SI-9920 Avoid linkage errors with captured local objects + self types

An outer parameter of a nested class is typed with the self type
of the enclosing class:

```
class C; trait T { _: C => def x = 42; class D { x } }
```

leads to:

```
    class D extends Object {
      def <init>($outer: C): T.this.D = {
        D.super.<init>();
        ()
      };
      D.this.$outer().$asInstanceOf[T]().x();
```

Note that a cast is inserted before the call to `x`.

If we modify that a little, to instead capture a local module:

```
class C; trait T { _: C => def y { object O; class D { O } } }
```

Scala 2.11 used to generate (after lambdalift):

```
    class D$1 extends Object {
      def <init>($outer: C, O$module$1: runtime.VolatileObjectRef): C#D$1 = {
        D$1.super.<init>();
        ()
      };
      D$1.this.$outer().O$1(O$module$1);
```

That isn't type correct, `D$1.this.$outer() : C` does not
have a member `O$1`.

However, the old trait encoding would rewrite this in mixin to:

```
T$class.O$1($outer, O$module$1);
```

Trait implementation methods also used to accept the self type:

```
trait T$class {
   final <stable> def O$1($this: C, O$module$1: runtime.VolatileObjectRef): T$O$2.type
}
```

So the problem was hidden.

This commit changes replaces manual typecheckin of the selection in LambdaLift with
a use of the local (erasure) typer, which will add casts as needed.

For `run/t9220.scala`, this changes the post LambdaLift AST as follows:

```
     class C1$1 extends Object {
       def <init>($outer: C0, Local$module$1: runtime.VolatileObjectRef): T#C1$1 = {
         C1$1.super.<init>();
         ()
       };
-      C1$1.this.$outer.Local$1(Local$module$1);
+      C1$1.this.$outer.$asInstanceOf[T]().Local$1(Local$module$1);
       <synthetic> <paramaccessor> <artifact> private[this] val $outer: C0 = _;
       <synthetic> <stable> <artifact> def $outer(): C0 = C1$1.this.$outer
     }
```

7 files changed, 84 insertions, 2 deletions

diff --git a/src/compiler/scala/tools/nsc/transform/ExplicitOuter.scala b/src/compiler/scala/tools/nsc/transform/ExplicitOuter.scala
index f3d5ceb0f0..7d50c12852 100644
--- a/src/compiler/scala/tools/nsc/transform/ExplicitOuter.scala
+++ b/src/compiler/scala/tools/nsc/transform/ExplicitOuter.scala
@@ -8,7 +8,7 @@ package tools.nsc
 package transform
 
 import symtab._
-import Flags.{ CASE => _, _ }
+import Flags.{CASE => _, _}
 import scala.collection.mutable.ListBuffer
 
 /** This class ...
diff --git a/src/compiler/scala/tools/nsc/transform/LambdaLift.scala b/src/compiler/scala/tools/nsc/transform/LambdaLift.scala
index 74e6c58388..10d9c5627b 100644
--- a/src/compiler/scala/tools/nsc/transform/LambdaLift.scala
+++ b/src/compiler/scala/tools/nsc/transform/LambdaLift.scala
@@ -319,7 +319,14 @@ abstract class LambdaLift extends InfoTransform {
           else if (clazz.isStaticOwner) gen.mkAttributedQualifier(clazz.thisType)
           else outerValue match {
             case EmptyTree => prematureSelfReference()
-            case o         => outerPath(o, currentClass.outerClass, clazz)
+            case o         =>
+              val path = outerPath(o, currentClass.outerClass, clazz)
+              if (path.tpe <:< clazz.tpeHK) path
+              else {
+                // SI-9920 The outer accessor might have an erased type of the self type of a trait,
+                //         rather than the trait itself. Add a cast if necessary.
+                gen.mkAttributedCast(path, clazz.tpeHK)
+              }
           }
         }
 
diff --git a/test/files/pos/t9920.scala b/test/files/pos/t9920.scala
new file mode 100644
index 0000000000..8612618cc4
--- /dev/null
+++ b/test/files/pos/t9920.scala
@@ -0,0 +1,6 @@
+object Test {
+  def o = {
+    def i: Int = { i; 0 }
+    i
+  }
+}
diff --git a/test/files/run/t9920.scala b/test/files/run/t9920.scala
new file mode 100644
index 0000000000..5dc32e99b7
--- /dev/null
+++ b/test/files/run/t9920.scala
@@ -0,0 +1,17 @@
+class C0
+trait T { self: C0 =>
+  def test = {
+    object Local
+
+    class C1 {
+      Local
+    }
+    new C1()
+  }
+}
+
+object Test extends C0 with T {
+  def main(args: Array[String]): Unit = {
+    test
+  }
+}
diff --git a/test/files/run/t9920b.scala b/test/files/run/t9920b.scala
new file mode 100644
index 0000000000..fab196b669
--- /dev/null
+++ b/test/files/run/t9920b.scala
@@ -0,0 +1,17 @@
+class C0
+trait T {
+  def test = {
+    object Local
+
+    class C1 {
+      Local
+    }
+    new C1()
+  }
+}
+
+object Test extends C0 with T {
+  def main(args: Array[String]): Unit = {
+    test
+  }
+}
diff --git a/test/files/run/t9920c.scala b/test/files/run/t9920c.scala
new file mode 100644
index 0000000000..9541dc650a
--- /dev/null
+++ b/test/files/run/t9920c.scala
@@ -0,0 +1,21 @@
+class C0
+trait T { self: C0 =>
+  def test = {
+    object Local
+
+    class C2 {
+      class C1 {
+        Local
+      }
+      T.this.toString
+      new C1
+    }
+    new C2()
+  }
+}
+
+object Test extends C0 with T {
+  def main(args: Array[String]): Unit = {
+    test
+  }
+}
diff --git a/test/files/run/t9920d.scala b/test/files/run/t9920d.scala
new file mode 100644
index 0000000000..debc99e199
--- /dev/null
+++ b/test/files/run/t9920d.scala
@@ -0,0 +1,14 @@
+class C { object O }
+trait T { _: C =>
+  def foo {
+    class D { O }
+    new D
+  }
+}
+
+
+object Test extends C with T {
+  def main(args: Array[String]): Unit = {
+    foo
+  }
+}