Add Case flag to synthetic unapply methods with -Yscala2-stdlib

Author: nicolasstucki

compiler/src/dotty/tools/dotc/ast/Desugar.scala

@@ -805,13 +805,15 @@ object desugar {
             else if caseClassInScala2StdLib then scala2LibCompatUnapplyRhs(unapplyParam.name)
             else Ident(unapplyParam.name)
           val unapplyResTp = if (arity == 0) Literal(Constant(true)) else TypeTree()
-
+          val unapplyMods =
+            if ctx.settings.Yscala2Stdlib.value then synthetic | Case
+            else synthetic
           DefDef(
             methName,
             joinParams(derivedTparams, (unapplyParam :: Nil) :: Nil),
             unapplyResTp,
             unapplyRHS
-          ).withMods(synthetic)
+          ).withMods(unapplyMods)
         }
         val toStringMeth =
           DefDef(nme.toString_, Nil, TypeTree(), Literal(Constant(className.toString))).withMods(Modifiers(Override | Synthetic))

compiler/src/dotty/tools/dotc/transform/PatternMatcher.scala

@@ -330,10 +330,7 @@ object PatternMatcher {
         lazy val caseAccessors = caseClass.caseAccessors.filter(sym => sym.is(Method) || sym.owner.is(Scala2Tasty))
 
         def isSyntheticScala2Unapply(sym: Symbol) =
-          sym.is(Synthetic) && (
-            (sym.is(Case) && sym.owner.is(Scala2x))
-            || sym.owner.is(Scala2Tasty)
-          )
+          sym.isAllOf(SyntheticCase) && sym.owner.is(Scala2x)
 
         def tupleApp(i: Int, receiver: Tree) = // manually inlining the call to NonEmptyTuple#apply, because it's an inline method
           ref(defn.RuntimeTuplesModule)