Case class desugaring extend Product & NameBasedPattern (not ProductN)

Ignoring compatibility, I would say we could entirely remove the `Product` superclass.

Replacing `ProductN` with `Product` require synthesizing `productArity` and
`productElement` methods, which is done here directly in desugaring. To be
able to complie scala.TupleN in their current definition we Unfortunately need
to synthesize these new methods with override, which is less than ideal given
then user defined productArity / productElement methods would be silently
ignored.

Author: OlivierBlanvillain

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

@@ -285,7 +285,8 @@ object desugar {
       case _ => false
     }
 
-    val isCaseClass = mods.is(Case) && !mods.is(Module)
+    val isCaseClass  = mods.is(Case) && !mods.is(Module)
+    val isCaseObject = mods.is(Case) && mods.is(Module)
     val isValueClass = parents.nonEmpty && isAnyVal(parents.head)
       // This is not watertight, but `extends AnyVal` will be replaced by `inline` later.
 
@@ -332,6 +333,7 @@ object desugar {
     // Methods to add to a case class C[..](p1: T1, ..., pN: Tn)(moreParams)
     //     def isDefined = true
     //     def productArity = N
+    //     def productElement(i: Int): Any = i match { ... }
     //     def _1 = this.p1
     //     ...
     //     def _N = this.pN
@@ -342,14 +344,35 @@ object desugar {
     // Note: copy default parameters need @uncheckedVariance; see
     // neg/t1843-variances.scala for a test case. The test would give
     // two errors without @uncheckedVariance, one of them spurious.
-    val caseClassMeths =
-      if (isCaseClass) {
-        def syntheticProperty(name: TermName, rhs: Tree) =
-          DefDef(name, Nil, Nil, TypeTree(), rhs).withMods(synthetic)
+    val caseClassMeths = {
+      def syntheticProperty(name: TermName, rhs: Tree) =
+        DefDef(name, Nil, Nil, TypeTree(), rhs).withMods(synthetic)
+      // The override here is less than ideal: user defined productArity / productElement
+      // methods would be silently ignored. This is necessary to compile `scala.TupleN`.
+      // The long term solution is to remove `ProductN` entirely from stdlib.
+      def productArity =
+        DefDef(nme.productArity, Nil, Nil, TypeTree(), Literal(Constant(arity)))
+          .withMods(Modifiers(Synthetic | Override))
+      def productElement = {
+        val param = makeSyntheticParameter(tpt = ref(defn.IntType))
+        // case N => _${N + 1}
+        val cases = 0.until(arity).map { i =>
+          CaseDef(Literal(Constant(i)), EmptyTree, Select(This(EmptyTypeIdent), nme.selectorName(i)))
+        }
+        val ioob  = ref(defn.IndexOutOfBoundsException.typeRef)
+        val error = Throw(New(ioob, List(List(Select(refOfDef(param), nme.toString_)))))
+        // case _ => throw new IndexOutOfBoundsException(i.toString)
+        val defaultCase = CaseDef(untpd.Ident(nme.WILDCARD), EmptyTree, error)
+        val body = Match(refOfDef(param), (cases :+ defaultCase).toList)
+        DefDef(nme.productElement, Nil, List(List(param)), TypeTree(defn.AnyType), body)
+          .withMods(Modifiers(Synthetic | Override))
+      }
+      def productElemMeths = {
         val caseParams = constrVparamss.head.toArray
-        val productElemMeths =
-          for (i <- 0 until arity if nme.selectorName(i) `ne` caseParams(i).name)
-          yield syntheticProperty(nme.selectorName(i), Select(This(EmptyTypeIdent), caseParams(i).name))
+        for (i <- 0 until arity if nme.selectorName(i) `ne` caseParams(i).name)
+        yield syntheticProperty(nme.selectorName(i), Select(This(EmptyTypeIdent), caseParams(i).name))
+      }
+      def copyMeths = {
         def isRepeated(tree: Tree): Boolean = tree match {
           case PostfixOp(_, nme.raw.STAR) => true
           case ByNameTypeTree(tree1) => isRepeated(tree1)
@@ -359,34 +382,32 @@ object desugar {
           case ValDef(_, tpt, _) => isRepeated(tpt)
           case _ => false
         })
-
-        val copyMeths =
-          if (mods.is(Abstract) || hasRepeatedParam) Nil  // cannot have default arguments for repeated parameters, hence copy method is not issued
-          else {
-            def copyDefault(vparam: ValDef) =
-              makeAnnotated("scala.annotation.unchecked.uncheckedVariance", refOfDef(vparam))
-            val copyFirstParams = derivedVparamss.head.map(vparam =>
-              cpy.ValDef(vparam)(rhs = copyDefault(vparam)))
-            val copyRestParamss = derivedVparamss.tail.nestedMap(vparam =>
-              cpy.ValDef(vparam)(rhs = EmptyTree))
-            DefDef(nme.copy, derivedTparams, copyFirstParams :: copyRestParamss, TypeTree(), creatorExpr)
-              .withMods(synthetic) :: Nil
-          }
-        copyMeths ::: productElemMeths.toList
+        if (mods.is(Abstract) || hasRepeatedParam) Nil // Cannot have default arguments for repeated parameters, hence copy method is not issued
+        else {
+          def copyDefault(vparam: ValDef) =
+            makeAnnotated("scala.annotation.unchecked.uncheckedVariance", refOfDef(vparam))
+          val copyFirstParams = derivedVparamss.head.map(vparam =>
+            cpy.ValDef(vparam)(rhs = copyDefault(vparam)))
+          val copyRestParamss = derivedVparamss.tail.nestedMap(vparam =>
+            cpy.ValDef(vparam)(rhs = EmptyTree))
+          DefDef(nme.copy, derivedTparams, copyFirstParams :: copyRestParamss, TypeTree(), creatorExpr)
+            .withMods(synthetic) :: Nil
+        }
       }
+
+      if (isCaseClass)
+        productElement :: productArity :: copyMeths ::: productElemMeths.toList
+      else if (isCaseObject)
+        productElement :: productArity :: Nil
       else Nil
+    }
 
