Do not generate _N members for case classes in stdlib-bootstrapped

Author: nicolasstucki

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

@@ -667,18 +667,20 @@ object desugar {
         DefDef(name, Nil, tpt, rhs).withMods(synthetic)
 
       def productElemMeths =
-        val caseParams = derivedVparamss.head.toArray
-        val selectorNamesInBody = normalizedBody.collect {
-          case vdef: ValDef if vdef.name.isSelectorName =>
-            vdef.name
-          case ddef: DefDef if ddef.name.isSelectorName && ddef.paramss.isEmpty =>
-            ddef.name
-        }
-        for i <- List.range(0, arity)
-            selName = nme.selectorName(i)
-            if (selName ne caseParams(i).name) && !selectorNamesInBody.contains(selName)
-        yield syntheticProperty(selName, caseParams(i).tpt,
-          Select(This(EmptyTypeIdent), caseParams(i).name))
+        if caseClassInScala2StdLib then Nil
+        else
+          val caseParams = derivedVparamss.head.toArray
+          val selectorNamesInBody = normalizedBody.collect {
+            case vdef: ValDef if vdef.name.isSelectorName =>
+              vdef.name
+            case ddef: DefDef if ddef.name.isSelectorName && ddef.paramss.isEmpty =>
+              ddef.name
+          }
+          for i <- List.range(0, arity)
+              selName = nme.selectorName(i)
+              if (selName ne caseParams(i).name) && !selectorNamesInBody.contains(selName)
+          yield syntheticProperty(selName, caseParams(i).tpt,
+            Select(This(EmptyTypeIdent), caseParams(i).name))
 
       def enumCaseMeths =
         if isEnumCase then
@@ -768,11 +770,13 @@ object desugar {
         val unapplyMeth = {
           def scala2LibCompatUnapplyRhs(unapplyParamName: Name) =
             assert(arity <= Definitions.MaxTupleArity, "Unexpected case class with tuple larger than 22: "+ cdef.show)
-            if arity == 1 then Apply(scalaDot(nme.Option), Select(Ident(unapplyParamName), nme._1))
-            else
-              val tupleApply = Select(Ident(nme.scala), s"Tuple$arity".toTermName)
-              val members = List.tabulate(arity) { n => Select(Ident(unapplyParamName), s"_${n+1}".toTermName) }
-              Apply(scalaDot(nme.Option), Apply(tupleApply, members))
+            derivedVparamss.head match
+              case vparam :: Nil =>
+                Apply(scalaDot(nme.Option), Select(Ident(unapplyParamName), vparam.name))
+              case vparams =>
+                val tupleApply = Select(Ident(nme.scala), s"Tuple$arity".toTermName)
+                val members = vparams.map(vparam => Select(Ident(unapplyParamName), vparam.name))
+                Apply(scalaDot(nme.Option), Apply(tupleApply, members))
 
           val hasRepeatedParam = constrVparamss.head.exists {
             case ValDef(_, tpt, _) => isRepeated(tpt)

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

@@ -161,7 +161,9 @@ class SyntheticMembers(thisPhase: DenotTransformer) {
         case nme.productArity => Literal(Constant(accessors.length))
         case nme.productPrefix if isEnumValue => nameRef
         case nme.productPrefix => ownName
-        case nme.productElement => productElementBody(accessors.length, vrefss.head.head)
+        case nme.productElement =>
+          if ctx.settings.Yscala2Stdlib.value then productElementBodyForScala2Compat(accessors.length, vrefss.head.head)
+          else productElementBody(accessors.length, vrefss.head.head)
         case nme.productElementName => productElementNameBody(accessors.length, vrefss.head.head)
       }
       report.log(s"adding $synthetic to $clazz at ${ctx.phase}")
@@ -185,7 +187,7 @@ class SyntheticMembers(thisPhase: DenotTransformer) {
      *  ```
      */
     def productElementBody(arity: Int, index: Tree)(using Context): Tree = {
-      // case N => _${N + 1}
+      // case N => this._${N + 1}
       val cases = 0.until(arity).map { i =>
         val sel = This(clazz).select(nme.selectorName(i), _.info.isParameterless)
         CaseDef(Literal(Constant(i)), EmptyTree, sel)
@@ -194,6 +196,33 @@ class SyntheticMembers(thisPhase: DenotTransformer) {
       Match(index, (cases :+ generateIOBECase(index)).toList)
     }
 
+    /** The class
+     *
+     *  ```
+     *  case class C(x: T, y: T)
+     *  ```
+     *
+     *  gets the `productElement` method:
+     *
+     *  ```
+     *  def productElement(index: Int): Any = index match {
+     *    case 0 => this.x
+     *    case 1 => this.y
+     *    case _ => throw new IndexOutOfBoundsException(index.toString)
+     *  }
+     *  ```
+     */
+    def productElementBodyForScala2Compat(arity: Int, index: Tree)(using Context): Tree = {
+      val paramNames = ctx.owner.owner.info.decls.toList.filter(_.flags.is(ParamAccessor)).map(_.name)
+      // case N => this.${paramNames(N)}
+      val cases = paramNames.zip(0.until(arity)).map { (name, i) =>
+        val sel = This(clazz).select(name, _.info.isParameterless)
+        CaseDef(Literal(Constant(i)), EmptyTree, sel)
+      }
+
+      Match(index, (cases :+ generateIOBECase(index)).toList)
+    }
+
     /** The class
      *
      *  ```

stdlib-bootstrapped-tasty-tests/src/Main.scala

@@ -36,7 +36,7 @@ object HelloWorld:
 
   def testScala2CaseClassUnderscoreMembers() = {
     val some: Some[Int] = Some(1)
-    // FIXME: assert(!typeChecks("some._1"))
+    assert(!typeChecks("some._1"))
   }
 
   def testScalaNumberUnderlying() = {