1 files changed, 43 insertions, 0 deletions
diff --git a/src/dotty/tools/dotc/core/TyperState.scala b/src/dotty/tools/dotc/core/TyperState.scala
new file mode 100644
index 000000000..e90070253
--- /dev/null
+++ b/src/dotty/tools/dotc/core/TyperState.scala
@@ -0,0 +1,43 @@
+package dotty.tools
+package dotc
+package core
+
+import Types._
+import Flags._
+import Contexts._
+import util.SimpleMap
+
+class TyperState extends DotClass {
+
+  /** The current constraint set */
+  def constraint: Constraint = new Constraint(SimpleMap.Empty)
+
+  /** The currently uninstantiated TypeVars */
+  def undetVars: List[TypeVar] = Nil
+
+  /** The currently outstanding errors, warnings, or infos */
+  def diagnostics: List[Diagnostic] = Nil
+
+  def constraint_=(c: Constraint): Unit = {}
+  def undetVars_=(vs: List[TypeVar]): Unit = unsupported("undetVars_=")
+  def diagnostics_=(ds: List[Diagnostic]): Unit = unsupported("diagnostics_=")
+
+  def fresh: TyperState = this
+}
+
+class MutableTyperState (previous: TyperState) extends TyperState {
+
+  private var myConstraint: Constraint = previous.constraint
+  private var myUndetVars: List[TypeVar] = previous.undetVars
+  private var myDiagnostics: List[Diagnostic] = Nil
+
+  override def constraint = myConstraint
+  override def undetVars = myUndetVars
+  override def diagnostics = myDiagnostics
+
+  override def constraint_=(c: Constraint) = myConstraint = c
+  override def undetVars_=(vs: List[TypeVar]) = myUndetVars = vs
+  override def diagnostics_=(ds: List[Diagnostic]) = myDiagnostics = ds
+
+  override def fresh: TyperState = new MutableTyperState(this)
+}