3 files changed, 130 insertions, 2 deletions
diff --git a/src/interactive/scala/tools/nsc/interactive/Global.scala b/src/interactive/scala/tools/nsc/interactive/Global.scala
index e308bc3bf0..7df809b6ff 100644
--- a/src/interactive/scala/tools/nsc/interactive/Global.scala
+++ b/src/interactive/scala/tools/nsc/interactive/Global.scala
@@ -64,6 +64,8 @@ trait InteractiveAnalyzer extends Analyzer {
     // that case the definitions that were already attributed as
     // well as any default parameters of such methods need to be
     // re-entered in the current scope.
+    //
+    // Tested in test/files/presentation/t8941b
     override def enterExistingSym(sym: Symbol, tree: Tree): Context = {
       if (sym != null && sym.owner.isTerm) {
         enterIfNotThere(sym)
@@ -737,7 +739,7 @@ class Global(settings: Settings, _reporter: Reporter, projectName: String = "")
     }
   }
 
-  private def reloadSource(source: SourceFile) {
+  private[interactive] def reloadSource(source: SourceFile) {
     val unit = new RichCompilationUnit(source)
     unitOfFile(source.file) = unit
     toBeRemoved -= source.file
@@ -786,7 +788,7 @@ class Global(settings: Settings, _reporter: Reporter, projectName: String = "")
   }
 
   /** A fully attributed tree located at position `pos` */
-  private def typedTreeAt(pos: Position): Tree = getUnit(pos.source) match {
+  private[interactive] def typedTreeAt(pos: Position): Tree = getUnit(pos.source) match {
     case None =>
       reloadSources(List(pos.source))
       try typedTreeAt(pos)
diff --git a/test/files/presentation/t8941b/IdempotencyTest.scala b/test/files/presentation/t8941b/IdempotencyTest.scala
new file mode 100644
index 0000000000..af01b36898
--- /dev/null
+++ b/test/files/presentation/t8941b/IdempotencyTest.scala
@@ -0,0 +1,73 @@
+package scala.tools.nsc
+package interactive
+package tests.core
+
+import reporters.{Reporter => CompilerReporter}
+import scala.tools.nsc.interactive.InteractiveReporter
+import scala.reflect.internal.util.SourceFile
+
+/** Determistically interrupts typechecking of `code` when a defintion named
+  * `MagicInterruptionMarker` is typechecked, and then performs a targetted
+  * typecheck of the tree at the specal comment marker marker
+  */  
+abstract class IdempotencyTest { self =>
+  private val settings = new Settings
+  settings.usejavacp.value = true
+
+  private object Break extends scala.util.control.ControlThrowable
+
+  private val compilerReporter: CompilerReporter = new InteractiveReporter {
+    override def compiler = self.compiler
+  }
+
+  object compiler extends Global(settings, compilerReporter) {
+    override def checkForMoreWork(pos: Position) {
+    }
+    override def signalDone(context: Context, old: Tree, result: Tree) {
+      // println("signalDone: " + old.toString.take(50).replaceAll("\n", "\\n"))
+      if (!interrupted && analyzer.lockedCount == 0 && interruptsEnabled && shouldInterrupt(result)) {
+        interrupted = true
+        val typed = typedTreeAt(markerPosition)
+        checkTypedTree(typed)
+        throw Break
+      }
+      super.signalDone(context, old, result)
+    }
+
+    // we're driving manually using our own thread, disable the check here.
+    override def assertCorrectThread() {}
+  }
+
+  import compiler._
+
+  private var interrupted = false
+
+  // Extension points
+  protected def code: String
+  protected def shouldInterrupt(tree: Tree): Boolean = {
+    tree.symbol != null && tree.symbol.name.toString == "MagicInterruptionMarker"
+  }
+  protected def checkTypedTree(tree: Tree): Unit = {}
+  
+
+  private val source: SourceFile = newSourceFile(code)
+  private def markerPosition: Position = source.position(code.indexOf("/*?*/"))
+
+  def assertNoProblems() {
+    val problems = getUnit(source).get.problems
+    assert(problems.isEmpty, problems.mkString("\n"))
+  }
+
+  def show() {
+    reloadSource(source)
+    try {
+      typedTree(source, true)
+      assert(false, "Expected to break out of typechecking.")
+    } catch {
+      case Break => // expected
+    }
+    assertNoProblems()
+  }
+
+  def main(args: Array[String]) { show() }
+}
diff --git a/test/files/presentation/t8941b/Test.scala b/test/files/presentation/t8941b/Test.scala
new file mode 100644
index 0000000000..7269a14286
--- /dev/null
+++ b/test/files/presentation/t8941b/Test.scala
@@ -0,0 +1,53 @@
+import scala.tools.nsc.interactive.tests.core.IdempotencyTest
+
+// At the time of writing this test, removing any part of `enterExistingSym` 
+// leads to a failure.
+object Test {
+  def main(args: Array[String]) {
+    test("""
+      object Foo {
+        def term {
+          def foo(c: String = "") = c
+          class MagicInterruptionMarker
+          foo()/*?*/
+        }
+      }
+    """)
+
+    test("""
+      object Foo {
+        def term {
+          def foo = 42
+          class MagicInterruptionMarker
+          foo/*?*/
+        }
+      }
+    """)
+
+    test("""
+      object Foo {
+        def term {
+          lazy val foo = 42
+          class MagicInterruptionMarker
+          foo/*?*/
+        }
+      }
+    """)
+
+    test("""
+      object Foo {
+        implicit class C(val a: String) extends AnyVal
+        class MagicInterruptionMarker
+        ""/*?*/
+      }
+    """)
+  }
+
+  def test(code0: String) {
+    val t = new IdempotencyTest {
+      def code = code0
+    }
+    t.show()
+  }
+}
+