Charge also dcs of local reaches to capture set of enclosing method

Fixes #21442

Author: odersky

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

@@ -394,7 +394,7 @@ class CheckCaptures extends Recheck, SymTransformer:
                 val isVisible = isVisibleFromEnv(refOwner)
                 if !isVisible
                     && (c.isReach || ref.isType)
-                    && refSym.is(Param)
+                    && (!ccConfig.useSealed || refSym.is(Param))
                     && refOwner == env.owner
                 then
                   if refSym.hasAnnotation(defn.UnboxAnnot) then

tests/neg-custom-args/captures/i21442.check

@@ -0,0 +1,8 @@
+-- Error: tests/neg-custom-args/captures/i21442.scala:9:13 -------------------------------------------------------------
+9 |  val io = x.unbox // error: local reach capability {x*} leaks
+  |           ^^^^^^^
+  |           Local reach capability x* leaks into capture scope of method foo
+-- Error: tests/neg-custom-args/captures/i21442.scala:17:14 ------------------------------------------------------------
+17 |  val io = x1.unbox // error
+   |           ^^^^^^^^
+   |           Local reach capability x1* leaks into capture scope of method bar

tests/neg-custom-args/captures/i21442.scala

@@ -0,0 +1,18 @@
+import language.experimental.captureChecking
+trait IO:
+  def use(): Unit
+case class Boxed[+T](unbox: T)
+
+// `foo` is a function that unboxes its parameter
+// and uses the capability boxed inside the parameter.
+def foo(x: Boxed[IO^]): Unit =
+  val io = x.unbox // error: local reach capability {x*} leaks
+  io.use()
+
+// `bar` is a function that does the same thing in a
+// slightly different way.
+// But, no type error reported.
+def bar(x: Boxed[IO^]): Unit =
+  val x1: Boxed[IO^] = x
+  val io = x1.unbox // error
+  io.use()

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

@@ -25,18 +25,10 @@
    |                           ^^^^^^^^^^^^
    |                       The expression's type box () => Unit is not allowed to capture the root capability `cap`.
    |                       This usually means that a capability persists longer than its allowed lifetime.
--- [E007] Type Mismatch Error: tests/neg-custom-args/captures/reaches.scala:53:2 ---------------------------------------
-53 |  val id: Id[Proc, Proc] = new Id[Proc, () -> Unit] // error
-   |  ^
-   |  Found:    box () => Unit
-   |  Required: () => Unit
-   |
-   |  Note that box () => Unit cannot be box-converted to () => Unit
-   |  since at least one of their capture sets contains the root capability `cap`
-54 |  usingFile: f =>
-55 |    id(() => f.write())
-   |
-   | longer explanation available when compiling with `-explain`
+-- Error: tests/neg-custom-args/captures/reaches.scala:55:6 ------------------------------------------------------------
+55 |    id(() => f.write()) // error
+   |    ^^^^^^^^^^^^^^^^^^^
+   |    Local reach capability id* leaks into capture scope of method test
 -- [E007] Type Mismatch Error: tests/neg-custom-args/captures/reaches.scala:62:27 --------------------------------------
 62 |    val f1: File^{id*} = id(f) // error, since now id(f): File^
    |                         ^^^^^
@@ -52,3 +44,9 @@
 79 |  ps.map((x, y) => compose1(x, y)) // error // error
    |             ^
    |             Local reach capability ps* leaks into capture scope of method mapCompose
+-- [E057] Type Mismatch Error: tests/neg-custom-args/captures/reaches.scala:53:51 --------------------------------------
+53 |  val id: Id[Proc, Proc] = new Id[Proc, () -> Unit] // error
+   |                                                   ^
+   |                                             Type argument () -> Unit does not conform to lower bound () => Unit
+   |
+   | longer explanation available when compiling with `-explain`

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

@@ -52,7 +52,7 @@ class Id[-A,  +B >: A]():
 def test =
   val id: Id[Proc, Proc] = new Id[Proc, () -> Unit] // error
   usingFile: f =>
-    id(() => f.write())
+    id(() => f.write()) // error
 
 def attack2 =
   val id: File^ -> File^ = x => x

tests/neg-custom-args/captures/unsound-reach.check

@@ -1,3 +1,8 @@
+-- Error: tests/neg-custom-args/captures/unsound-reach.scala:18:21 -----------------------------------------------------
+18 |        boom.use(f): (f1: File^{backdoor*}) => // error
+   |                     ^
+   |                     Local reach capability backdoor* leaks into capture scope of method bad
+19 |            escaped = f1
 -- [E164] Declaration Error: tests/neg-custom-args/captures/unsound-reach.scala:10:8 -----------------------------------
 10 |    def use(x: File^)(op: File^ => Unit): Unit = op(x) // error, was OK using sealed checking
    |        ^

tests/neg-custom-args/captures/unsound-reach.scala

@@ -15,6 +15,6 @@ def bad(): Unit =
 
     var escaped: File^{backdoor*} = null
     withFile("hello.txt"): f =>
-        boom.use(f): (f1: File^{backdoor*}) => // was error before existentials
+        boom.use(f): (f1: File^{backdoor*}) => // error
             escaped = f1