Fix for SI-6706, Symbol breakage under GC.

Ensure the map key and the String in the Symbol are the
same reference by removing the old key before updating the
map with the new key -> symbol relation.

2 files changed, 19 insertions, 0 deletions

diff --git a/src/library/scala/Symbol.scala b/src/library/scala/Symbol.scala
index 723c05a30c..4fead7a50c 100644
--- a/src/library/scala/Symbol.scala
+++ b/src/library/scala/Symbol.scala
@@ -69,6 +69,11 @@ private[scala] abstract class UniquenessCache[K, V >: Null]
         val res = cached()
         if (res != null) res
         else {
+          // If we don't remove the old String key from the map, we can
+          // wind up with one String as the key and a different String as
+          // as the name field in the Symbol, which can lead to surprising
+          // GC behavior and duplicate Symbols. See SI-6706.
+          map remove name
           val sym = valueFromKey(name)
           map.put(name, new WeakReference(sym))
           sym
diff --git a/test/files/run/t6706.scala b/test/files/run/t6706.scala
new file mode 100644
index 0000000000..905494ca8d
--- /dev/null
+++ b/test/files/run/t6706.scala
@@ -0,0 +1,14 @@
+object Test {
+  var name = "foo" + 1
+  var s1 = Symbol(name)
+  s1 = null
+  System.gc
+  val s2 = Symbol("foo1")
+  name = null
+  System.gc
+  val s3 = Symbol("foo1")
+
+  def main(args: Array[String]): Unit = {
+    assert(s2 eq s3, ((s2, System.identityHashCode(s2), s3, System.identityHashCode(s3))))
+  }
+}