Fix hashCode method for value classes

Before this commit, we used the same implementation than for case classes.
After this commit, we use the hashCode of the underlying type as defined
in SIP-15.

Author: smarter

src/dotty/tools/dotc/transform/SyntheticMethods.scala

@@ -76,7 +76,8 @@ class SyntheticMethods extends MiniPhaseTransform with IdentityDenotTransformer
         ref(defn.runtimeMethod("_" + sym.name.toString)).appliedToArgs(This(clazz) :: vrefss.head)
 
       def syntheticRHS(implicit ctx: Context): List[List[Tree]] => Tree = synthetic.name match {
-        case nme.hashCode_ => vrefss => hashCodeBody
+        case nme.hashCode_ if isDerivedValueClass(clazz) => vrefss => valueHashCodeBody
+        case nme.hashCode_ => vrefss => caseHashCodeBody
         case nme.toString_ => forwardToRuntime
         case nme.equals_ => vrefss => equalsBody(vrefss.head.head)
         case nme.canEqual_ => vrefss => canEqualBody(vrefss.head.head)
@@ -120,11 +121,24 @@ class SyntheticMethods extends MiniPhaseTransform with IdentityDenotTransformer
       }
     }
 
+    /** The class
+     *
+     *  class C(x: T) extends AnyVal
+     *
+     *  gets the hashCode method:
+     *
+     *    def hashCode: Int = x.hashCode()
+     */
+    def valueHashCodeBody(implicit ctx: Context): Tree = {
+      assert(accessors.length == 1)
+      ref(accessors.head).select(nme.hashCode_).ensureApplied
+    }
+
     /** The class
      *
      *    case class C(x: T, y: T)
      *
-     *  get the hashCode method:
+     *  gets the hashCode method:
      *
      *    def hashCode: Int = {
      *      <synthetic> var acc: Int = 0xcafebabe;
@@ -133,7 +147,7 @@ class SyntheticMethods extends MiniPhaseTransform with IdentityDenotTransformer
      *      Statics.finalizeHash(acc, 2)
      *    }
      */
-    def hashCodeBody(implicit ctx: Context): Tree = {
+    def caseHashCodeBody(implicit ctx: Context): Tree = {
       val acc = ctx.newSymbol(ctx.owner, "acc".toTermName, Mutable | Synthetic, defn.IntType, coord = ctx.owner.pos)
       val accDef = ValDef(acc, Literal(Constant(0xcafebabe)))
       val mixes = for (accessor <- accessors.toList) yield