Add boxmap test from paper

Author: odersky

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

@@ -80,7 +80,7 @@ class CheckCaptures extends Recheck:
     import ast.tpd.*
 
     override def reinfer(tp: Type)(using Context): Type =
-      CapturingType(tp, CaptureSet.Var())
+      CapturingType(tp, CaptureSet.Var()) // ^^^ go deep
 
     private var curEnv: Env = Env(NoSymbol, CaptureSet.empty, false, null)
 

tests/pos-custom-args/captures/boxmap-paper.scala

@@ -0,0 +1,38 @@
+infix type ==> [A, B] = (A => B) retains *
+
+type Cell[+T] = [K] => (T ==> K) => K
+
+def cell[T](x: T): Cell[T] =
+  [K] => (k: T ==> K) => k(x)
+
+def get[T](c: Cell[T]): T = c[T](identity[T]) // TODO: drop [T]
+
+def map[A, B](c: Cell[A])(f: A ==> B): Cell[B]
+  = c[Cell[B]]((x: A) => cell(f(x)))
+
+def pureMap[A, B](c: Cell[A])(f: A => B): Cell[B]
+  = c[Cell[B]]((x: A) => cell(f(x)))
+
+def lazyMap[A, B](c: Cell[A])(f: A ==> B): {f} () => Cell[B]
+  = () => c[Cell[B]]((x: A) => cell(f(x)))
+
+trait IO:
+  def print(s: String): Unit
+
+def test(io: {*} IO) =
+
+  val loggedOne: {io} () => Int = () => { io.print("1"); 1 }
+
+  val c: Cell[{io} () => Int]
+      = cell[{io} () => Int](loggedOne)
+
+  val g = (f: {io} () => Int) =>
+    val x = f(); io.print(" + ")
+    val y = f(); io.print(s" = ${x + y}")
+
+  val r = lazyMap[{io} () => Int, Unit](c)(f => g(f))
+  val r2 = lazyMap[{io} () => Int, Unit](c)(g)
+  //  val r3 = lazyMap(c)(g)  not yet
+  val _ = r()
+  val _ = r2()
+  // val _ = r3()