diff --git a/compiler/src/dotty/tools/dotc/cc/CheckCaptures.scala b/compiler/src/dotty/tools/dotc/cc/CheckCaptures.scala
index 74279197d45d..714f769dacd9 100644
--- a/compiler/src/dotty/tools/dotc/cc/CheckCaptures.scala
+++ b/compiler/src/dotty/tools/dotc/cc/CheckCaptures.scala
@@ -738,20 +738,20 @@ class CheckCaptures extends Recheck, SymTransformer:
        *  the innermost capturing type. The outer capture annotations can be
        *  reconstructed with the returned function.
        */
-      def destructCapturingType(tp: Type, reconstruct: Type => Type = x => x): ((Type, CaptureSet, Boolean), Type => Type) =
+      def destructCapturingType(tp: Type, reconstruct: Type => Type = x => x): (Type, CaptureSet, Boolean, Type => Type) =
         tp.dealias match
           case tp @ CapturingType(parent, cs) =>
             if parent.dealias.isCapturingType then
               destructCapturingType(parent, res => reconstruct(tp.derivedCapturingType(res, cs)))
             else
-              ((parent, cs, tp.isBoxed), reconstruct)
+              (parent, cs, tp.isBoxed, reconstruct)
           case actual =>
-            ((actual, CaptureSet(), false), reconstruct)
+            (actual, CaptureSet(), false, reconstruct)
 
       def adapt(actual: Type, expected: Type, covariant: Boolean): Type = trace(adaptInfo(actual, expected, covariant), recheckr, show = true) {
         if expected.isInstanceOf[WildcardType] then actual
         else
-          val ((parent, cs, actualIsBoxed), recon) = destructCapturingType(actual)
+          val (parent, cs, actualIsBoxed, recon) = destructCapturingType(actual)
 
           val needsAdaptation = actualIsBoxed != expected.isBoxedCapturing
           val insertBox = needsAdaptation && covariant != actualIsBoxed
diff --git a/tests/neg-custom-args/captures/PENDING-box-override.scala b/tests/neg-custom-args/captures/PENDING-box-override.scala
new file mode 100644
index 000000000000..efb7bfaea1ac
--- /dev/null
+++ b/tests/neg-custom-args/captures/PENDING-box-override.scala
@@ -0,0 +1,18 @@
+abstract class A[X]:
+  def foo(x: X): X
+
+class IO
+class C
+def test(io: {*} IO) =
+  class B extends A[{io} C]:    // error, but should work
+    override def foo(x: {io} C): {io} C = ???
+
+/* -- Error: box-override.scala:7:8 -----------------------------------------------
+7 |  class B extends A[{io} C]:
+  |        ^
+  |class B needs to be abstract, since def foo(x: X): X in class A is not defined
+  |(Note that
+  | parameter X in def foo(x: X): X in class A does not match
+  | parameter {io} C in override def foo(x: {io} C): {io} C in class B
+  | )
+*/