Classify SharedCapabilities as non-exclusive

Author: odersky

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

@@ -434,10 +434,12 @@ object Capabilities:
 
     /** An exclusive capability is a capability that derives
      *  indirectly from a maximal capability without going through
-     *  a read-only capability first.
+     *  a read-only capability or a capability classified as SharedCapability first.
      */
     final def isExclusive(using Context): Boolean =
-      !isReadOnly && (isTerminalCapability || captureSetOfInfo.isExclusive)
+      !isReadOnly
+      && !classifier.isSubClass(defn.Caps_SharedCapability)
+      && (isTerminalCapability || captureSetOfInfo.isExclusive)
 
     /** Similar to isExlusive, but also includes capabilties with capture
      *  set variables in their info whose status is still open.

tests/pos-custom-args/captures/mutable-capturing-sharable.scala

@@ -0,0 +1,22 @@
+import language.experimental.captureChecking
+import language.experimental.separationChecking
+import caps.*
+class Logger extends SharedCapability:
+  def log(msg: String): Unit = ()
+class TracedRef(logger: Logger^{cap.only[SharedCapability]}) extends Mutable:
+  private var _data: Int = 0
+  update def set(newValue: Int): Unit =
+    logger.log("set")
+    _data = newValue
+  def get: Int =
+    logger.log("get")  // error, but should it be allowed?
+    _data
+
+class TracedRef2(logger: Logger) extends Mutable:
+  private var _data: Int = 0
+  update def set(newValue: Int): Unit =
+    logger.log("set")
+    _data = newValue
+  def get: Int =
+    logger.log("get")  // error, but should it be allowed?
+    _data