Support cases with type parameters that extend a non-parameterized base

Support cases with type parameters that implicitly extend a non-parameterized base
without needing their own extends clause. The proposal has been updated to make clear
that this is supported.

Also address other reviewers comments.

Author: odersky

Diff for: compiler/src/dotty/tools/dotc/ast/Desugar.scala

@@ -296,11 +296,16 @@ object desugar {
     val isValueClass = parents.nonEmpty && isAnyVal(parents.head)
       // This is not watertight, but `extends AnyVal` will be replaced by `inline` later.
 
+    lazy val reconstitutedTypeParams = reconstitutedEnumTypeParams(cdef.pos.startPos)
+
     val originalTparams =
       if (isEnumCase && parents.isEmpty) {
-        if (constr1.tparams.nonEmpty)
-          ctx.error(em"case with type parameters needs extends clause", constr1.tparams.head.pos)
-        reconstitutedEnumTypeParams(cdef.pos.startPos)
+        if (constr1.tparams.nonEmpty) {
+          if (reconstitutedTypeParams.nonEmpty)
+            ctx.error(em"case with type parameters needs extends clause", constr1.tparams.head.pos)
+          constr1.tparams
+        }
+        else reconstitutedTypeParams
       }
       else constr1.tparams
     val originalVparamss = constr1.vparamss
@@ -339,7 +344,9 @@ object desugar {
     // a reference to the class type bound by `cdef`, with type parameters coming from the constructor
     val classTypeRef = appliedRef(classTycon)
     // a reference to `enumClass`, with type parameters coming from the constructor
-    lazy val enumClassTypeRef = appliedRef(enumClassRef)
+    lazy val enumClassTypeRef =
+      if (reconstitutedTypeParams.isEmpty) enumClassRef
+      else appliedRef(enumClassRef)
 
     // new C[Ts](paramss)
     lazy val creatorExpr = New(classTypeRef, constrVparamss nestedMap refOfDef)
@@ -516,10 +523,7 @@ object desugar {
       case _ =>
     }
 
-    val result = flatTree(cdef1 :: companions ::: implicitWrappers)
-    //if (isEnum) println(i"enum $cdef\n --->\n$result")
-    //if (isEnumCase) println(i"enum case $cdef\n --->\n$result")
-    result
+    flatTree(cdef1 :: companions ::: implicitWrappers)
   }
 
   val AccessOrSynthetic = AccessFlags | Synthetic

Diff for: compiler/src/dotty/tools/dotc/ast/DesugarEnums.scala

@@ -14,14 +14,17 @@ object DesugarEnums {
   import untpd._
   import desugar.DerivedFromParamTree
 
-  val EnumCaseCount = new Property.Key[Int]
+  /** Attachment containing: The number of enum cases seen so far, and whether a
+   *  simple enum case was already seen.
+   */
+  val EnumCaseCount = new Property.Key[(Int, Boolean)]
 
   def enumClass(implicit ctx: Context) = ctx.owner.linkedClass
 
-  def nextEnumTag(implicit ctx: Context): Int = {
-    val result = ctx.tree.removeAttachment(EnumCaseCount).getOrElse(0)
-    ctx.tree.pushAttachment(EnumCaseCount, result + 1)
-    result
+  def nextEnumTag(isSimpleCase: Boolean)(implicit ctx: Context): (Int, Boolean) = {
+    val (count, simpleSeen) = ctx.tree.removeAttachment(EnumCaseCount).getOrElse((0, false))
+    ctx.tree.pushAttachment(EnumCaseCount, (count + 1, simpleSeen | isSimpleCase))
+    (count, simpleSeen)
   }
 
   def isLegalEnumCase(tree: MemberDef)(implicit ctx: Context): Boolean =
@@ -54,7 +57,8 @@ object DesugarEnums {
   }
 
   def enumTagMeth(implicit ctx: Context) =
-    DefDef(nme.enumTag, Nil, Nil, TypeTree(), Literal(Constant(nextEnumTag)))
+    DefDef(nme.enumTag, Nil, Nil, TypeTree(),
+        Literal(Constant(nextEnumTag(isSimpleCase = false)._1)))
 
   def enumClassRef(implicit ctx: Context) = TypeTree(enumClass.typeRef)
 
@@ -120,8 +124,8 @@ object DesugarEnums {
   def expandSimpleEnumCase(name: TermName, mods: Modifiers, pos: Position)(implicit ctx: Context): Tree = {
     if (reconstitutedEnumTypeParams(pos).nonEmpty)
       ctx.error(i"illegal enum value of generic $enumClass: an explicit `extends' clause is needed", pos)
-    val tag = nextEnumTag
-    val prefix = if (tag == 0) enumScaffolding else Nil
+    val (tag, simpleSeen) = nextEnumTag(isSimpleCase = true)
+    val prefix = if (simpleSeen) Nil else enumScaffolding
     val creator = Apply(Ident(nme.DOLLAR_NEW), List(Literal(Constant(tag)), Literal(Constant(name.toString))))
     val vdef = ValDef(name, enumClassRef, creator).withMods(mods | Final).withPos(pos)
     flatTree(prefix ::: vdef :: Nil).withPos(pos.startPos)

Diff for: tests/run/enum-HList.scala

@@ -0,0 +1,22 @@
+enum HLst {
+  case HCons[+Hd, +Tl <: HLst](hd: Hd, tl: Tl)
+  case HNil
+}
+
+object Test {
+  import HLst._
+  def length(hl: HLst): Int = hl match {
+    case HCons(_, tl) => 1 + length(tl)
+    case HNil => 0
+  }
+  def sumInts(hl: HLst): Int = hl match {
+    case HCons(x: Int, tl) => x + sumInts(tl)
+    case HCons(_, tl) => sumInts(tl)
+    case HNil => 0
+  }
+  def main(args: Array[String]) = {
+    val hl = HCons(1, HCons("A", HNil))
+    assert(length(hl) == 2, length(hl))
+    assert(sumInts(hl) == 1)
+  }
+}