot reliably find outer acessors anymore
         ctx.log(i"outer to $toCls of $tree: ${tree.tpe}, looking for ${outerAccName(treeCls.asClass)(outerAccessorCtx)} in $treeCls")
-        if (treeCls == toCls || outOfContext && toCls.derivesFrom(treeCls)) tree
+        if (count == 0 || count < 0 && treeCls == toCls) tree
         else {
           val acc = outerAccessor(treeCls.asClass)(outerAccessorCtx)
           assert(acc.exists,
               i"failure to construct path from ${ctx.owner.ownersIterator.toList}%/% to `this` of ${toCls.showLocated};\n${treeCls.showLocated} does not have an outer accessor")
-          loop(tree.select(acc).ensureApplied)
+          loop(tree.select(acc).ensureApplied, count - 1)
         }
       }
       ctx.log(i"computing outerpath to $toCls from ${ctx.outersIterator.map(_.owner).toList}")
-      loop(start)
+      loop(start, count)
     } catch {
       case ex: ClassCastException =>
         throw new ClassCastException(i"no path exists from ${ctx.owner.enclosingClass} to $toCls")

compiler/src/dotty/tools/dotc/transform/LambdaLift.scala

@@ -440,10 +440,10 @@ class LambdaLift extends MiniPhase with IdentityDenotTransformer { thisTransform
           singleton(clazz.thisType)
         else if (ctx.owner.isConstructor)
           outerParam.get(ctx.owner) match {
-            case Some(param) => outer.path(clazz, Ident(param.termRef))
-            case _ => outer.path(clazz)
+            case Some(param) => outer.path(start = Ident(param.termRef), toCls = clazz)
+            case _ => outer.path(toCls = clazz)
           }
-        else outer.path(clazz)
+        else outer.path(toCls = clazz)
       transformFollowingDeep(qual.select(sym))
     }
 

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

@@ -396,29 +396,29 @@ class Inliner(call: tpd.Tree, rhs: tpd.Tree)(implicit ctx: Context) {
     def classOf(selfSym: Symbol) = selfSym.info.widen.classSymbol
 
     // The name of the outer selector that computes the rhs of `selfSym`
-    def outerSelector(selfSym: Symbol): TermName = classOf(selfSym).name.toTermName ++ nme.OUTER_SELECT
+    def outerSelector(n: Int): TermName = n.toString.toTermName ++ nme.OUTER_SELECT
 
     // The total nesting depth of the class represented by `selfSym`.
     def outerLevel(selfSym: Symbol): Int = classOf(selfSym).ownersIterator.length
 
-    // All needed this-proxies, sorted by nesting depth of the classes they represent (innermost first)
-    val accessedSelfSyms =
-      thisProxy.toList.sortBy {
-        case (cls, proxy) => -outerLevel(cls)
-      } map {
-        case (cls, proxy) => proxy.symbol
-      }
+    // All needed this-proxies, paired-with and sorted-by nesting depth of
+    // the classes they represent (innermost first)
+    val sortedProxies = thisProxy.toList.map {
+      case (cls, proxy) => (outerLevel(cls), proxy.symbol)
+    } sortBy (-_._1)
 
     // Compute val-definitions for all this-proxies and append them to `bindingsBuf`
     var lastSelf: Symbol = NoSymbol
-    for (selfSym <- accessedSelfSyms) {
+    var lastLevel: Int = 0
+    for ((level, selfSym) <- sortedProxies) {
       val rhs =
         if (!lastSelf.exists)
           prefix
         else
-          untpd.Select(ref(lastSelf), outerSelector(selfSym)).withType(selfSym.info)
+          untpd.Select(ref(lastSelf), outerSelector(lastLevel - level)).withType(selfSym.info)
       bindingsBuf += ValDef(selfSym.asTerm, rhs)
       lastSelf = selfSym
+      lastLevel = level
     }
 
     // The type map to apply to the inlined tree. This maps references to this-types

tests/run/i1990b.check

@@ -0,0 +1 @@
+C()

tests/run/i1990b.scala

@@ -0,0 +1,20 @@
+trait A { self =>
+  class Foo {
+    inline def inlineMeth: Unit = {
+      println(self)
+    }
+  }
+}
+
+case class A2() extends A {
+  case class C() extends Foo with A
+
+  val c = new C
+  (new c.Foo).inlineMeth
+}
+
+object Test {
+  def main(args: Array[String]): Unit = {
+    new A2
+  }
+}