Allow traits to extend jl.Enum, and classes to do so in migration mode

Author: TheElectronWill

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

@@ -17,7 +17,7 @@ import scala.util.{Try, Failure, Success}
 import config.{ScalaVersion, NoScalaVersion}
 import Decorators._
 import typer.ErrorReporting._
-import config.Feature.warnOnMigration
+import config.Feature.{warnOnMigration, migrateTo3}
 import reporting._
 
 object RefChecks {
@@ -106,8 +106,13 @@ object RefChecks {
       for (reqd <- cinfo.cls.givenSelfType.classSymbols)
         checkSelfConforms(reqd, "missing requirement", "required")
 
-      if !cls.is(Enum) && parents.exists(_.classSymbol == defn.JavaEnumClass) then
+      // Prevent wrong `extends` of java.lang.Enum
+      if !migrateTo3 &&
+         !cls.isOneOf(Enum | Trait) &&
+         parents.exists(_.classSymbol == defn.JavaEnumClass)
+      then
         report.error(CannotExtendJavaEnum(cls), cls.sourcePos)
+
     case _ =>
   }
 

compiler/test/dotty/tools/dotc/CompilationTests.scala

@@ -66,6 +66,7 @@ class CompilationTests extends ParallelTesting {
       compileFile("tests/pos-custom-args/i5498-postfixOps.scala", defaultOptions withoutLanguageFeature "postfixOps"),
       compileFile("tests/pos-custom-args/i8875.scala", defaultOptions.and("-Xprint:getters")),
       compileFile("tests/pos-custom-args/i9267.scala", defaultOptions.and("-Ystop-after:erasure")),
+      compileFile("tests/pos-special/extend-java-enum.scala", defaultOptions.and("-source", "3.0-migration")),
     ).checkCompile()
   }
 

tests/neg/extend-java-enum.scala

@@ -4,4 +4,6 @@ class C1 extends jl.Enum[C1] // error: class C1 cannot extend java.lang.Enum
 
 class C2(name: String, ordinal: Int) extends jl.Enum[C2](name, ordinal) // error: class C2 cannot extend java.lang.Enum
 
-trait T extends jl.Enum[T] // error: trait T cannot extend java.lang.Enum
+trait T extends jl.Enum[T] // ok
+
+class C3 extends T // error: class C3 cannot extend java.lang.Enum

tests/pos-special/extend-java-enum.scala

@@ -0,0 +1,21 @@
+import java.{lang => jl}
+
+final class ConfigSyntax private (name: String, ordinal: Int)
+  extends jl.Enum[ConfigSyntax](name, ordinal)
+
+object ConfigSyntax {
+
+  final val JSON = new ConfigSyntax("JSON", 0)
+  final val CONF = new ConfigSyntax("CONF", 1)
+  final val PROPERTIES = new ConfigSyntax("PROPERTIES", 2)
+
+  private[this] final val _values: Array[ConfigSyntax] =
+    Array(JSON, CONF, PROPERTIES)
+
+  def values: Array[ConfigSyntax] = _values.clone()
+
+  def valueOf(name: String): ConfigSyntax =
+    _values.find(_.name == name).getOrElse {
+      throw new IllegalArgumentException("No enum const ConfigSyntax." + name)
+    }
+}

tests/pos/trait-java-enum.scala

@@ -0,0 +1,5 @@
+trait T extends java.lang.Enum[T]
+
+enum MyEnum extends T {
+  case A, B
+}