Make owningState a weak reference

`owningState` of `TypeVar` was still a source of memory leaks even though
we null it out when a TypeVar gets instantiated. The problem is that uninstantiated
TypeVars can sit as parts of types in caches.

Author: odersky

compiler/src/dotty/tools/dotc/core/TyperState.scala

@@ -11,6 +11,7 @@ import printing.{Showable, Printer}
 import printing.Texts._
 import config.Config
 import collection.mutable
+import java.lang.ref.WeakReference
 
 class TyperState(r: Reporter) extends DotClass with Showable {
 
@@ -143,8 +144,7 @@ extends TyperState(r) {
       if (targetState.constraint eq previousConstraint) constraint
       else targetState.constraint & constraint
     constraint foreachTypeVar { tvar =>
-      if (tvar.owningState eq this)
-        tvar.owningState = targetState
+      if (tvar.owningState.get eq this) tvar.owningState = new WeakReference(targetState)
     }
     targetState.ephemeral |= ephemeral
     targetState.gc()
@@ -157,7 +157,7 @@ extends TyperState(r) {
     constraint foreachTypeVar { tvar =>
       if (!tvar.inst.exists) {
         val inst = instType(tvar)
-        if (inst.exists && (tvar.owningState eq this)) {
+        if (inst.exists && (tvar.owningState.get eq this)) {
           tvar.inst = inst
           val lam = tvar.origin.binder
           if (constraint.isRemovable(lam)) toCollect += lam

compiler/src/dotty/tools/dotc/core/Types.scala

@@ -36,6 +36,7 @@ import Flags.FlagSet
 import language.implicitConversions
 import scala.util.hashing.{ MurmurHash3 => hashing }
 import config.Printers.{core, typr, cyclicErrors}
+import java.lang.ref.WeakReference
 
 object Types {
 
@@ -3186,7 +3187,7 @@ object Types {
     /** The state owning the variable. This is at first `creatorState`, but it can
      *  be changed to an enclosing state on a commit.
      */
-    private[core] var owningState = creatorState
+    private[core] var owningState = new WeakReference(creatorState)
 
     /** The instance type of this variable, or NoType if the variable is currently
      *  uninstantiated
@@ -3204,7 +3205,7 @@ object Types {
     private def instantiateWith(tp: Type)(implicit ctx: Context): Type = {
       assert(tp ne this, s"self instantiation of ${tp.show}, constraint = ${ctx.typerState.constraint.show}")
       typr.println(s"instantiating ${this.show} with ${tp.show}")
-      if ((ctx.typerState eq owningState) && !ctx.typeComparer.subtypeCheckInProgress)
+      if ((ctx.typerState eq owningState.get) && !ctx.typeComparer.subtypeCheckInProgress)
         inst = tp
       ctx.typerState.constraint = ctx.typerState.constraint.replace(origin, tp)
       tp