Fix #9473: Add test

[liufengyun]

Fix #9473: Add test

[smarter]

Should the example in https://dotty.epfl.ch/docs/reference/contextual/derivation.html also be updated?

[liufengyun]

Should the example in https://dotty.epfl.ch/docs/reference/contextual/derivation.html also be updated?

The example there actually works well, I mean the Opt example. It does not work with the example Tree in the report.

[liufengyun]

@OlivierBlanvillain I'm not sure if it is the best way to do it. Could you have a look?

[OlivierBlanvillain]

These changes make sense, now that we have by name implicits they are cannonical way to deal with recursive data types.

I don't see a reason not to update the example from the doc, the updated version is more robust and having it that way in the docs can only help!

[liufengyun]

test performance please

[dottybot]

performance test scheduled: 11 job(s) in queue, 1 running.

[dottybot]

Performance test finished successfully:

Visit http://dotty-bench.epfl.ch/9565/ to see the changes.

Benchmarks is based on merging with master (6149e92)

[odersky]

The last commit can be dropped, since equivalent code is now merged in master.