Add List encoding test

Author: odersky

compiler/src/dotty/tools/dotc/core/CaptureSet.scala

@@ -85,7 +85,7 @@ object CaptureSet:
     val collect = new TypeAccumulator[Refs]:
       var localBinders: SimpleIdentitySet[BindingType] = SimpleIdentitySet.empty
       var seenLazyRefs: SimpleIdentitySet[LazyRef] = SimpleIdentitySet.empty
-      def apply(elems: Refs, tp: Type): Refs = trace(i"capt $elems, $tp", show = true) {
+      def apply(elems: Refs, tp: Type): Refs = trace(i"capt $elems, $tp", capt, show = true) {
         tp match
           case tp: NamedType =>
             if variance < 0 then elems

tests/pos-special/captures/list-encoding.scala

@@ -0,0 +1,22 @@
+type Top = Any retains *
+class Cap extends Retains[*]
+
+type Op[T <: Top, C <: Top] =
+  ((v: T) => ((s: C) => C) retains *) retains *
+
+type List[T <: Top] =
+  [C <: Top] => (op: Op[T, C]) => ((s: C) => C) retains op.type
+
+def nil[T <: Top]: List[T] =
+  [C <: Top] => (op: Op[T, C]) => (s: C) => s
+
+def cons[T <: Top](hd: T, tl: List[T]): List[T] =
+  [C <: Top] => (op: Op[T, C]) => (s: C) => op(hd)(tl(op)(s))
+
+def foo(c: Cap) =
+  def f(x: String retains c.type, y: String retains c.type) =
+    cons(x, cons(y, nil))
+  def g(x: String retains c.type, y: Any) =
+    cons(x, cons(y, nil))
+  def h(x: String, y: Any retains c.type) =
+    cons(x, cons(y, nil))