Include possible candidate classes in "not found" error

src/compiler/scala/tools/nsc/backend/JavaPlatform.scala

@@ -21,6 +21,27 @@ trait JavaPlatform extends Platform {
 
   private[nsc] var currentClassPath: Option[MergedClassPath[AbstractFile]] = None
 
+  val forceIndexPackages: collection.mutable.Set[String] = collection.mutable.Set.empty
+  val classPathIndexByClassName: collection.mutable.Map[String, Set[String]] = collection.mutable.Map.empty
+
+  def addForceIndexPackages(packages: Set[String]) = {
+    forceIndexPackages ++= packages
+  }
+
+  def updateIndex() = {
+    def addIfInitialized(pkg: Symbol): Unit = {
+      if (pkg.hasCompleteInfo || forceIndexPackages.exists(p => p startsWith pkg.fullName)) {
+        for (m <- pkg.info.members if m != pkg) { // _root_ is member of _root_ it seems, so filter m != pkg
+          if (m.isClass)
+            classPathIndexByClassName(m.name.toString) = classPathIndexByClassName.getOrElse(m.name.toString, Set.empty) + m.fullName
+          if (m.moduleClass.isPackageClass)
+            addIfInitialized(m.moduleClass)
+        }
+      }
+    }
+    addIfInitialized(rootMirror.getPackage("_root_": TermName))
+  }
+
   def classPath: ClassPath[AbstractFile] = {
     assert(settings.YclasspathImpl.value == ClassPathRepresentationType.Recursive,
       "To use recursive classpath representation you must enable it with -YclasspathImpl:recursive compiler option.")

src/compiler/scala/tools/nsc/typechecker/ContextErrors.scala

@@ -245,7 +245,15 @@ trait ContextErrors {
         NormalTypeError(tree, "reference to " + name + " is ambiguous;\n" + msg)
 
       def SymbolNotFoundError(tree: Tree, name: Name, owner: Symbol, startingIdentCx: Context) = {
-        NormalTypeError(tree, "not found: "+decodeWithKind(name, owner))
+        platform.updateIndex()
+        lazy val candidatesString = name match {
+          case tn: TypeName =>
+            val cs = platform.classPathIndexByClassName.getOrElse(tn.toString, Set.empty)
+            if (cs.nonEmpty) cs.mkString(". You may want to import one of the following classes from your classpath:\n  ", "\n  ", "")
+            else ""
+          case _ => ""
+        }
+        NormalTypeError(tree, "not found: "+decodeWithKind(name, owner) + candidatesString)
       }
 
       // typedAppliedTypeTree

src/repl/scala/tools/nsc/interpreter/IMain.scala

@@ -64,6 +64,8 @@ import java.io.File
 class IMain(@BeanProperty val factory: ScriptEngineFactory, initialSettings: Settings, protected val out: JPrintWriter) extends AbstractScriptEngine with Compilable with Imports {
   imain =>
 
+  val api = new PresentationCompilerAPI(this)
+
   setBindings(createBindings, ScriptContext.ENGINE_SCOPE)
   object replOutput extends ReplOutput(settings.Yreploutdir) { }
 

src/repl/scala/tools/nsc/interpreter/PresentationCompilerAPI.scala

@@ -0,0 +1,11 @@
+package scala.tools.nsc.interpreter
+
+class PresentationCompilerAPI(interpreter: IMain) {
+  /**
+   * Index the top-level classes in the the provided packages for the `not found: Class` error
+   * message.
+   */
+  def scanPackagesForClassNotFoundMessage(packages: Set[String]): Unit = {
+    interpreter.global.platform.addForceIndexPackages(packages)
+  }
+}