Add Id example to reaches neg test

Author: odersky

tests/neg-custom-args/captures/reaches.check

@@ -0,0 +1,36 @@
+-- [E007] Type Mismatch Error: tests/neg-custom-args/captures/reaches.scala:22:11 --------------------------------------
+22 |    cur = (() => f.write()) :: Nil // error since {f*} !<: {xs*}
+   |           ^^^^^^^^^^^^^^^^^^^^^^^
+   |           Found:    List[box () ->{f} Unit]
+   |           Required: List[box () ->{xs*} Unit]
+   |
+   | longer explanation available when compiling with `-explain`
+-- [E007] Type Mismatch Error: tests/neg-custom-args/captures/reaches.scala:33:7 ---------------------------------------
+33 |      (() => f.write()) :: Nil // error since {f*} !<: {xs*}
+   |       ^^^^^^^^^^^^^^^^^^^^^^^
+   |       Found:    List[box () ->{f} Unit]
+   |       Required: box List[box () ->{xs*} Unit]^?
+   |
+   |       Note that reference (f : File^), defined in method $anonfun
+   |       cannot be included in outer capture set {xs*} of value cur which is associated with method runAll1
+   |
+   | longer explanation available when compiling with `-explain`
+-- Error: tests/neg-custom-args/captures/reaches.scala:36:6 ------------------------------------------------------------
+36 |  var cur: List[Proc] = xs // error: Illegal type for var
+   |      ^
+   |      Mutable variable cur cannot have type List[box () => Unit] since
+   |      the part box () => Unit of that type captures the root capability `cap`.
+-- Error: tests/neg-custom-args/captures/reaches.scala:43:15 -----------------------------------------------------------
+43 |  val cur = Ref[List[Proc]](xs) // error: illegal type for type argument to Ref
+   |            ^^^^^^^^^^^^^^^
+   |            Sealed type variable T cannot be instantiated to List[box () => Unit] since
+   |            the part box () => Unit of that type captures the root capability `cap`.
+   |            This is often caused by a local capability in an argument of constructor Ref
+   |            leaking as part of its result.
+-- Error: tests/neg-custom-args/captures/reaches.scala:53:31 -----------------------------------------------------------
+53 |  val id: Id[Proc, Proc] = new Id[Proc, () -> Unit] // error
+   |                               ^^^^^^^^^^^^^^^^^^^^
+   |                               Sealed type variable A cannot be instantiated to box () => Unit since
+   |                               that type captures the root capability `cap`.
+   |                               This is often caused by a local capability in an argument of constructor Id
+   |                               leaking as part of its result.

tests/neg-custom-args/captures/reaches.scala

@@ -46,3 +46,10 @@ def runAll3(xs: List[Proc]): Unit =
     next()
     cur.set(cur.get.tail: List[Proc])
 
+class Id[sealed -A, sealed +B >: A]():
+  def apply(a: A): B = a
+
+def test =
+  val id: Id[Proc, Proc] = new Id[Proc, () -> Unit] // error
+  usingFile: f =>
+    id(() => f.write())  // escape, if it was not for the error above