Fix dcs for invariant type parameters

Also: add test that reach capabilities are contained inside boxes

Author: odersky

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

@@ -1116,13 +1116,15 @@ object CaptureSet:
   /** The deep capture set of a type is the union of all covariant occurrences of
    *  capture sets. Nested existential sets are approximated with `cap`.
    *  NOTE: The traversal logic needs to be in sync with narrowCaps in CaptureOps, which
-   *  replaces caps with reach capabilties.
+   *  replaces caps with reach capabilties. The one exception to this is invariant
+   *  arguments. This have to be included to be conservative in dcs but must be
+   *  excluded in narrowCaps.
    */
   def ofTypeDeeply(tp: Type)(using Context): CaptureSet =
     val collect = new TypeAccumulator[CaptureSet]:
       val seen = util.HashSet[Symbol]()
       def apply(cs: CaptureSet, t: Type) =
-        if variance <= 0 then cs
+        if variance < 0 then cs
         else t.dealias match
           case t @ CapturingType(p, cs1) =>
             this(cs, p) ++ cs1

tests/pos-custom-args/captures/boxed-use.scala

@@ -0,0 +1,17 @@
+class Box[A](val elem: A)
+class CoBox[+A](val elem: A)
+
+def applyAll[A](fs: Box[A => Unit], x: A): Box[() ->{fs*} Unit] =
+  Box(() => fs.elem(x))
+
+def applyAllCo[A](fs: CoBox[A => Unit], x: A): CoBox[() ->{fs*} Unit] =
+  CoBox(() => fs.elem(x))
+
+// Same with inferred result types
+def test =
+  def applyAll[A](fs: Box[A => Unit], x: A) =
+    Box(() => fs.elem(x))
+
+  def applyAllCo[A](fs: CoBox[A => Unit], x: A) =
+    CoBox(() => fs.elem(x))
+