introduces failsafe against endless type printing

The parent commit works around a particular problem that led to a compiler
freeze in SI-8158, whereas this commit introduces a general solution -
a cache that tracks all types that we've recursed into during printing.

I can't immediately come up with an example of a type that would be caught
by this safety net, but unknown unknowns are the worst of them all, so why not
guard against them while we can.

3 files changed, 16 insertions, 1 deletions

diff --git a/src/reflect/scala/reflect/internal/tpe/TypeToStrings.scala b/src/reflect/scala/reflect/internal/tpe/TypeToStrings.scala
index ebc4394d25..b08aabef85 100644
--- a/src/reflect/scala/reflect/internal/tpe/TypeToStrings.scala
+++ b/src/reflect/scala/reflect/internal/tpe/TypeToStrings.scala
@@ -3,6 +3,8 @@ package reflect
 package internal
 package tpe
 
+import scala.collection.mutable.HashSet
+
 private[internal] trait TypeToStrings {
   self: SymbolTable =>
 
@@ -14,8 +16,15 @@ private[internal] trait TypeToStrings {
   def tostringRecursions = _tostringRecursions
   def tostringRecursions_=(value: Int) = _tostringRecursions = value
 
+  private var _tostringSubjects = HashSet[Type]()
+  def tostringSubjects = _tostringSubjects
+
   protected def typeToString(tpe: Type): String =
-    if (tostringRecursions >= maxTostringRecursions) {
+    if (tostringSubjects contains tpe) {
+      // handles self-referential anonymous classes and who knows what else
+      "..."
+    }
+    else if (tostringRecursions >= maxTostringRecursions) {
       devWarning("Exceeded recursion depth attempting to print " + util.shortClassOfInstance(tpe))
       if (settings.debug)
         (new Throwable).printStackTrace
@@ -25,8 +34,10 @@ private[internal] trait TypeToStrings {
     else
       try {
         tostringRecursions += 1
+        tostringSubjects += tpe
         tpe.safeToString
       } finally {
+        tostringSubjects -= tpe
         tostringRecursions -= 1
       }
 }
diff --git a/src/reflect/scala/reflect/runtime/JavaUniverseForce.scala b/src/reflect/scala/reflect/runtime/JavaUniverseForce.scala
index 6b3985d434..ac6dd0783d 100644
--- a/src/reflect/scala/reflect/runtime/JavaUniverseForce.scala
+++ b/src/reflect/scala/reflect/runtime/JavaUniverseForce.scala
@@ -42,6 +42,7 @@ trait JavaUniverseForce { self: runtime.JavaUniverse  =>
     // inaccessible: this._glbResults
     // inaccessible: this._indent
     // inaccessible: this._tostringRecursions
+    // inaccessible: this._tostringSubjects
     // inaccessible: this.atomicIds
     // inaccessible: this.atomicExistentialIds
     // inaccessible: this._recursionTable
diff --git a/src/reflect/scala/reflect/runtime/SynchronizedTypes.scala b/src/reflect/scala/reflect/runtime/SynchronizedTypes.scala
index de78e527a7..12ada07a56 100644
--- a/src/reflect/scala/reflect/runtime/SynchronizedTypes.scala
+++ b/src/reflect/scala/reflect/runtime/SynchronizedTypes.scala
@@ -85,6 +85,9 @@ private[reflect] trait SynchronizedTypes extends internal.Types { self: SymbolTa
   override def tostringRecursions = _tostringRecursions.get
   override def tostringRecursions_=(value: Int) = _tostringRecursions.set(value)
 
+  private lazy val _tostringSubjects = mkThreadLocalStorage(new mutable.HashSet[Type])
+  override def tostringSubjects = _tostringSubjects.get
+
   /* The idea of caches is as follows.
    * When in reflexive mode, a cache is either null, or one sentinal
    * value representing undefined or the final defined