SI-7402 List extends Serializable

While we are all aware of the issues around Serialization,
I think in this case it is perfectly sound and safe to make
List serializable:

- List is not an interface, it is the base type of an ADT.
  Common behavior of its members should be reflected in the base type.
- List is sealed, there is no chance of an user providing a new
  non-serializable subtype of List.

src/library/scala/collection/immutable/List.scala

@@ -85,7 +85,8 @@ sealed abstract class List[+A] extends AbstractSeq[A]
                                   with LinearSeq[A]
                                   with Product
                                   with GenericTraversableTemplate[A, List]
-                                  with LinearSeqOptimized[A, List[A]] {
+                                  with LinearSeqOptimized[A, List[A]]
+                                  with Serializable {
   override def companion: GenericCompanion[List] = List
 
   import scala.collection.{Iterable, Traversable, Seq, IndexedSeq}

test/files/run/lub-visibility.check

@@ -8,7 +8,8 @@ scala> // should infer List[scala.collection.immutable.Seq[Nothing]]
 scala> // but reverted that for SI-5534.
 
 scala> val x = List(List(), Vector())
-x: List[scala.collection.immutable.Seq[Nothing] with scala.collection.AbstractSeq[Nothing]{def companion: scala.collection.generic.GenericCompanion[scala.collection.immutable.Seq with scala.collection.AbstractSeq]; def dropRight(n: Int): scala.collection.immutable.Seq[Nothing] with scala.collection.AbstractSeq[Nothing]{def companion: scala.collection.generic.GenericCompanion[scala.collection.immutable.Seq with scala.collection.AbstractSeq]; def dropRight(n: Int): scala.collection.immutable.Seq[Nothing] with scala.collection.AbstractSeq[Nothing]{def dropRight(n: Int): scala.collection.immutable.Seq[Nothing] with scala.collection.AbstractSeq[Nothing]; def takeRight(n: Int): scala.collection.immutable.Seq[Nothing] with scala.collection.AbstractSeq[Nothing]; def drop(n: Int): scala.collecti...
+x: List[scala.collection.immutable.Seq[Nothing] with scala.collection.AbstractSeq[Nothing] with java.io.Serializable] = List(List(), Vector())
+
 scala> 
 
 scala> 

test/files/run/repl-colon-type.check

@@ -79,7 +79,7 @@ TypeRef(
     )
     TypeRef(
       TypeSymbol(
-        sealed abstract class List[+A] extends AbstractSeq[A] with LinearSeq[A] with Product with GenericTraversableTemplate[A,List] with LinearSeqOptimized[A,List[A]]
+        sealed abstract class List[+A] extends AbstractSeq[A] with LinearSeq[A] with Product with GenericTraversableTemplate[A,List] with LinearSeqOptimized[A,List[A]] with Serializable
 
       )
       args = List(
@@ -146,7 +146,7 @@ TypeRef(
       args = List(
         TypeRef(
           TypeSymbol(
-            sealed abstract class List[+A] extends AbstractSeq[A] with LinearSeq[A] with Product with GenericTraversableTemplate[A,List] with LinearSeqOptimized[A,List[A]]
+            sealed abstract class List[+A] extends AbstractSeq[A] with LinearSeq[A] with Product with GenericTraversableTemplate[A,List] with LinearSeqOptimized[A,List[A]] with Serializable
 
           )
           args = List(
@@ -179,7 +179,7 @@ PolyType(
           args = List(
             TypeRef(
               TypeSymbol(
-                sealed abstract class List[+A] extends AbstractSeq[A] with LinearSeq[A] with Product with GenericTraversableTemplate[A,List] with LinearSeqOptimized[A,List[A]]
+                sealed abstract class List[+A] extends AbstractSeq[A] with LinearSeq[A] with Product with GenericTraversableTemplate[A,List] with LinearSeqOptimized[A,List[A]] with Serializable
 
               )
               args = List(TypeParamTypeRef(TypeParam(T <: AnyVal)))
@@ -202,7 +202,7 @@ PolyType(
     params = List(TermSymbol(x: T), TermSymbol(y: List[U]))
     resultType = TypeRef(
       TypeSymbol(
-        sealed abstract class List[+A] extends AbstractSeq[A] with LinearSeq[A] with Product with GenericTraversableTemplate[A,List] with LinearSeqOptimized[A,List[A]]
+        sealed abstract class List[+A] extends AbstractSeq[A] with LinearSeq[A] with Product with GenericTraversableTemplate[A,List] with LinearSeqOptimized[A,List[A]] with Serializable
 
