Don't recurse without bound in outerPath.

outerSelect returns EmptyTree if it hits a Symbol with no outer
accessor. outerPath doesn't check for this and will happily hand the
returned EmptyTree back to outerSelect, and so on until stack overflow.
The fix is to check for an EmptyTree result and terminate.

Fixes scala#10035.

Author: milessabin

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

@@ -264,7 +264,11 @@ abstract class ExplicitOuter extends InfoTransform
     protected def outerPath(base: Tree, from: Symbol, to: Symbol): Tree = {
       //Console.println("outerPath from "+from+" to "+to+" at "+base+":"+base.tpe)
       if (from == to) base
-      else outerPath(outerSelect(base), from.outerClass, to)
+      else {
+        val outerSel = outerSelect(base)
+        if (outerSel.isEmpty) EmptyTree
+        else outerPath(outerSel, from.outerClass, to)
+      }
     }
 
 

test/files/pos/t10035.scala

@@ -0,0 +1,11 @@
+trait Inner {
+  def f(): Outer
+}
+
+class Outer(o: Set[Inner]) {
+  def this() = this(Set(1).map{
+    case k => new Inner {
+      def f(): Outer = Outer.this
+    }
+  })
+}