Small refactorings related to TyperState

- Add Context#withTyperState
- Add a committable parameter to TyperState#fresh

Author: smarter

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

@@ -526,9 +526,11 @@ object Contexts {
     final def withOwner(owner: Symbol): Context =
       if (owner ne this.owner) fresh.setOwner(owner) else this
 
+    final def withTyperState(typerState: TyperState): Context =
+      if typerState ne this.typerState then fresh.setTyperState(typerState) else this
+
     final def withUncommittedTyperState: Context =
-      val ts = typerState.uncommittedAncestor
-      if ts ne typerState then fresh.setTyperState(ts) else this
+      withTyperState(typerState.uncommittedAncestor)
 
     final def withProperty[T](key: Key[T], value: Option[T]): Context =
       if (property(key) == value) this
@@ -599,8 +601,8 @@ object Contexts {
       this.scope = newScope
       this
     def setTyperState(typerState: TyperState): this.type = { this.typerState = typerState; this }
-    def setNewTyperState(): this.type = setTyperState(typerState.fresh().setCommittable(true))
-    def setExploreTyperState(): this.type = setTyperState(typerState.fresh().setCommittable(false))
+    def setNewTyperState(): this.type = setTyperState(typerState.fresh(committable = true))
+    def setExploreTyperState(): this.type = setTyperState(typerState.fresh(committable = false))
     def setReporter(reporter: Reporter): this.type = setTyperState(typerState.fresh().setReporter(reporter))
     def setTyper(typer: Typer): this.type = { this.scope = typer.scope; setTypeAssigner(typer) }
     def setGadt(gadt: GadtConstraint): this.type =

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

@@ -103,11 +103,12 @@ class TyperState() {
     this
 
   /** A fresh typer state with the same constraint as this one. */
-  def fresh(reporter: Reporter = StoreReporter(this.reporter)): TyperState =
+  def fresh(reporter: Reporter = StoreReporter(this.reporter),
+      committable: Boolean = this.isCommittable): TyperState =
     util.Stats.record("TyperState.fresh")
     TyperState().init(this, this.constraint)
       .setReporter(reporter)
-      .setCommittable(this.isCommittable)
+      .setCommittable(committable)
 
   /** The uninstantiated variables */
   def uninstVars: collection.Seq[TypeVar] = constraint.uninstVars
@@ -189,9 +190,7 @@ class TyperState() {
   def mergeConstraintWith(that: TyperState)(using Context): Unit =
     that.ensureNotConflicting(constraint)
 
-    val comparingCtx =
-      if ctx.typerState == this then ctx
-      else ctx.fresh.setTyperState(this)
+    val comparingCtx = ctx.withTyperState(this)
 
     comparing(typeComparer =>
       val other = that.constraint