Fix #23223: Add handling for classes deriving from Capability during Setup

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

@@ -735,7 +735,12 @@ class Setup extends PreRecheck, SymTransformer, SetupAPI:
                   // Infer the self type for the rest, which is all classes without explicit
                   // self types (to which we also add nested module classes), provided they are
                   // neither pure, nor are publicily extensible with an unconstrained self type.
-                  CapturingType(cinfo.selfType, CaptureSet.Var(cls, level = ccState.currentLevel))
+                  val cs = CaptureSet.Var(cls, level = ccState.currentLevel)
+                  if cls.derivesFrom(defn.Caps_Capability) then
+                    // If cls is a capability class, we need to add a fresh readonly capability to
+                    // ensure we cannot treat the class as pure.
+                    CaptureSet.fresh(Origin.InDecl(cls)).readOnly.subCaptures(cs)
+                  CapturingType(cinfo.selfType, cs)
 
               // Compute new parent types
               val ps1 = inContext(ctx.withOwner(cls)):

tests/neg-custom-args/captures/i23223.scala

@@ -0,0 +1,12 @@
+import language.experimental.captureChecking
+import caps.*
+
+class A:
+  def a: A = this
+
+class B extends A, Capability // error
+
+def leak(b: B): A = b.a
+
+class C extends Capability:
+  def c: C^{} = this // error

tests/pos-custom-args/captures/i20237.scala

@@ -1,8 +1,12 @@
 import language.experimental.captureChecking
+import caps.*
 
 class Cap extends caps.Capability:
   def use[T](body: Cap ?=> T) = body(using this)
 
+class Cap2 extends caps.Capability:
+  def use[T](body: Cap2 => T) = body(this)
+
 class Box[T](body: Cap ?=> T):
   inline def open(using cap: Cap) = cap.use(body)