Add missing Case flag on Scala 2 library unapply

Author: nicolasstucki

.github/workflows/ci.yaml

@@ -144,7 +144,7 @@ jobs:
           ./project/scripts/sbt ";sjsSandbox/run ;sjsSandbox/test ;sjsJUnitTests/test ;set sjsJUnitTests/scalaJSLinkerConfig ~= switchToESModules ;sjsJUnitTests/test ;sjsCompilerTests/test"
 
       - name: Test with Scala 2 library TASTy (fast)
-        run: ./project/scripts/sbt ";set ThisBuild/Build.useScala2LibraryTasty := true ;scala3-bootstrapped/testCompilation i5; scala3-bootstrapped/testCompilation tests/run/typelevel-peano.scala" # only test a subset of test to avoid doubling the CI execution time
+        run: ./project/scripts/sbt ";set ThisBuild/Build.useScala2LibraryTasty := true ;scala3-bootstrapped/testCompilation i5; scala3-bootstrapped/testCompilation tests/run/typelevel-peano.scala; scala3-bootstrapped/testOnly dotty.tools.backend.jvm.DottyBytecodeTests" # only test a subset of test to avoid doubling the CI execution time
 
   test_scala2_library_tasty:
     runs-on: [self-hosted, Linux]

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

@@ -805,13 +805,16 @@ object desugar {
             else if caseClassInScala2Library then scala2LibCompatUnapplyRhs(unapplyParam.name)
             else Ident(unapplyParam.name)
           val unapplyResTp = if (arity == 0) Literal(Constant(true)) else TypeTree()
+          val unapplyMods =
+            if ctx.settings.YcompileScala2Library.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))