Make caps.cap a given for caps.Root

Author: odersky

library/src/scala/caps.scala

@@ -13,6 +13,8 @@ import annotation.experimental
   /** The universal capture reference */
   val cap: Root = ()
 
+  given Root = cap
+
   object unsafe:
 
     extension [T](x: T)

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

@@ -1,8 +1,10 @@
--- Error: tests/neg-custom-args/captures/levels.scala:15:11 ------------------------------------------------------------
+-- [E007] Type Mismatch Error: tests/neg-custom-args/captures/levels.scala:15:11 ---------------------------------------
 15 |    r.setV(g) // error
    |           ^
-   |           reference (cap3 : CC^) is not included in the allowed capture set ?
-   |           of an enclosing function literal with expected type box (x$0: String) ->? String
+   |           Found:    box (x: String) ->{cap3} String
+   |           Required: box (x$0: String) ->? String
    |
    |           Note that reference (cap3 : CC^), defined at level 2
    |           cannot be included in outer capture set ?, defined at level 1 in method test
+   |
+   | longer explanation available when compiling with `-explain`

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

@@ -1,26 +1,50 @@
--- Error: tests/neg-custom-args/captures/vars.scala:13:6 ---------------------------------------------------------------
-13 |  var a: String => String = f // error
-   |  ^^^^^^^^^^^^^^^^^^^^^^^^^^^
-   |  Mutable variable a cannot have type box String => String since
-   |  that type captures the root capability `cap`.
-   |  This restriction serves to prevent local capabilities from escaping the scope where they are defined.
--- Error: tests/neg-custom-args/captures/vars.scala:14:6 ---------------------------------------------------------------
-14 |  var b: List[String => String] = Nil // error
-   |  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-   |  Mutable variable b cannot have type List[String => String] since
-   |  the part String => String of that type captures the root capability `cap`.
-   |  This restriction serves to prevent local capabilities from escaping the scope where they are defined.
 -- [E007] Type Mismatch Error: tests/neg-custom-args/captures/vars.scala:11:24 -----------------------------------------
 11 |  val z2c: () -> Unit = z2  // error
    |                        ^^
    |                        Found:    () ->{z2} Unit
    |                        Required: () -> Unit
    |
    | longer explanation available when compiling with `-explain`
--- Error: tests/neg-custom-args/captures/vars.scala:32:2 ---------------------------------------------------------------
-32 |  local { cap3 => // error
+-- Error: tests/neg-custom-args/captures/vars.scala:23:14 --------------------------------------------------------------
+23 |    a = x => g(x)      // error
+   |             ^^^^
+   |             reference (cap3 : CC^) is not included in the allowed capture set {cap[test]}
+   |             of an enclosing function literal with expected type box String ->{cap[test]} String
+-- [E007] Type Mismatch Error: tests/neg-custom-args/captures/vars.scala:24:8 ------------------------------------------
+24 |    a = g      // error
+   |        ^
+   |        Found:    box (x: String) ->{cap3} String
+   |        Required: box (x$0: String) ->{cap[test]} String
+   |
+   | longer explanation available when compiling with `-explain`
+-- [E007] Type Mismatch Error: tests/neg-custom-args/captures/vars.scala:26:12 -----------------------------------------
+26 |    b = List(g) // error
+   |        ^^^^^^^
+   |        Found:    List[box (x$0: String) ->{cap3} String]
+   |        Required: List[String ->{cap[test]} String]
+   |
+   | longer explanation available when compiling with `-explain`
+-- [E007] Type Mismatch Error: tests/neg-custom-args/captures/vars.scala:30:10 -----------------------------------------
+30 |  val s = scope // error (but should be OK, we need to allow poly-captures)
+   |          ^^^^^
+   |          Found:    (x$0: String) ->{cap[scope]} String
+   |          Required: (x$0: String) ->? String
+   |
+   |          Note that reference (cap[scope] : caps.Root), defined at level 2
+   |          cannot be included in outer capture set ?, defined at level 1 in method test
+   |
+   | longer explanation available when compiling with `-explain`
+-- [E007] Type Mismatch Error: tests/neg-custom-args/captures/vars.scala:31:29 -----------------------------------------
+31 |  val sc: String => String = scope // error (but should also be OK)
+   |                             ^^^^^
+   |                             Found:    (x$0: String) ->{cap[scope]} String
+   |                             Required: String => String
+   |
+   |                             Note that reference (cap[scope] : caps.Root), defined at level 2
+   |                             cannot be included in outer capture set ?, defined at level 1 in method test
+   |
+   | longer explanation available when compiling with `-explain`
+-- Error: tests/neg-custom-args/captures/vars.scala:35:2 ---------------------------------------------------------------
+35 |  local { root => cap3 => // error
    |  ^^^^^
-   |  Sealed type variable T cannot be instantiated to box (x$0: String) => String since
-   |  that type captures the root capability `cap`.
-   |  This is often caused by a local capability in an argument of method local
-   |  leaking as part of its result.
+   |  escaping local reference root.type