Change enum Desugarings

compiler/src/dotty/tools/dotc/ast/Desugar.scala

@@ -355,10 +355,14 @@ object desugar {
     val originalVparamss = constr1.vparamss
     lazy val derivedEnumParams = enumClass.typeParams.map(derivedTypeParam)
     val impliedTparams =
-      if (isEnumCase && originalTparams.isEmpty)
-        derivedEnumParams.map(tdef => tdef.withFlags(tdef.mods.flags | PrivateLocal))
-      else
-        originalTparams
+      if (isEnumCase) {
+        val tparamReferenced = typeParamIsReferenced(
+            enumClass.typeParams, originalTparams, originalVparamss, parents)
+        if (originalTparams.isEmpty && (parents.isEmpty || tparamReferenced))
+          derivedEnumParams.map(tdef => tdef.withFlags(tdef.mods.flags | PrivateLocal))
+        else originalTparams
+      }
+      else originalTparams
     val constrTparams = impliedTparams.map(toDefParam)
     val constrVparamss =
       if (originalVparamss.isEmpty) { // ensure parameter list is non-empty
@@ -594,10 +598,11 @@ object desugar {
           if (constrTparams.nonEmpty ||
               constrVparamss.length > 1 ||
               mods.is(Abstract) ||
-              restrictedAccess) anyRef
+              restrictedAccess ||
+              isEnumCase) anyRef
           else
             // todo: also use anyRef if constructor has a dependent method type (or rule that out)!
-            (constrVparamss :\ (if (isEnumCase) applyResultTpt else classTypeRef)) (
+            (constrVparamss :\ classTypeRef) (
               (vparams, restpe) => Function(vparams map (_.tpt), restpe))
         def widenedCreatorExpr =
           (creatorExpr /: widenDefs)((rhs, meth) => Apply(Ident(meth.name), rhs :: Nil))
@@ -738,8 +743,11 @@ object desugar {
 
     if (mods is Package)
       PackageDef(Ident(moduleName), cpy.ModuleDef(mdef)(nme.PACKAGE, impl).withMods(mods &~ Package) :: Nil)
-    else if (isEnumCase)
+    else if (isEnumCase) {
+      typeParamIsReferenced(enumClass.typeParams, Nil, Nil, impl.parents)
+        // used to check there are no illegal references to enum's type parameters in parents
       expandEnumModule(moduleName, impl, mods, mdef.span)
+    }
     else {
       val clsName = moduleName.moduleClassName
       val clsRef = Ident(clsName)

compiler/src/dotty/tools/dotc/ast/DesugarEnums.scala

@@ -168,6 +168,59 @@ object DesugarEnums {
     }
   }
 
+  /** Is a type parameter in `enumTypeParams` referenced from an enum class case that has
+   *  given type parameters `caseTypeParams`, value parameters `vparamss` and parents `parents`?
+   *  Issues an error if that is the case but the reference is illegal.
+   *  The reference could be illegal for two reasons:
+   *   - explicit type parameters are given
+   *   - it's a value case, i.e. no value parameters are given
+   */
+  def typeParamIsReferenced(
+    enumTypeParams: List[TypeSymbol],
+    caseTypeParams: List[TypeDef],
+    vparamss: List[List[ValDef]],
+    parents: List[Tree])(implicit ctx: Context): Boolean = {
+
+    object searchRef extends UntypedTreeAccumulator[Boolean] {
+      var tparamNames = enumTypeParams.map(_.name).toSet[Name]
+      def underBinders(binders: List[MemberDef], op: => Boolean): Boolean = {
+        val saved = tparamNames
+        tparamNames = tparamNames -- binders.map(_.name)
+        try op
+        finally tparamNames = saved
+      }
+      def apply(x: Boolean, tree: Tree)(implicit ctx: Context): Boolean = x || {
+        tree match {
+          case Ident(name) =>
+            val matches = tparamNames.contains(name)
+            if (matches && (caseTypeParams.nonEmpty || vparamss.isEmpty))
+              ctx.error(i"illegal reference to type parameter $name from enum case", tree.sourcePos)
+            matches
+          case LambdaTypeTree(lambdaParams, body) =>
+            underBinders(lambdaParams, foldOver(x, tree))
+          case RefinedTypeTree(parent, refinements) =>
+            val refinementDefs = refinements collect { case r: MemberDef => r }
+            underBinders(refinementDefs, foldOver(x, tree))
+          case _ => foldOver(x, tree)
+        }
+      }
+      def apply(tree: Tree)(implicit ctx: Context): Boolean =
+        underBinders(caseTypeParams, apply(false, tree))
+    }
+
+    def typeHasRef(tpt: Tree) = searchRef(tpt)
+    def valDefHasRef(vd: ValDef) = typeHasRef(vd.tpt)
+    def parentHasRef(parent: Tree): Boolean = parent match {
+      case Apply(fn, _) => parentHasRef(fn)
+      case TypeApply(_, targs) => targs.exists(typeHasRef)
+      case Select(nu, nme.CONSTRUCTOR) => parentHasRef(nu)
+      case New(tpt) => typeHasRef(tpt)
+      case parent => parent.isType && typeHasRef(parent)
+    }
+
+    vparamss.exists(_.exists(valDefHasRef)) || parents.exists(parentHasRef)
+  }
+
   /** A pair consisting of
    *   - the next enum tag
    *   - scaffolding containing the necessary definitions for singleton enum cases

