Fix -Ytest-pickler bug with private[this] (#16538)

Author: dwijnand

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

@@ -157,7 +157,7 @@ class OrderingConstraint(private val boundsMap: ParamBounds,
   type This = OrderingConstraint
 
   /** A new constraint with given maps and given set of hard typevars */
-  def newConstraint( // !!! Dotty problem: Making newConstraint `private` causes -Ytest-pickler failure.
+  private def newConstraint(
     boundsMap: ParamBounds = this.boundsMap,
     lowerMap: ParamOrdering = this.lowerMap,
     upperMap: ParamOrdering = this.upperMap,

compiler/src/dotty/tools/dotc/typer/Applications.scala

@@ -272,6 +272,7 @@ object Applications {
     else
       def selectGetter(qual: Tree): Tree =
         val getterDenot = qual.tpe.member(getterName)
+          .accessibleFrom(qual.tpe.widenIfUnstable) // to reset Local
         if (getterDenot.exists) qual.select(TermRef(qual.tpe, getterName, getterDenot))
         else EmptyTree
       if !meth.isClassConstructor then

tests/pos/test-pickler-private-this.scala

@@ -0,0 +1,3 @@
+class Test:
+  def callMeth(test: Test) = test.meth(-1)
+  private def meth(foo: Int, bar: Int = 1) = foo + bar