Make productArity a synthetic method which can be overridden

Like other synthetic methods, productArity will not be generated
if an explicit definition is given. Used to be generated unconditionally
in desugaring.

3 files changed, 5 insertions, 4 deletions

diff --git a/src/dotty/tools/dotc/ast/Desugar.scala b/src/dotty/tools/dotc/ast/Desugar.scala
index 9f37326f6..e2152066f 100644
--- a/src/dotty/tools/dotc/ast/Desugar.scala
+++ b/src/dotty/tools/dotc/ast/Desugar.scala
@@ -287,7 +287,6 @@ object desugar {
         def syntheticProperty(name: TermName, rhs: Tree) =
           DefDef(synthetic, name, Nil, Nil, TypeTree(), rhs)
         val isDefinedMeth = syntheticProperty(nme.isDefined, Literal(Constant(true)))
-        val productArityMeth = syntheticProperty(nme.productArity, Literal(Constant(arity)))
         val caseParams = constrVparamss.head.toArray
         val productElemMeths = for (i <- 0 until arity) yield
           syntheticProperty(nme.selectorName(i), Select(This(EmptyTypeName), caseParams(i).name))
@@ -302,7 +301,7 @@ object desugar {
               cpy.ValDef(vparam)(rhs = EmptyTree))
             DefDef(synthetic, nme.copy, derivedTparams, copyFirstParams :: copyRestParamss, TypeTree(), creatorExpr) :: Nil
           }
-        copyMeths ::: isDefinedMeth :: productArityMeth :: productElemMeths.toList
+        copyMeths ::: isDefinedMeth :: productElemMeths.toList
       }
       else Nil
 
diff --git a/src/dotty/tools/dotc/core/Definitions.scala b/src/dotty/tools/dotc/core/Definitions.scala
index 1bc60f053..4914f667a 100644
--- a/src/dotty/tools/dotc/core/Definitions.scala
+++ b/src/dotty/tools/dotc/core/Definitions.scala
@@ -296,6 +296,7 @@ class Definitions {
   lazy val ComparableClass              = ctx.requiredClass("java.lang.Comparable")
   lazy val ProductClass                 = ctx.requiredClass("scala.Product")
     lazy val Product_canEqual = ProductClass.requiredMethod(nme.canEqual_)
+    lazy val Product_productArity = ProductClass.requiredMethod(nme.productArity)
   lazy val LanguageModuleClass          = ctx.requiredModule("dotty.language").moduleClass.asClass
 
   // Annotation base classes
diff --git a/src/dotty/tools/dotc/transform/SyntheticMethods.scala b/src/dotty/tools/dotc/transform/SyntheticMethods.scala
index 0af3ef895..34cca2872 100644
--- a/src/dotty/tools/dotc/transform/SyntheticMethods.scala
+++ b/src/dotty/tools/dotc/transform/SyntheticMethods.scala
@@ -41,14 +41,14 @@ class SyntheticMethods extends MiniPhaseTransform with IdentityDenotTransformer
 
   override def init(implicit ctx: Context, info: TransformerInfo) = {
     valueSymbols = List(defn.Any_hashCode, defn.Any_equals)
-    caseSymbols = valueSymbols ++ List(defn.Any_toString, defn.Product_canEqual)
+    caseSymbols = valueSymbols ++ List(defn.Any_toString, defn.Product_canEqual, defn.Product_productArity)
   }
 
   /** The synthetic methods of the case or value class `clazz`.
    */
   def syntheticMethods(clazz: ClassSymbol)(implicit ctx: Context): List[Tree] = {
     val clazzType = clazz.typeRef
-    def accessors = clazz.decls.filter(_ is CaseAccessor)
+    def accessors = clazz.decls.filter(_ is CaseAccessor).toList
 
     val symbolsToSynthesize: List[Symbol] =
       if (clazz.is(Case)) caseSymbols
@@ -75,6 +75,7 @@ class SyntheticMethods extends MiniPhaseTransform with IdentityDenotTransformer
         case nme.toString_ => forwardToRuntime
         case nme.equals_ => vrefss => equalsBody(vrefss.head.head)
         case nme.canEqual_ => vrefss => canEqualBody(vrefss.head.head)
+        case nme.productArity => vrefss => Literal(Constant(accessors.length))
       }
       ctx.log(s"adding $synthetic to $clazz at ${ctx.phase}")
       DefDef(synthetic, syntheticRHS(ctx.withOwner(synthetic)))