compiler/src/dotty/tools/dotc/ast/Trees.scala

@@ -1419,7 +1419,7 @@ object Trees {
       }
 
       def foldMoreCases(x: X, tree: Tree)(implicit ctx: Context): X = {
-        assert(ctx.reporter.errorsReported || ctx.mode.is(Mode.Interactive))
+        assert(ctx.reporter.errorsReported || ctx.mode.is(Mode.Interactive), tree)
           // In interactive mode, errors might come from previous runs.
           // In case of errors it may be that typed trees point to untyped ones.
           // The IDE can still traverse inside such trees, either in the run where errors

docs/docs/reference/enums/desugarEnums.md

@@ -25,9 +25,9 @@ some terminology and notational conventions:
 
 The desugaring rules imply that class cases are mapped to case classes, and singleton cases are mapped to `val` definitions.
 
-There are eight desugaring rules. Rule (1) desugar enum definitions. Rules
+There are nine desugaring rules. Rule (1) desugar enum definitions. Rules
 (2) and (3) desugar simple cases. Rules (4) to (6) define extends clauses for cases that
-are missing them. Rules (7) and (8) define how such cases with extends clauses
+are missing them. Rules (7) to (9) define how such cases with extends clauses
 map into case classes or vals.
 
 1.  An `enum` definition
@@ -80,7 +80,7 @@ map into case classes or vals.
        case C extends E[B1, ..., Bn]
 
    where `Bi` is `Li` if `Vi = '+'` and `Ui` if `Vi = '-'`. This result is then further
-   rewritten with rule (7). Simple cases of enums with non-variant type
+   rewritten with rule (8). Simple cases of enums with non-variant type
    parameters are not permitted.
 
 5. A class case without an extends clause
@@ -91,7 +91,7 @@ map into case classes or vals.
 
         case C <type-params> <value-params> extends E
 
-   This result is then further rewritten with rule (8).
+   This result is then further rewritten with rule (9).
 
 6. If `E` is an enum with type parameters `Ts`, a class case with neither type parameters nor an extends clause
 
@@ -101,9 +101,20 @@ map into case classes or vals.
 
         case C[Ts] <value-params> extends E[Ts]
 
-   This result is then further rewritten with rule (8). For class cases that have type parameters themselves, an extends clause needs to be given explicitly.
+   This result is then further rewritten with rule (9). For class cases that have type parameters themselves, an extends clause needs to be given explicitly.
 
