Make hashcode of enum items stable (#23218)

Fixes #19177.

Author: sjrd

compiler/src/dotty/tools/dotc/transform/SyntheticMembers.scala

@@ -69,7 +69,7 @@ class SyntheticMembers(thisPhase: DenotTransformer) {
       myCaseSymbols = defn.caseClassSynthesized
       myCaseModuleSymbols = myCaseSymbols.filter(_ ne defn.Any_equals)
       myEnumValueSymbols = List(defn.Product_productPrefix)
-      myNonJavaEnumValueSymbols = myEnumValueSymbols :+ defn.Any_toString :+ defn.Enum_ordinal
+      myNonJavaEnumValueSymbols = myEnumValueSymbols :+ defn.Any_toString :+ defn.Enum_ordinal :+ defn.Any_hashCode
     }
 
   def valueSymbols(using Context): List[Symbol] = { initSymbols; myValueSymbols }
@@ -117,6 +117,12 @@ class SyntheticMembers(thisPhase: DenotTransformer) {
     def syntheticDefIfMissing(sym: Symbol): List[Tree] =
       if (existingDef(sym, clazz).exists) Nil else syntheticDef(sym) :: Nil
 
+    def identifierRef: Tree =
+      if isSimpleEnumValue then // owner is `def $new(_$ordinal: Int, $name: String) = new MyEnum { ... }`
+        ref(clazz.owner.paramSymss.head.find(_.name == nme.nameDollar).get)
+      else // assume owner is `val Foo = new MyEnum { def ordinal = 0 }`
+        Literal(Constant(clazz.owner.name.toString))
+
     def syntheticDef(sym: Symbol): Tree = {
       val synthetic = sym.copy(
         owner = clazz,
@@ -136,12 +142,6 @@ class SyntheticMembers(thisPhase: DenotTransformer) {
         else
           identifierRef
 
-      def identifierRef: Tree =
-        if isSimpleEnumValue then // owner is `def $new(_$ordinal: Int, $name: String) = new MyEnum { ... }`
-          ref(clazz.owner.paramSymss.head.find(_.name == nme.nameDollar).get)
-        else // assume owner is `val Foo = new MyEnum { def ordinal = 0 }`
-          Literal(Constant(clazz.owner.name.toString))
-
       def ordinalRef: Tree =
         if isSimpleEnumValue then // owner is `def $new(_$ordinal: Int, $name: String) = new MyEnum { ... }`
           ref(clazz.owner.paramSymss.head.find(_.name == nme.ordinalDollar_).get)
@@ -358,7 +358,8 @@ class SyntheticMembers(thisPhase: DenotTransformer) {
      * For case classes with primitive paramters, see [[caseHashCodeBody]].
      */
     def chooseHashcode(using Context) =
-      if (accessors.isEmpty) Literal(Constant(ownName.hashCode))
+      if (isNonJavaEnumValue) identifierRef.select(nme.hashCode_).appliedToTermArgs(Nil)
+      else if (accessors.isEmpty) Literal(Constant(ownName.hashCode))
       else if (accessors.exists(_.info.finalResultType.classSymbol.isPrimitiveValueClass))
         caseHashCodeBody
       else

tests/run/stable-enum-hashcodes.check

@@ -0,0 +1,12 @@
+65
+65
+66
+66
+67
+67
+68
+68
+-1449359058
+-1449359058
+194551161
+194551161

tests/run/stable-enum-hashcodes.scala

@@ -0,0 +1,23 @@
+enum Enum:
+  case A
+  case B
+  case C()
+  case D()
+  case E(x: Int)
+
+@main def Test =
+  // Enum values (were not stable from run to run before #23218)
+  println(Enum.A.hashCode)
+  println(Enum.A.hashCode)
+  println(Enum.B.hashCode)
+  println(Enum.B.hashCode)
+
+  // Other enum cases (were already stable from run to run)
+  println(Enum.C().hashCode)
+  println(Enum.C().hashCode)
+  println(Enum.D().hashCode)
+  println(Enum.D().hashCode)
+  println(Enum.E(1).hashCode)
+  println(Enum.E(1).hashCode)
+  println(Enum.E(2).hashCode)
+  println(Enum.E(2).hashCode)