Fix Scala 2 unpickler: do not load case accessor vals

The class file contains both a case accessor `private val` and a case
accessor `def`. We only need to load the `def` accessor.

Author: nicolasstucki

.github/workflows/ci.yaml

@@ -143,7 +143,7 @@ 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
-        run: ./project/scripts/sbt ";set ThisBuild/Build.useScala2LibraryTasty := true ;scala3-bootstrapped/testCompilation i5" # 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" # only test a subset of test to avoid doubling the CI execution time
 
   test_windows_fast:
     runs-on: [self-hosted, Windows]

compiler/src/dotty/tools/dotc/core/unpickleScala2/Scala2Unpickler.scala

@@ -450,6 +450,8 @@ class Scala2Unpickler(bytes: Array[Byte], classRoot: ClassDenotation, moduleClas
        // Scala 2 sometimes pickle the same type parameter symbol multiple times
        // (see i11173 for an example), but we should only unpickle it once.
        || tag == TYPEsym && flags.is(TypeParam) && symScope(owner).lookup(name.asTypeName).exists
+       // We discard the private val representing a case accessor. We only load the case accessor def.
+       || flags.isAllOf(CaseAccessor| Private, butNot = Method)
     then
       // skip this member
       return NoSymbol

compiler/src/dotty/tools/dotc/inlines/InlineReducer.scala

@@ -329,7 +329,7 @@ class InlineReducer(inliner: Inliner)(using Context):
               val paramCls = paramType.classSymbol
               if (paramCls.is(Case) && unapp.symbol.is(Synthetic) && scrut <:< paramType) {
                 val caseAccessors =
-                  if (paramCls.is(Scala2x)) paramCls.caseAccessors.filter(_.is(Method))
+                  if paramCls.is(Scala2x) then paramCls.caseAccessors
                   else paramCls.asClass.paramAccessors
                 val selectors =
                   for (accessor <- caseAccessors)

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

@@ -327,7 +327,7 @@ object PatternMatcher {
       /** Plan for matching the result of an unapply against argument patterns `args` */
       def unapplyPlan(unapp: Tree, args: List[Tree]): Plan = {
         def caseClass = unapp.symbol.owner.linkedClass
-        lazy val caseAccessors = caseClass.caseAccessors.filter(_.is(Method))
+        lazy val caseAccessors = caseClass.caseAccessors
 
         def isSyntheticScala2Unapply(sym: Symbol) =
           sym.isAllOf(SyntheticCase) && sym.owner.is(Scala2x)

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

@@ -409,7 +409,7 @@ class Synthesizer(typer: Typer)(using @constructorOnly c: Context):
       New(defn.RuntimeTupleMirrorTypeRef, Literal(Constant(arity)) :: Nil)
 
     def makeProductMirror(pre: Type, cls: Symbol, tps: Option[List[Type]]): TreeWithErrors =
-      val accessors = cls.caseAccessors.filterNot(_.isAllOf(PrivateLocal))
+      val accessors = cls.caseAccessors
       val elemLabels = accessors.map(acc => ConstantType(Constant(acc.name.toString)))
       val typeElems = tps.getOrElse(accessors.map(mirroredType.resultType.memberInfo(_).widenExpr))
       val nestedPairs = TypeOps.nestedPairs(typeElems)