Disallow inheriting both SharedCapability and Mutable

Author: odersky

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

@@ -438,7 +438,7 @@ object Capabilities:
      */
     final def isExclusive(using Context): Boolean =
       !isReadOnly
-      && !classifier.isSubClass(defn.Caps_SharedCapability)
+      && !classifier.derivesFrom(defn.Caps_SharedCapability)
       && (isTerminalCapability || captureSetOfInfo.isExclusive)
 
     /** Similar to isExlusive, but also includes capabilties with capture

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

@@ -702,7 +702,7 @@ class CheckCaptures extends Recheck, SymTransformer:
      *  type `pt` to `ref`. Expand the marked tree accordingly to take account of
      *  the added path. Example:
      *  If we have `x` and the expected type says we select that with `.a.b`
-     *  where `b` is a read-only method, we charge `x.a.b.rd` for tree `x.a.b`
+     *  where `b` is a read-only method, we charge `x.a.rd` for tree `x.a.b`
      *  instead of just charging `x`.
      */
     private def markPathFree(ref: TermRef | ThisType, pt: Type, tree: Tree)(using Context): Unit = pt match

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

@@ -952,10 +952,12 @@ class Setup extends PreRecheck, SymTransformer, SetupAPI:
       case c :: cs1 =>
         for c1 <- cs1 do
           if !c.derivesFrom(c1) && !c1.derivesFrom(c) then
-            report.error(i"$cls inherits two unrelated classifier traits: $c and $c1", cls.srcPos)
+            report.error(em"$cls inherits two unrelated classifier traits: $c and $c1", cls.srcPos)
         recur(cs1)
       case Nil =>
     recur(cls.baseClasses.filter(_.isClassifiedCapabilityClass).distinct)
+    if cls.derivesFrom(defn.Caps_SharedCapability) && cls.derivesFrom(defn.Caps_Mutable) then
+      report.error(em"$cls cannot inheit from both SharaedCapability and Mutable", cls.srcPos)
 
   // ------ Checks to run after main capture checking --------------------------
 

tests/neg-custom-args/captures/classified-inheritance2.check

@@ -0,0 +1,4 @@
+-- Error: tests/neg-custom-args/captures/classified-inheritance2.scala:4:6 ---------------------------------------------
+4 |class Logger extends SharedCapability, Mutable:  // error (1) does this make sense?
+  |      ^
+  |      class Logger cannot inheit from both SharaedCapability and Mutable

tests/neg-custom-args/captures/classified-inheritance2.scala

@@ -0,0 +1,12 @@
+import language.experimental.captureChecking
+import language.experimental.separationChecking
+import caps.*
+class Logger extends SharedCapability, Mutable:  // error (1) does this make sense?
+  private var _state: Int = 0
+  update def log(msg: String): Unit = ()
+
+def onlyShared(x: Object^{cap.only[SharedCapability]}): Unit = ()
+
+def main(): Unit =
+  onlyShared(Logger())  // even if we allow (1), why would this type check?
+  val t: Logger^{} = Logger()  // and this type checks too, thus the above line I guess