SI-10026 Fix endless cycle in runtime reflection

56f23af introduced a call to `baseTypeSeq` of `scala.collection.mutable.ArrayOps.ofRef[?T]{}`
in `findMember`. This exposed a latent bug in the synchronized wrapper of `BaseTypeSeq`,
demonstrated below with an older version of Scala:

```
Welcome to Scala 2.11.8 (Java HotSpot(TM) 64-Bit Server VM, Java 1.8.0_112).
Type in expressions for evaluation. Or try :help.

scala> val symtab = reflect.runtime.universe.asInstanceOf[scala.reflect.internal.SymbolTable]
symtab: scala.reflect.internal.SymbolTable = scala.reflect.runtime.JavaUniverse@458544e0

scala> import symtab._
import symtab._

scala> val ArrayOps_ofRef_Class = symtab.symbolOf[scala.collection.mutable.ArrayOps.ofRef[AnyRef]]
ArrayOps_ofRef_Class: symtab.TypeSymbol = class ofRef

scala> appliedType(symbolOf[Set[Any]], symbolOf[Set[Any]].typeParams.map(TypeVar(_)))
res2: symtab.Type = Set[?A]

scala> .narrow
res3: symtab.Type = <none>.<refinement>.type

scala> .baseTypeSeq
java.lang.StackOverflowError
  at scala.reflect.runtime.Gil$class.gilSynchronized(Gil.scala:21)
  at scala.reflect.runtime.JavaUniverse.gilSynchronized(JavaUniverse.scala:16)
  at scala.reflect.runtime.SynchronizedOps$SynchronizedBaseTypeSeq$class.map(SynchronizedOps.scala:27)
  at scala.reflect.runtime.SynchronizedOps$SynchronizedBaseTypeSeq$$anon$2.map(SynchronizedOps.scala:34)
  at scala.reflect.runtime.SynchronizedOps$SynchronizedBaseTypeSeq$class.lateMap(SynchronizedOps.scala:34)
  at scala.reflect.runtime.SynchronizedOps$SynchronizedBaseTypeSeq$$anon$2.lateMap(SynchronizedOps.scala:34)
  at scala.reflect.internal.BaseTypeSeqs$MappedBaseTypeSeq.map(BaseTypeSeqs.scala:235)
  at scala.reflect.runtime.SynchronizedOps$SynchronizedBaseTypeSeq$$anon$2.scala$reflect$runtime$SynchronizedOps$SynchronizedBaseTypeSeq$$super$map(SynchronizedOps.scala:34)
  at scala.reflect.runtime.SynchronizedOps$SynchronizedBaseTypeSeq$$anonfun$map$1.apply(SynchronizedOps.scala:27)
  at scala.reflect.runtime.SynchronizedOps$SynchronizedBaseTypeSeq$$anonfun$map$1.apply(SynchronizedOps.scala:27)
  at scala.reflect.runtime.Gil$class.gilSynchronized(Gil.scala:19)
  at scala.reflect.runtime.JavaUniverse.gilSynchronized(JavaUniverse.scala:16)
  at scala.reflect.runtime.SynchronizedOps$SynchronizedBaseTypeSeq$class.map(SynchronizedOps.scala:27)
  at scala.reflect.runtime.SynchronizedOps$SynchronizedBaseTypeSeq$$anon$2.map(SynchronizedOps.scala:34)
  at scala.reflect.runtime.SynchronizedOps$SynchronizedBaseTypeSeq$class.lateMap(SynchronizedOps.scala:34)
  at scala.reflect.runtime.SynchronizedOps$SynchronizedBaseTypeSeq$$anon$2.lateMap(SynchronizedOps.scala:34)
  at scala.reflect.internal.BaseTypeSeqs$MappedBaseTypeSeq.map(BaseTypeSeqs.scala:235)
```

The infinite cycle involves:

```
class MappedBaseTypeSeq(orig: BaseTypeSeq, f: Type => Type) extends BaseTypeSeq(orig.parents map f, orig.elems) {
    ...
    override def map(g: Type => Type) = lateMap(g)
    override def lateMap(g: Type => Type) = orig.lateMap(x => g(f(x)))
  }

  trait SynchronizedBaseTypeSeq extends BaseTypeSeq {
    ...
    override def map(f: Type => Type): BaseTypeSeq = gilSynchronized { super.map(f) }
    override def lateMap(f: Type => Type): BaseTypeSeq =
      // only need to synchronize BaseTypeSeqs if they contain refined types
      if (map(f).toList.exists(_.isInstanceOf[RefinedType])) new MappedBaseTypeSeq(this, f) with SynchronizedBaseTypeSeq
      else new MappedBaseTypeSeq(this, f)
  }
```

This commit creates a new factory method for `MappedBaseTypeSeq`-s to break the cycle.

