Avoid name table pollution with fresh existentials

During large compilations runs, the large numbers of globally unique
fresh names for existentials captured from prefixes of `asSeenFrom`.
is a) somewhat wasteful (all these names are interned in the name table)
, and, b) form a pathological case for the current implementation of
`Names#hashValue`, which leads to overfull hash-buckets in the name table.

`hashValue` should probably be improved, but my attempts to do so have
shown a small performance degradation in some benchmarks. So this commit
starts by being more frugal with these names, only uniquely naming
within an `asSeenFrom` operation.

References scala/scala-dev#246

2 files changed, 2 insertions, 2 deletions

diff --git a/src/compiler/scala/tools/nsc/transform/patmat/Logic.scala b/src/compiler/scala/tools/nsc/transform/patmat/Logic.scala
index 4ae97ce281..cb3759e5fa 100644
--- a/src/compiler/scala/tools/nsc/transform/patmat/Logic.scala
+++ b/src/compiler/scala/tools/nsc/transform/patmat/Logic.scala
@@ -682,7 +682,7 @@ trait ScalaLogic extends Interface with Logic with TreeAndTypeAnalysis {
       private[TreesAndTypesDomain] def uniqueTpForTree(t: Tree): Type = {
         def freshExistentialSubtype(tp: Type): Type = {
           // SI-8611 tp.narrow is tempting, but unsuitable. See `testRefinedTypeSI8611` for an explanation.
-          NoSymbol.freshExistential("").setInfo(TypeBounds.upper(tp)).tpe
+          NoSymbol.freshExistential("", 0).setInfo(TypeBounds.upper(tp)).tpe
         }
 
         if (!t.symbol.isStable) {
diff --git a/src/compiler/scala/tools/nsc/typechecker/PatternTypers.scala b/src/compiler/scala/tools/nsc/typechecker/PatternTypers.scala
index 1df3449ce6..cd0c292d90 100644
--- a/src/compiler/scala/tools/nsc/typechecker/PatternTypers.scala
+++ b/src/compiler/scala/tools/nsc/typechecker/PatternTypers.scala
@@ -123,7 +123,7 @@ trait PatternTypers {
     }
 
     private def boundedArrayType(bound: Type): Type = {
-      val tparam = context.owner freshExistential "" setInfo (TypeBounds upper bound)
+      val tparam = context.owner.freshExistential("", 0) setInfo (TypeBounds upper bound)
       newExistentialType(tparam :: Nil, arrayType(tparam.tpe_*))
     }