Drop syntax sugar for capture set member

Author: noti0na1

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

@@ -143,6 +143,9 @@ trait CaptureRef extends TypeProxy, ValueType:
           || viaInfo(y.info)(subsumingRefs(this, _))
         case MaybeCapability(y1) => this.stripMaybe.subsumes(y1)
         case y: TypeRef if y.derivesFrom(defn.Caps_CapSet) =>
+          // The upper and lower bounds don't have to be in the form of `CapSet^{...}`.
+          // They can be other capture set variables, which are bounded by `CapSet`,
+          // like `def test[X^, Y^, Z >: X <: Y]`.
           y.info match
             case TypeBounds(_, hi: CaptureRef) => this.subsumes(hi)
             case _ => y.captureSetOfInfo.elems.forall(this.subsumes)

compiler/src/dotty/tools/dotc/parsing/Parsers.scala

@@ -4057,11 +4057,8 @@ object Parsers {
               || sourceVersion.isAtLeast(`3.6`) && in.isColon =>
             makeTypeDef(typeAndCtxBounds(tname))
           case _ =>
-            if in.isIdent(nme.UPARROW) && Feature.ccEnabled then
-              makeTypeDef(typeAndCtxBounds(tname))
-            else
-              syntaxErrorOrIncomplete(ExpectedTypeBoundOrEquals(in.token))
-              return EmptyTree // return to avoid setting the span to EmptyTree
+            syntaxErrorOrIncomplete(ExpectedTypeBoundOrEquals(in.token))
+            return EmptyTree // return to avoid setting the span to EmptyTree
         }
       }
     }

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

@@ -3,7 +3,7 @@ import caps.*
 trait Foo extends Capability
 
 trait CaptureSet:
-  type C^
+  type C >: CapSet <: CapSet^
 
 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/i21868.scala

@@ -5,9 +5,9 @@ trait AbstractWrong:
   def f(): Unit^{C^} // error
 
 trait Abstract1:
-  type C^
+  type C >: CapSet <: CapSet^
   def f(): Unit^{C^}
 
-class Abstract2:
-  type C >: CapSet <: CapSet^
-  def f(): Unit^{C^}
+// class Abstract2:
+//   type C^
+//   def f(): Unit^{C^}