-7. A value case
+7. If `E` is an enum with type parameters `Ts`, a class case without type parameters but with an extends clause
+
+       case C <value-params> extends <parents>
+
+   expands to
+
+       case C[Ts] <value-params> extends <parents>
+
+   provided at least one of the parameters `Ts` is mentioned in a parameter type in
+   `<value-params>` or in a type argument in `<parents>`.
+
+8. A value case
 
        case C extends <parents>
 
@@ -116,7 +127,10 @@ map into case classes or vals.
    as one of the `enumValues` of the enumeration (see below). `$values` is a
    compiler-defined private value in the companion object.
 
-8. A class case
+   It is an error if a value case refers to a type parameter of the enclosing `enum`
+   in a type argument of `<parents>`.
+
+9. A class case
 
        case C <params> extends <parents>
 
@@ -134,6 +148,11 @@ map into case classes or vals.
    where `n` is the ordinal number of the case in the companion object,
    starting from 0.
 
+   It is an error if a value case refers to a type parameter of the enclosing `enum`
+   in a parameter type in `<params>` or in a type argument of `<parents>`, unless that parameter is already
+   a type parameter of the case, i.e. the parameter name is defined in `<params>`.
+
+
 ### Translation of Enumerations
 
 Non-generic enums `E` that define one or more singleton cases

tests/neg/enum-tparams.scala

@@ -0,0 +1,40 @@
+object Test {
+
+  enum Opt[+T] {
+    case S(x: T) extends Opt[T]
+    case I(x: Int) extends Opt[Int]
+    case V() extends Opt[`T`]
+    case P(x: List[T]) extends Opt[String]
+    case N extends Opt[Nothing]
+  }
+
+  type Id[_]
+
+  enum E[F[_], G[_]] {
+    case C1() extends E[[X] => X, Id]
+    case C2() extends E[[F] => F, Id]
+    case C3() extends E[[X] => { type Y = F[Int] }, Id]
+    case C4() extends E[[X] => { type F = Int }, Id]
+    case C5() extends E[[F] => G[Int], Id]
+  }
+
+  Opt.S[Int](1) // OK
+  Opt.S(1)      // OK
+  Opt.I[Int](1) // error: does not take type parameters
+  Opt.I(1)      // OK
+  Opt.V[Int]()  // OK
+  Opt.V()       // OK
+  Opt.P[Int](List(1, 2, 3)) // OK
+  Opt.P(List(1, 2, 3)) // OK
+
+  E.C1[List, Id]()  // error: does not take type parameters
+  E.C1()            // OK
+  E.C2[List, Id]()  // error: does not take type parameters
+  E.C2()            // OK
+  E.C3[List, Id]()  // OK
+  E.C3()            // OK
+  E.C4[List, Id]()  // error: does not take type parameters
+  E.C4()            // OK
+  E.C5[List, Id]()  // OK
+  E.C5()            // OK
+}

tests/neg/enums.scala

@@ -19,11 +19,22 @@ enum E3[-T <: Ordered[T]] {
 
 enum E4 {
   case C
+  case C4(x: Int)
+}
+object E4 {
+  val x1: Int => E4 = C4    // error: found: C4, required: Int => E4
+  val x2: Int => E4 = C4(_) // ok
 }
 
 case class C4() extends E4 // error: cannot extend enum
 case object O4 extends E4 // error: cannot extend enum
 
+enum Captured[T] {
+  case Case1[U](x: T) extends Captured[U] // error: illegal reference to type parameter T from enum case
+  case Case2[U]()     extends Captured[T] // error: illegal reference to type parameter T from enum case
+  case Case3          extends Captured[T] // error: illegal reference to type parameter T from enum case
+}
+
 enum Option[+T] derives Eql {
   case Some(x: T)
   case None

tests/pos/enums-capture.scala

@@ -0,0 +1,7 @@
+class T
+
+enum Foo[T](val foo: Any) {
+  case Case1(x: Int) extends Foo(new T)
+  case Case2[U](x: U) extends Foo(new T)
+  case Case3 extends Foo(new T)
+}

tests/run/enums.scala

@@ -105,6 +105,7 @@ object Test6 {
     case Green  extends Color(3)
     case Red    extends Color(2)
     case Violet extends Color(Green.x + Red.x)
+    case RGB(xx: Int) extends Color(xx)
   }
 }