SI-7782 Derive type skolems at the ground level

Rather than at the current value of `skolemizationLevel`,
which could be influenced by an in-flight existential
subtype computation.

This method is called in `PolyTypeCompleter`, which
could be constructed by the lazy type completer of the
enclosing class. So currently it is closing over a mutable
variable; hence the Heisenbug.

This issue was exposed by the changes in b74c33eb860,
which was introduced in Scala 2.10.1.

1 files changed, 8 insertions, 1 deletions

diff --git a/src/reflect/scala/reflect/internal/ExistentialsAndSkolems.scala b/src/reflect/scala/reflect/internal/ExistentialsAndSkolems.scala
index 3bcb793926..2c2ed351c9 100644
--- a/src/reflect/scala/reflect/internal/ExistentialsAndSkolems.scala
+++ b/src/reflect/scala/reflect/internal/ExistentialsAndSkolems.scala
@@ -19,6 +19,9 @@ trait ExistentialsAndSkolems {
    *  can be deskolemized to the original type parameter. (A skolem is a
    *  representation of a bound variable when viewed inside its scope.)
    *  !!!Adriaan: this does not work for hk types.
+   *
+   *  Skolems will be created at level 0, rather than the current value
+   *  of `skolemizationLevel`. (See SI-7782)
    */
   def deriveFreshSkolems(tparams: List[Symbol]): List[Symbol] = {
     class Deskolemizer extends LazyType {
@@ -30,7 +33,11 @@ trait ExistentialsAndSkolems {
         sym setInfo sym.deSkolemize.info.substSym(typeParams, typeSkolems)
       }
     }
-    (new Deskolemizer).typeSkolems
+
+    val saved = skolemizationLevel
+    skolemizationLevel = 0
+    try new Deskolemizer().typeSkolems
+    finally skolemizationLevel = saved
   }
 
   def isRawParameter(sym: Symbol) = // is it a type parameter leaked by a raw type?