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.

1 files changed, 22 insertions, 0 deletions

diff --git a/tests/run/enum-HList.scala b/tests/run/enum-HList.scala
new file mode 100644
index 000000000..c019cb6cc
--- /dev/null
+++ b/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)
+  }
+}