As an independent change, I have also removed the attempt to conditionally synchronize them,
as the condition was eagerly applying the map function (and throwing away the result!).

I've appeased MiMa with new whitelist entries, but I'm confident that this is deep enough
in the bowels of our runtime reflection implementation that there is no way that user code
will be calling these methods directly.

2 files changed, 10 insertions, 6 deletions

diff --git a/src/reflect/scala/reflect/internal/BaseTypeSeqs.scala b/src/reflect/scala/reflect/internal/BaseTypeSeqs.scala
index 0ef52213e5..1cdefff2e9 100644
--- a/src/reflect/scala/reflect/internal/BaseTypeSeqs.scala
+++ b/src/reflect/scala/reflect/internal/BaseTypeSeqs.scala
@@ -33,6 +33,9 @@ trait BaseTypeSeqs {
   protected def newBaseTypeSeq(parents: List[Type], elems: Array[Type]) =
     new BaseTypeSeq(parents, elems)
 
+  protected def newMappedBaseTypeSeq(orig: BaseTypeSeq, f: Type => Type) =
+    new MappedBaseTypeSeq(orig, f)
+
   /** Note: constructor is protected to force everyone to use the factory method newBaseTypeSeq instead.
    *  This is necessary because when run from reflection every base type sequence needs to have a
    *  SynchronizedBaseTypeSeq as mixin.
@@ -125,7 +128,7 @@ trait BaseTypeSeqs {
       newBaseTypeSeq(parents, arr)
     }
 
-    def lateMap(f: Type => Type): BaseTypeSeq = new MappedBaseTypeSeq(this, f)
+    def lateMap(f: Type => Type): BaseTypeSeq = newMappedBaseTypeSeq(this, f)
 
     def exists(p: Type => Boolean): Boolean = elems exists p
 
diff --git a/src/reflect/scala/reflect/runtime/SynchronizedOps.scala b/src/reflect/scala/reflect/runtime/SynchronizedOps.scala
index f0d96e0fd6..eadafc8abb 100644
--- a/src/reflect/scala/reflect/runtime/SynchronizedOps.scala
+++ b/src/reflect/scala/reflect/runtime/SynchronizedOps.scala
@@ -18,6 +18,12 @@ private[reflect] trait SynchronizedOps extends internal.SymbolTable
     if (elems.exists(_.isInstanceOf[RefinedType])) new BaseTypeSeq(parents, elems) with SynchronizedBaseTypeSeq
     else new BaseTypeSeq(parents, elems)
 
+  override protected def newMappedBaseTypeSeq(orig: BaseTypeSeq, f: Type => Type) =
+    // MappedBaseTypeSeq's are used rarely enough that we unconditionally mixin the synchronized
+    // wrapper, rather than doing this conditionally. A previous attempt to do that broke the "late"
+    // part of the "lateMap" contract in inspecting the mapped elements.
+    new MappedBaseTypeSeq(orig, f) with SynchronizedBaseTypeSeq
+
   trait SynchronizedBaseTypeSeq extends BaseTypeSeq {
     override def apply(i: Int): Type = gilSynchronized { super.apply(i) }
     override def rawElem(i: Int) = gilSynchronized { super.rawElem(i) }
@@ -28,11 +34,6 @@ private[reflect] trait SynchronizedOps extends internal.SymbolTable
     override def exists(p: Type => Boolean): Boolean = gilSynchronized { super.exists(p) }
     override lazy val maxDepth = gilSynchronized { maxDepthOfElems }
     override def toString = gilSynchronized { super.toString }
-
-    override def lateMap(f: Type => Type): BaseTypeSeq =
-      // only need to synchronize BaseTypeSeqs if they contain refined types
-      if (map(f).toList.exists(_.isInstanceOf[RefinedType])) new MappedBaseTypeSeq(this, f) with SynchronizedBaseTypeSeq
-      else new MappedBaseTypeSeq(this, f)
   }
 
 // Scopes