Disabllow type parameters in OnlyCapability annotations

Author: noti0na1

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

@@ -922,8 +922,12 @@ object OnlyCapability:
 
   def unapply(tree: AnnotatedType)(using Context): Option[(Type, ClassSymbol)] = tree match
     case AnnotatedType(parent: Type, ann) if ann.hasSymbol(defn.OnlyCapabilityAnnot) =>
-      ann.tree.tpe.argTypes.head.classSymbol match
-        case cls: ClassSymbol => Some((parent, cls))
+      val arg = ann.tree.tpe.argTypes.head
+      arg match
+        case tp: TypeRef if tp.symbol.isClass =>
+          tp.symbol match
+            case cls: ClassSymbol => Some((parent, cls))
+            case _ => None
         case _ => None
     case _ => None
 end OnlyCapability

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

@@ -0,0 +1,12 @@
+-- Error: tests/neg-custom-args/captures/classified-tp.scala:10:17 -----------------------------------------------------
+10 |  val c: AnyRef^{any.only[C]} = new AnyRef // error
+   |                 ^^^^^^^^^^^
+   |                 scala.caps.any.type @onlyCapability[C] is not a legal element of a capture set
+   |
+   |                 where:    C is a type in method test with bounds <: B
+-- Error: tests/neg-custom-args/captures/classified-tp.scala:11:17 -----------------------------------------------------
+11 |  val d: AnyRef^{any.only[D]} = new AnyRef // error
+   |                 ^^^^^^^^^^^
+   |                 scala.caps.any.type @onlyCapability[D] is not a legal element of a capture set
+   |
+   |                 where:    D is a type in method test with bounds <: scala.caps.Classifier

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

@@ -0,0 +1,11 @@
+import language.experimental.captureChecking
+import caps.*
+
+trait A extends Classifier, SharedCapability
+trait B extends A, Classifier
+
+def test[C <: B, D <: Classifier] =
+  val a: AnyRef^{any.only[A]} = new AnyRef
+  val b: AnyRef^{any.only[B]} = new AnyRef
+  val c: AnyRef^{any.only[C]} = new AnyRef // error
+  val d: AnyRef^{any.only[D]} = new AnyRef // error