       )
       args = List(TypeParamTypeRef(TypeParam(U >: T)))

test/files/run/t2251b.check

@@ -6,6 +6,6 @@ TypeTag[List[scala.collection.Set[_ >: G with F <: B[_ >: G with F <: B[_ >: G w
 TypeTag[List[scala.collection.Set[_ >: G with F <: B[_ >: G with F <: B[_ >: G with F <: A]]]]]
 TypeTag[List[Seq[B[_ >: G with F <: B[_ >: G with F <: A]]]]]
 TypeTag[List[scala.collection.Map[_ >: F with C <: B[_ >: F with C <: B[_ >: F with C <: A]], B[_ >: G with D <: B[_ >: G with D <: A]]]]]
-TypeTag[List[scala.collection.AbstractSeq[B[_ >: G with F <: B[_ >: G with F <: A]]] with scala.collection.LinearSeq[B[_ >: G with F <: B[_ >: G with F <: A]]]{def companion: scala.collection.generic.GenericCompanion[scala.collection.AbstractSeq with scala.collection.LinearSeq]; def dropRight(n: Int): scala.collection.AbstractSeq[B[_ >: G with F <: A]] with scala.collection.LinearSeq[B[_ >: G with F <: A]]{def companion: scala.collection.generic.GenericCompanion[scala.collection.AbstractSeq with scala.collection.LinearSeq]; def dropRight(n: Int): scala.collection.AbstractSeq[A] with scala.collection.LinearSeq[A]; def drop(n: Int): scala.collection.AbstractSeq[A] with scala.collection.LinearSeq[A]; def take(n: Int): scala.collection.AbstractSeq[A] with scala.collection.LinearSeq[A]; def slice(from: Int,until: Int): scala.collection.AbstractSeq[A] with scala.collection.LinearSeq[A]}; def drop(n: Int): scala.collection.AbstractSeq[B[_ >: G with F <: A]] with scala.collection.LinearSeq[B[_ >: G with F <: A]]{def companion: scala.collection.generic.GenericCompanion[scala.collection.AbstractSeq with scala.collection.LinearSeq]; def dropRight(n: Int): scala.collection.AbstractSeq[A] with scala.collection.LinearSeq[A]; def drop(n: Int): scala.collection.AbstractSeq[A] with scala.collection.LinearSeq[A]; def take(n: Int): scala.collection.AbstractSeq[A] with scala.collection.LinearSeq[A]; def slice(from: Int,until: Int): scala.collection.AbstractSeq[A] with scala.collection.LinearSeq[A]}; def take(n: Int): scala.collection.AbstractSeq[B[_ >: G with F <: A]] with scala.collection.LinearSeq[B[_ >: G with F <: A]]{def companion: scala.collection.generic.GenericCompanion[scala.collection.AbstractSeq with scala.collection.LinearSeq]; def dropRight(n: Int): scala.collection.AbstractSeq[A] with scala.collection.LinearSeq[A]; def drop(n: Int): scala.collection.AbstractSeq[A] with scala.collection.LinearSeq[A]; def take(n: Int): scala.collection.AbstractSeq[A] with scala.collection.LinearSeq[A]; def slice(from: Int,until: Int): scala.collection.AbstractSeq[A] with scala.collection.LinearSeq[A]}; def slice(from: Int,until: Int): scala.collection.AbstractSeq[B[_ >: G with F <: A]] with scala.collection.LinearSeq[B[_ >: G with F <: A]]{def companion: scala.collection.generic.GenericCompanion[scala.collection.AbstractSeq with scala.collection.LinearSeq]; def dropRight(n: Int): scala.collection.AbstractSeq[A] with scala.collection.LinearSeq[A]; def drop(n: Int): scala.collection.AbstractSeq[A] with scala.collection.LinearSeq[A]; def take(n: Int): scala.collection.AbstractSeq[A] with scala.collection.LinearSeq[A]; def slice(from: Int,until: Int): scala.collection.AbstractSeq[A] with scala.collection.LinearSeq[A]}}]]
+TypeTag[List[scala.collection.AbstractSeq[B[_ >: G with F <: B[_ >: G with F <: A]]] with scala.collection.LinearSeq[B[_ >: G with F <: B[_ >: G with F <: A]]] with java.io.Serializable]]
 TypeTag[List[Seq[B[_ >: G with F <: B[_ >: G with F <: A]]]]]
 TypeTag[List[Seq[B[_ >: G with F <: B[_ >: G with F <: A]]]]]