Fix 2.13.9 regression which broke binary compatibility of case classes which are also value classes

src/compiler/scala/tools/nsc/typechecker/SyntheticMethods.scala

@@ -24,6 +24,7 @@ import symtab.Flags._
  *    def productArity: Int
  *    def productElement(n: Int): Any
  *    def productPrefix: String
+ *    def productIterator: Iterator[Any]   // required for binary compatibility of value classes
  *
  *  Selectively added to case classes/objects, unless a non-default
  *  implementation already exists:
@@ -113,6 +114,10 @@ trait SyntheticMethods extends ast.TreeDSL {
         (m0 ne meth) && !m0.isDeferred && !m0.isSynthetic && (m0.owner != AnyValClass) && (typeInClazz(m0) matches typeInClazz(meth))
       }
     }
+    def productIteratorMethod =
+      createMethod(nme.productIterator, iteratorOfType(AnyTpe))(_ =>
+        gen.mkMethodCall(ScalaRunTimeModule, nme.typedProductIterator, List(AnyTpe), List(mkThis))
+      )
 
     def perElementMethod(name: Name, returnType: Type)(caseFn: Symbol => Tree): Tree = 
       createSwitchMethod(name, accessors.indices, returnType)(idx => caseFn(accessors(idx)))
@@ -259,6 +264,7 @@ trait SyntheticMethods extends ast.TreeDSL {
         Product_productPrefix -> (() => constantNullary(nme.productPrefix, clazz.name.decode)),
         Product_productArity -> (() => constantNullary(nme.productArity, arity)),
         Product_productElement -> (() => perElementMethod(nme.productElement, AnyTpe)(mkThisSelect)),
+        Product_iterator -> (() => productIteratorMethod),
         Product_canEqual -> (() => canEqualMethod)
       )
     }

test/files/run/idempotency-case-classes.check

@@ -20,6 +20,7 @@ C(2,3)
       case 1 => C.this.y
       case _ => scala.runtime.Statics.ioobe[Any](x$1)
     };
+    override <synthetic> def productIterator: Iterator[Any] = scala.runtime.ScalaRunTime.typedProductIterator[Any](C.this);
     <synthetic> def canEqual(x$1: Any): Boolean = x$1.$isInstanceOf[C]();
     override <synthetic> def productElementName(x$1: Int): String = x$1 match {
       case 0 => "x"

test/files/scalap/caseClass.check

@@ -6,6 +6,7 @@ case class CaseClass[A <: scala.Seq[scala.Int]](i: A, s: scala.Predef.String) ex
   override def productPrefix: java.lang.String = { /* compiled code */ }
   def productArity: scala.Int = { /* compiled code */ }
   def productElement(x$1: scala.Int): scala.Any = { /* compiled code */ }
+  override def productIterator: scala.collection.Iterator[scala.Any] = { /* compiled code */ }
   def canEqual(x$1: scala.Any): scala.Boolean = { /* compiled code */ }
   override def productElementName(x$1: scala.Int): java.lang.String = { /* compiled code */ }
   override def hashCode(): scala.Int = { /* compiled code */ }

test/files/scalap/caseObject.check

@@ -3,6 +3,7 @@ case object CaseObject extends scala.AnyRef with scala.Product with scala.Serial
   override def productPrefix: java.lang.String = { /* compiled code */ }
   def productArity: scala.Int = { /* compiled code */ }
   def productElement(x$1: scala.Int): scala.Any = { /* compiled code */ }
+  override def productIterator: scala.collection.Iterator[scala.Any] = { /* compiled code */ }
   def canEqual(x$1: scala.Any): scala.Boolean = { /* compiled code */ }
   override def hashCode(): scala.Int = { /* compiled code */ }
   override def toString(): java.lang.String = { /* compiled code */ }

test/scaladoc/run/implicits-chaining.scala

@@ -23,15 +23,15 @@ object Test extends ScaladocModelTest {
     val A = base._class("A")
 
     conv = A._conversion(base.qualifiedName + ".convertToZ")
-    assertEquals(3, conv.members.length)
+    assertEquals(2, conv.members.length)
     assertEquals(1, conv.constraints.length)
 
 //// class B ///////////////////////////////////////////////////////////////////////////////////////////////////////////
 
     val B = base._class("B")
 
     conv = B._conversion(base.qualifiedName + ".convertToZ")
-    assertEquals(3, conv.members.length)
+    assertEquals(2, conv.members.length)
     assertEquals(0, conv.constraints.length)
 
 //// class C ///////////////////////////////////////////////////////////////////////////////////////////////////////////
@@ -45,15 +45,15 @@ object Test extends ScaladocModelTest {
     val D = base._class("D")
 
     conv = D._conversion(base.qualifiedName + ".convertToZ")
-    assertEquals(3, conv.members.length)
+    assertEquals(2, conv.members.length)
     assertEquals(0, conv.constraints.length)
 
 //// class E ///////////////////////////////////////////////////////////////////////////////////////////////////////////
 
     val E = base._class("E")
 
     conv = E._conversion(base.qualifiedName + ".convertToZ")
-    assertEquals(3, conv.members.length)
+    assertEquals(2, conv.members.length)
     assertEquals(0, conv.constraints.length)
 
 //// class F ///////////////////////////////////////////////////////////////////////////////////////////////////////////