Fixing #22005

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

@@ -101,7 +101,7 @@ trait CaptureRef extends TypeProxy, ValueType:
    *   TODO: Document path cases
    */
   final def subsumes(y: CaptureRef)(using Context): Boolean =
-
+    
     def subsumingRefs(x: Type, y: Type): Boolean = x match
       case x: CaptureRef => y match
         case y: CaptureRef => x.subsumes(y)
@@ -135,13 +135,21 @@ trait CaptureRef extends TypeProxy, ValueType:
               case _ => false
           || viaInfo(y.info)(subsumingRefs(this, _))
         case MaybeCapability(y1) => this.stripMaybe.subsumes(y1)
+        case y: TypeRef if y.symbol.info.derivesFrom(defn.Caps_CapSet) =>
+          y.info match
+            case _: TypeAlias => y.captureSetOfInfo.elems.exists(this.subsumes)
+            case TypeBounds(_, hi: CaptureRef) => this.subsumes(hi)
+            case _ => false
         case _ => false
     || this.match
         case ReachCapability(x1) => x1.subsumes(y.stripReach)
         case x: TermRef => viaInfo(x.info)(subsumingRefs(_, y))
         case x: TermParamRef => subsumesExistentially(x, y)
         case x: TypeRef if x.symbol.info.derivesFrom(defn.Caps_CapSet) =>
-          x.captureSetOfInfo.elems.exists(_.subsumes(y))
+          x.info match
+            case _: TypeAlias => x.captureSetOfInfo.elems.exists(_.subsumes(y))
+            case TypeBounds(lo: CaptureRef, _) => lo.subsumes(y)
+            case _ => false
         case x: TypeRef => assumedContainsOf(x).contains(y)
         case _ => false
   end subsumes

tests/neg-custom-args/captures/capture-poly.scala

@@ -3,7 +3,7 @@ import caps.*
 trait Foo extends Capability
 
 trait CaptureSet:
-  type C <: CapSet^
+  type C^
 
 def capturePoly[C^](a: Foo^{C^}): Foo^{C^} = a
 def capturePoly2(c: CaptureSet)(a: Foo^{c.C^}): Foo^{c.C^} = a

tests/neg-custom-args/captures/i22005.scala

@@ -0,0 +1,8 @@
+import caps.*
+
+class IO
+class File(io: IO^)
+
+class Handler[C^]:
+  def f(file: File^): File^{C^} = file // error
+  def g(file: File^{C^}): File^ = file // ok