Revert r24960, "Change Types#narrow to create a...

Revert r24960, "Change Types#narrow to create an existential rather than
a refinement type."

For many days I have been haunted by the knowledge that the 20 seconds
I took off quick.comp between r24893 and r24920 all came roaring back
around r24960 for no reason which I could discern. To verify r24960 was
not at fault I had compared it against r24959 more than once, and the
difference was negligible.

It was negligible, that is, until I compared under -optimise, and then
the wheels came off. In fact the differences there are so spectacular
(quick.lib under -optimise goes from 8:50 to 4:26 with this patch,
quick.comp from 8:34 to 6:30) that there must be some major disconnect
between what the benchmark charts are measuring and what I am measuring.

In any case, the charts and I do agree on the direction of the arrow.
For reasons which remain to be determined, this commit was bad on the
nanos. Review by odersky.

2 files changed, 16 insertions, 17 deletions

diff --git a/src/compiler/scala/reflect/internal/Types.scala b/src/compiler/scala/reflect/internal/Types.scala
index cb3ec1bfeb..3264c34d4b 100644
--- a/src/compiler/scala/reflect/internal/Types.scala
+++ b/src/compiler/scala/reflect/internal/Types.scala
@@ -79,6 +79,8 @@ trait Types /*extends reflect.generic.Types*/ { self: SymbolTable =>
   private var explainSwitch = false
   private final val emptySymbolSet = immutable.Set.empty[Symbol]
 
+  private final val alternativeNarrow = false
+
   private final val LogPendingSubTypesThreshold = 50
   private final val LogPendingBaseTypesThreshold = 50
   private final val LogVolatileThreshold = 50
@@ -338,12 +340,21 @@ trait Types /*extends reflect.generic.Types*/ { self: SymbolTable =>
 
     /** Map to a singleton type which is a subtype of this type.
      *  The fallback implemented here gives
-     *    T.narrow  = T' forSome { type T' <: T with Singleton }
+     *    T.narrow  =  (T {}).this.type
      *  Overridden where we know more about where types come from.
+     *
+     *  todo: change to singleton type of an existentially defined variable
+     *  of the right type instead of making this a `this` of a refined type.
      */
     def narrow: Type =
       if (phase.erasedTypes) this
-      else commonOwner(this) freshExistential ".type" setInfo singletonBounds(this) tpe
+      else if (alternativeNarrow) { // investigate why this does not work!
+        val tparam = commonOwner(this) freshExistential ".type" setInfo singletonBounds(this)
+        tparam.tpe
+      } else {
+        val cowner = commonOwner(this)
+        refinedType(List(this), cowner, EmptyScope, cowner.pos).narrow
+      }
 
     /** For a TypeBounds type, itself;
      *  for a reference denoting an abstract type, its bounds,
diff --git a/src/compiler/scala/tools/nsc/matching/MatrixAdditions.scala b/src/compiler/scala/tools/nsc/matching/MatrixAdditions.scala
index 2d31281e34..d75670fd38 100644
--- a/src/compiler/scala/tools/nsc/matching/MatrixAdditions.scala
+++ b/src/compiler/scala/tools/nsc/matching/MatrixAdditions.scala
@@ -142,23 +142,11 @@ trait MatrixAdditions extends ast.TreeDSL {
       import Flags.{ MUTABLE, ABSTRACT, SEALED }
 
       private case class Combo(index: Int, sym: Symbol) {
+        val isBaseClass = sym.tpe.baseClasses.toSet
+
         // is this combination covered by the given pattern?
         def isCovered(p: Pattern) = {
-          def coversSym = {
-            val lhs = decodedEqualsType(p.tpe)
-            val rhs = sym.tpe
-            // This logic, arrived upon after much struggle, attempts to find the
-            // the route through the type maze which let us issue precise exhaustiveness
-            // warnings against narrowed types (see test case sealed-java-enums.scala)
-            // while retaining the necessary pattern matching behavior that case _: List[_] =>
-            // matches both "object Nil" and "class ::[T]".
-            //
-            // Doubtless there is a more direct/correct expression of it.
-            if (rhs.typeSymbol.isSingletonExistential)
-              lhs <:< rhs
-            else
-              rhs.baseClasses contains lhs.typeSymbol
-          }
+          def coversSym = isBaseClass(decodedEqualsType(p.tpe).typeSymbol)
 
           cond(p.tree) {
             case _: UnApply | _: ArrayValue => true