chore: Allow to write tasty for Scala2 symbols under -Ycompile-scala2-library

Author: hamzaremmal

compiler/src/dotty/tools/dotc/core/tasty/TreePickler.scala

@@ -861,7 +861,7 @@ class TreePickler(pickler: TastyPickler, attributes: Attributes) {
       assert(isModifierTag(tag))
       writeByte(tag)
     }
-    assert(!flags.is(Scala2x))
+    if flags.is(Scala2x) then assert(attributes.scala2StandardLibrary)
     if (flags.is(Private)) writeModTag(PRIVATE)
     if (flags.is(Protected)) writeModTag(PROTECTED)
     if (flags.is(Final, butNot = Module)) writeModTag(FINAL)

project/Scala2LibraryBootstrappedMiMaFilters.scala

@@ -66,8 +66,6 @@ object Scala2LibraryBootstrappedMiMaFilters {
         ProblemFilters.exclude[FinalMethodProblem]("scala.io.Source.NoPositioner"),
         ProblemFilters.exclude[FinalMethodProblem]("scala.io.Source.RelaxedPosition"),
         ProblemFilters.exclude[FinalMethodProblem]("scala.io.Source.RelaxedPositioner"),
-        ProblemFilters.exclude[IncompatibleResultTypeProblem]("scala.reflect.ManifestFactory#NothingManifest.newArray"),
-        ProblemFilters.exclude[IncompatibleResultTypeProblem]("scala.reflect.ManifestFactory#NullManifest.newArray"),
         ProblemFilters.exclude[MissingFieldProblem]("scala.collection.ArrayOps#ReverseIterator.xs"),
         ProblemFilters.exclude[MissingFieldProblem]("scala.runtime.NonLocalReturnControl.value"),
       ) ++