[RFC] Polymorphic deep copy (aka PR46174)

[Janus Weil]

>>> The attached new version of the patch finally is free of regressions
>>> (on x86_64-unknown-linux-gnu). Ok for trunk?
>>
>> OK; but please add (or "dg-do run" enable) a test case for the user-defined
>> assignment.
>>
>> You could also add PR 45451 to the changelog as the patch seemingly fixes
>> the last remaining issue of that PR.
>
> Thanks for the review. I'll make the changes you propose and commit tonight.

Committed as r166368.

I'll open a follow-up PR for the $free case (i.e. polymorphic
deallocation), which is really a separate issue.

Cheers,
Janus