Turn off withCapAsRoot exception for box adaptation

TODO: Check that the new error in box-adapt-contra makes sense
TODO: error messages need to be improved

Author: odersky

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

@@ -1453,7 +1453,7 @@ class CheckCaptures extends Recheck, SymTransformer:
           val cs = actual.captureSet
           if covariant then cs ++ leaked
           else
-            if CCState.withCapAsRoot: // Not sure this is OK, actually
+            if // CCState.withCapAsRoot: // Not sure withCapAsRoot is OK here, actually
               !leaked.subCaptures(cs).isOK
             then
               report.error(

tests/neg-custom-args/captures/box-adapt-contra.check

@@ -0,0 +1,15 @@
+-- Error: tests/neg-custom-args/captures/box-adapt-contra.scala:9:52 ---------------------------------------------------
+9 |  val f: (Cap^{c} -> Unit) -> Unit = useCap[Cap^{c}](c)  // error
+  |                                     ^^^^^^^^^^^^^^^^^^
+  |           Cap^{c} -> Unit cannot be box-converted to box Cap^{c} ->{c} Unit
+  |           since the additional capture set {c} resulted from box conversion is not allowed in box Cap^{c} -> Unit
+-- Error: tests/neg-custom-args/captures/box-adapt-contra.scala:13:57 --------------------------------------------------
+13 |  val f1: (Cap^{c} => Unit) ->{c} Unit = useCap1[Cap^{c}](c)  // error, was ok when cap was a root
+   |                                         ^^^^^^^^^^^^^^^^^^^
+   |         Cap^{c} => Unit cannot be box-converted to box Cap^{c} ->{cap, c} Unit
+   |         since the additional capture set {c} resulted from box conversion is not allowed in box Cap^{c} => Unit
+-- Error: tests/neg-custom-args/captures/box-adapt-contra.scala:19:54 --------------------------------------------------
+19 |  val f3: (Cap^{c} -> Unit) => Unit = useCap3[Cap^{c}](c)  // error
+   |                                      ^^^^^^^^^^^^^^^^^^^
+   |      Cap^{c} -> Unit cannot be box-converted to box Cap^{c} ->{d, c} Unit
+   |      since the additional capture set {c} resulted from box conversion is not allowed in box Cap^{c} ->{d} Unit

tests/neg-custom-args/captures/box-adapt-contra.scala

@@ -10,7 +10,7 @@ def test1(c: Cap^): Unit =
 
 def test2(@consume c: Cap^, d: Cap^): Unit =
   def useCap1[X](x: X): (X => Unit) -> Unit = ???
-  val f1: (Cap^{c} => Unit) ->{c} Unit = useCap1[Cap^{c}](c)  // ok
+  val f1: (Cap^{c} => Unit) ->{c} Unit = useCap1[Cap^{c}](c)  // error, was ok when cap was a root
 
   def useCap2[X](x: X): (X ->{c} Unit) -> Unit = ???
   val f2: (Cap^{c} -> Unit) ->{c} Unit = useCap2[Cap^{c}](c)  // ok

tests/neg-custom-args/captures/i15772.check

@@ -3,25 +3,21 @@
    |                          ^
    |                          reference (x : C^) is not included in the allowed capture set {}
    |                          of an enclosing function literal with expected type () -> Int
--- [E007] Type Mismatch Error: tests/neg-custom-args/captures/i15772.scala:22:46 ---------------------------------------
+-- Error: tests/neg-custom-args/captures/i15772.scala:22:46 ------------------------------------------------------------
 22 |    val boxed1 : ((C^) => Unit) -> Unit = box1(c)  // error
    |                                          ^^^^^^^
-   |                                          Found:    (C{val arg: C^}^{c} => Unit) ->{c} Unit
-   |                                          Required: (C^ => Unit) -> Unit
-   |
-   | longer explanation available when compiling with `-explain`
+   |C^ => Unit cannot be box-converted to box C{val arg: C^}^{c} ->{cap, c} Unit
+   |since the additional capture set {c} resulted from box conversion is not allowed in box C{val arg: C^}^{c} => Unit
 -- Error: tests/neg-custom-args/captures/i15772.scala:28:26 ------------------------------------------------------------
 28 |    val c : C^{x} = new C(x) // error
    |                          ^
    |                          reference (x : C^) is not included in the allowed capture set {}
    |                          of an enclosing function literal with expected type () -> Int
--- [E007] Type Mismatch Error: tests/neg-custom-args/captures/i15772.scala:29:35 ---------------------------------------
+-- Error: tests/neg-custom-args/captures/i15772.scala:29:35 ------------------------------------------------------------
 29 |    val boxed2 : Observe[C^] = box2(c)  // error
    |                               ^^^^^^^
-   |                               Found:    (C{val arg: C^}^{c} => Unit) ->{c} Unit
-   |                               Required: (C^ => Unit) -> Unit
-   |
-   | longer explanation available when compiling with `-explain`
+   |C^ => Unit cannot be box-converted to box C{val arg: C^}^{c} ->{cap, c} Unit
+   |since the additional capture set {c} resulted from box conversion is not allowed in box C{val arg: C^}^{c} => Unit
 -- [E007] Type Mismatch Error: tests/neg-custom-args/captures/i15772.scala:35:34 ---------------------------------------
 35 |  val boxed2 : Observe[C]^ = box2(c) // error
    |                                  ^