Fix #9011: Make single enum values inherit from Product

compiler/src/dotty/tools/dotc/core/ConstraintHandling.scala

@@ -300,6 +300,8 @@ trait ConstraintHandling[AbstractContext] {
    *      (i.e. `inst.widenSingletons <:< bound` succeeds with satisfiable constraint)
    *   2. If `inst` is a union type, approximate the union type from above by an intersection
    *      of all common base types, provided the result is a subtype of `bound`.
+   *   3. If `inst` is an intersection with some protected base types, drop
+   *      the protected base types from the intersection, provided the result is a subtype of `bound`.
    *
    *  Don't do these widenings if `bound` is a subtype of `scala.Singleton`.
    *  Also, if the result of these widenings is a TypeRef to a module class,
@@ -309,26 +311,43 @@ trait ConstraintHandling[AbstractContext] {
    * At this point we also drop the @Repeated annotation to avoid inferring type arguments with it,
    * as those could leak the annotation to users (see run/inferred-repeated-result).
    */
-  def widenInferred(inst: Type, bound: Type)(implicit actx: AbstractContext): Type = {
-    def widenOr(tp: Type) = {
+  def widenInferred(inst: Type, bound: Type)(implicit actx: AbstractContext): Type =
+
+    def isProtected(tp: Type) = tp.typeSymbol == defn.EnumValueClass // for now, to be generalized later
+
+    def dropProtected(tp: Type): Type = tp.dealias match
+      case tp @ AndType(tp1, tp2) =>
+        if isProtected(tp1) then tp2
+        else if isProtected(tp2) then tp1
+        else tp.derivedAndType(dropProtected(tp1), dropProtected(tp2))
+      case _ =>
+        tp
+
+    def widenProtected(tp: Type) =
+      val tpw = dropProtected(tp)
+      if (tpw ne tp) && (tpw <:< bound) then tpw else tp
+
+    def widenOr(tp: Type) =
       val tpw = tp.widenUnion
       if (tpw ne tp) && (tpw <:< bound) then tpw else tp
-    }
-    def widenSingle(tp: Type) = {
+
+    def widenSingle(tp: Type) =
       val tpw = tp.widenSingletons
       if (tpw ne tp) && (tpw <:< bound) then tpw else tp
-    }
+
     def isSingleton(tp: Type): Boolean = tp match
       case WildcardType(optBounds) => optBounds.exists && isSingleton(optBounds.bounds.hi)
       case _ => isSubTypeWhenFrozen(tp, defn.SingletonType)
+
     val wideInst =
-      if isSingleton(bound) then inst else widenOr(widenSingle(inst))
+      if isSingleton(bound) then inst
+      else widenProtected(widenOr(widenSingle(inst)))
     wideInst match
       case wideInst: TypeRef if wideInst.symbol.is(Module) =>
         TermRef(wideInst.prefix, wideInst.symbol.sourceModule)
       case _ =>
         wideInst.dropRepeatedAnnot
-  }
+  end widenInferred
 
   /** The instance type of `param` in the current constraint (which contains `param`).
    *  If `fromBelow` is true, the instance type is the lub of the parameter's

compiler/src/dotty/tools/dotc/core/Definitions.scala

@@ -639,6 +639,7 @@ class Definitions {
   @tu lazy val EnumClass: ClassSymbol = ctx.requiredClass("scala.Enum")
     @tu lazy val Enum_ordinal: Symbol = EnumClass.requiredMethod(nme.ordinal)
 
+  @tu lazy val EnumValueClass: ClassSymbol = ctx.requiredClass("scala.EnumValue")
   @tu lazy val EnumValuesClass: ClassSymbol = ctx.requiredClass("scala.runtime.EnumValues")
   @tu lazy val ProductClass: ClassSymbol = ctx.requiredClass("scala.Product")
     @tu lazy val Product_canEqual          : Symbol = ProductClass.requiredMethod(nme.canEqual_)

tests/neg/enumvalues.scala

@@ -0,0 +1,11 @@
+enum Color:
+  case Red, Green, Blue
+
+@main def Test(c: Boolean) =
+  // These currently give errors. But maybe we should make the actual
+  // enum values carry the `EnumValue` trait, and only strip it from
+  // user-defined vals and defs?
+  val x: EnumValue = if c then Color.Red else Color.Blue // error // error
+  val y: EnumValue = Color.Green        // error
+
+