SI-8085 Fix BrowserTraverser for package objects

A source file like:

    import foo.bar
    package object baz

Is parsed into:

    package <empty> {
       import foo.bar
       package baz {
          object `package`
       }
    }

A special case in Namers compensates by adjusting the owner of
`baz` to be `<root>`, rather than `<empty>`.

This wasn't being accounted for in `BrowserTraverser`, which
underpins `-sourcepath`, and allows the presentation compiler to
load top level symbols from sources outside those passes as
the list of sources to compile.

This bug did not appear in sources like:

    package p1
    package object p2 { ... }

... because the parser does not wrap this in the `package <empty> {}`

This goes some way to explaining why it has gone unnoticed for
so long.

src/compiler/scala/tools/nsc/symtab/BrowsingLoaders.scala

@@ -64,8 +64,10 @@ abstract class BrowsingLoaders extends SymbolLoaders {
           addPackagePrefix(pre)
           packagePrefix += ("." + name)
         case Ident(name) =>
-          if (packagePrefix.length != 0) packagePrefix += "."
-          packagePrefix += name
+          if (name != nme.EMPTY_PACKAGE_NAME) { // mirrors logic in Namers, see createPackageSymbol
+            if (packagePrefix.length != 0) packagePrefix += "."
+            packagePrefix += name
+          }
         case _ =>
           throw new MalformedInput(pkg.pos.point, "illegal tree node in package prefix: "+pkg)
       }

test/files/presentation/t8085.check

@@ -1,3 +1,3 @@
 reload: NodeScalaSuite.scala
-prefixes differ: <empty>.nodescala,nodescala
-value always is not a member of object scala.concurrent.Future
+open package module: package nodescala
+Test OK

test/files/presentation/t8085/Test.scala

@@ -5,7 +5,6 @@ import scala.tools.nsc.interactive.Response
 object Test extends InteractiveTest {
 
   override def execute(): Unit = {
-    // loadSourceAndWaitUntilTypechecked("package.scala") // <-- uncomment this line and the test succeed
     val src = loadSourceAndWaitUntilTypechecked("NodeScalaSuite.scala")
     checkErrors(src)
   }

test/files/presentation/t8085/src/nodescala/NodeScalaSuite.scala

@@ -1,12 +1,10 @@
 package nodescala
 
-import scala.concurrent.Future
-
 class NodeScalaSuite {
-  Future.always(517)
+  "".rich
 
   // This is here only to prove that the presentation compiler is instantiated with the
   // correct `sourcepath` value (if it wasn't, you would see a `not found: type Foo` in
   // the test's output
   println(new Foo())
-}
+}

test/files/presentation/t8085/src/nodescala/package.scala

@@ -1,8 +1,7 @@
-import scala.concurrent.Future // <-- if you move the import *inside* the package object, then it all works fine!!
+import scala.Some // <-- if you move the import *inside* the package object, then it all works fine!!
 
 package object nodescala {
-  implicit class FutureCompanionOps[T](val f: Future.type) extends AnyVal {
-    def always[T](value: T): Future[T] = Promise[T].success(value).future
+  implicit class StringOps(val f: String) {
+    def rich = 0
   }
 }
-

test/files/presentation/t8085b.check

@@ -0,0 +1,3 @@
+reload: NodeScalaSuite.scala
+open package module: package nodescala
+Test OK

test/files/presentation/t8085b.flags

@@ -0,0 +1 @@
+-sourcepath src

test/files/presentation/t8085b/Test.scala

@@ -0,0 +1,27 @@
+import scala.tools.nsc.interactive.tests.InteractiveTest
+import scala.reflect.internal.util.SourceFile
+import scala.tools.nsc.interactive.Response
+
+object Test extends InteractiveTest {
+
+  override def execute(): Unit = {
+    val src = loadSourceAndWaitUntilTypechecked("NodeScalaSuite.scala")
+    checkErrors(src)
+  }
+
+  private def loadSourceAndWaitUntilTypechecked(sourceName: String): SourceFile = {
+    val sourceFile = sourceFiles.find(_.file.name == sourceName).head
+    askReload(List(sourceFile)).get
+    askLoadedTyped(sourceFile).get
+    sourceFile
+  }
+
+  private def checkErrors(source: SourceFile): Unit = compiler.getUnitOf(source) match {
+    case Some(unit) =>
+      val problems = unit.problems.toList
+      if(problems.isEmpty) reporter.println("Test OK")
+      else problems.foreach(problem => reporter.println(problem.msg))
+
+    case None => reporter.println("No compilation unit found for " + source.file.name)
+  }
+}

test/files/presentation/t8085b/src/p1/nodescala/Foo.scala

@@ -0,0 +1,4 @@
+package p1
+package nodescala
+
+class Foo

test/files/presentation/t8085b/src/p1/nodescala/NodeScalaSuite.scala

@@ -0,0 +1,11 @@
+package p1
+package nodescala
+
+class NodeScalaSuite {
+  "".rich
+
+  // This is here only to prove that the presentation compiler is instantiated with the
+  // correct `sourcepath` value (if it wasn't, you would see a `not found: type Foo` in
+  // the test's output
+  println(new Foo())
+}

test/files/presentation/t8085b/src/p1/nodescala/package.scala

@@ -0,0 +1,9 @@
+import scala.Some // <-- if you move the import *inside* the package object, then it all works fine!!
+
+package p1 {
+  package object nodescala {
+    implicit class StringOps(val f: String) {
+      def rich = 0
+    }
+  }
+}