Support refinement type on enum case

This used to fail with:
    trait <refinement> in value x extends enum EC, but extending enums is prohibited.

Author: smarter

compiler/src/dotty/tools/dotc/typer/Typer.scala

@@ -3166,7 +3166,7 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
         .withType(dummy.termRef)
       if (!cls.isOneOf(AbstractOrTrait) && !ctx.isAfterTyper)
         checkRealizableBounds(cls, cdef.sourcePos.withSpan(cdef.nameSpan))
-      if cls.isEnum || firstParentTpe.classSymbol.isEnum then
+      if cls.isEnum || !cls.isRefinementClass && firstParentTpe.classSymbol.isEnum then
         checkEnum(cdef, cls, firstParent)
       val cdef1 = assignType(cpy.TypeDef(cdef)(name, impl1), cls)
 

tests/pos/enum-refinement.scala

@@ -0,0 +1,12 @@
+enum Enum:
+  case EC(val x: Int)
+
+val a: Enum.EC { val x: 1 } = Enum.EC(1).asInstanceOf[Enum.EC { val x: 1 }]
+
+import scala.language.experimental.modularity
+
+enum EnumT:
+  case EC(tracked val x: Int)
+
+val b: EnumT.EC { val x: 1 } = EnumT.EC(1)
+