Test for #16415

Author: odersky

tests/pos-custom-args/captures/i16415.scala

@@ -0,0 +1,8 @@
+abstract class A[X]:
+  def foo(x: X): X
+
+class IO
+class C
+def test(io: IO^) =
+  class B extends A[C^{io}]:    // error, but should work
+    override def foo(x: C^{io}): C^{io} = ???