     def anyRef = ref(defn.AnyRefAlias.typeRef)
-    def productConstr(n: Int) = {
-      val tycon = scalaDot((tpnme.Product.toString + n).toTypeName)
-      val targs = constrVparamss.head map (_.tpt)
-      if (targs.isEmpty) tycon else AppliedTypeTree(tycon, targs)
-    }
 
-    // Case classes and case objects get a ProductN parent
-    var parents1 = parents
-    if (mods.is(Case) && arity <= Definitions.MaxTupleArity)
-      parents1 = parents1 :+ productConstr(arity)
+    // Case classes and case objects get NameBasedPattern and Product parents
+    val parents1: List[Tree] =
+      if (mods.is(Case)) parents :+ scalaDot(nme.Product.toTypeName) :+ scalaDot(nme.NameBasedPattern.toTypeName)
+      else parents
 
     // The thicket which is the desugared version of the companion object
     //     synthetic object C extends parentTpt { defs }

compiler/src/dotty/tools/dotc/core/Definitions.scala

@@ -459,6 +459,7 @@ class Definitions {
 
   lazy val JavaCloneableClass        = ctx.requiredClass("java.lang.Cloneable")
   lazy val NullPointerExceptionClass = ctx.requiredClass("java.lang.NullPointerException")
+  lazy val IndexOutOfBoundsException = ctx.requiredClass("java.lang.IndexOutOfBoundsException")
   lazy val ClassClass                = ctx.requiredClass("java.lang.Class")
   lazy val BoxedNumberClass          = ctx.requiredClass("java.lang.Number")
   lazy val ThrowableClass            = ctx.requiredClass("java.lang.Throwable")