1 files changed, 36 insertions, 6 deletions
diff --git a/src/dotty/tools/dotc/core/TyperState.scala b/src/dotty/tools/dotc/core/TyperState.scala
index e492eee60..b00e55e29 100644
--- a/src/dotty/tools/dotc/core/TyperState.scala
+++ b/src/dotty/tools/dotc/core/TyperState.scala
@@ -14,10 +14,17 @@ class TyperState(val reporter: Reporter = ThrowingReporter) extends DotClass {
   def constraint: Constraint = new Constraint(SimpleMap.Empty)
 
   /** The currently uninstantiated TypeVars */
-  def undetVars: List[TypeVar] = Nil
+  def undetVars: Set[TypeVar] = Set()
+
+  /** A map that records for instantiated type vars their instance type.
+   *  Used only in a temporary way for contexts that may be retracted
+   *  without also retracting the type var. 
+   */
+  def instType: SimpleMap[TypeVar, Type] = SimpleMap.Empty
 
   def constraint_=(c: Constraint): Unit = {}
-  def undetVars_=(vs: List[TypeVar]): Unit = unsupported("undetVars_=")
+  def undetVars_=(vs: Set[TypeVar]): Unit = unsupported("undetVars_=")
+  def instType_=(m: SimpleMap[TypeVar, Type]): Unit = unsupported("instType_=")
 
   def fresh: TyperState = this
 
@@ -28,19 +35,42 @@ class MutableTyperState(previous: TyperState, reporter: Reporter)
 extends TyperState(reporter) {
 
   private var myConstraint: Constraint = previous.constraint
-  private var myUndetVars: List[TypeVar] = previous.undetVars
+  private var myUndetVars: Set[TypeVar] = previous.undetVars
+  private var myInstType: SimpleMap[TypeVar, Type] = previous.instType
 
   override def constraint = myConstraint
   override def undetVars = myUndetVars
+  override def instType = myInstType
 
   override def constraint_=(c: Constraint) = myConstraint = c
-  override def undetVars_=(vs: List[TypeVar]) = myUndetVars = vs
+  override def undetVars_=(vs: Set[TypeVar]) = myUndetVars = vs
+  override def instType_=(m: SimpleMap[TypeVar, Type]): Unit = myInstType = m
 
   override def fresh: TyperState = new MutableTyperState(this, new StoreReporter)
 
+  /** Commit typer state so that its information is copied into current typer state
+   *  In addition (1) the owning state of undetermined or temporarily instantiated
+   *  type variables changes from this typer state to the current one. (2) Variables
+   *  that were temporarily instantiated in the current typer state are permanently
+   *  instantiated instead.
+   */
   override def commit()(implicit ctx: Context) = {
-    ctx.typerState.constraint = constraint
-    ctx.typerState.undetVars = undetVars
+    var targetState = ctx.typerState
+    targetState.constraint = constraint
+    targetState.undetVars = undetVars
+    targetState.instType = instType
+
+    def adjustOwningState(tvar: TypeVar) =
+      if (tvar.owningState eq this) tvar.owningState = targetState
+    undetVars foreach adjustOwningState
+    instType foreachKey { case tvar: TypeVar =>
+      adjustOwningState(tvar)
+      if (tvar.owningState == targetState) {
+        tvar.inst = instType(tvar)
+        targetState.instType = targetState.instType remove tvar
+      }
+    }
+
     reporter.flush()
   }
 }