A problematic test showing interaction of boxing and inheritance

[odersky]

Plus a small simplification in CheckCaptures

[odersky]

This test currently fails:

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 = ???

with the error message

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 | ) */ 

Arguably there should be a way to achieve such overridings. You can think of it as inserting a bridge method in B that implements the method in A by forwarding to the method in B and box-adapting argument and result. However, it seems that such a box adaptation could in general require some capability in B that was not required before. So this would need to be worked into the general capability propagation framework.

[odersky]

Looks like this test compiles now. I am adding the test to #18192 and am closing this one.