On Sat, 20 Mar 2021 at 17:54, Phil Bouchard via Std-Proposals
<std-proposals_at_[hidden]> wrote:
> And that's why deduction probably shouldn't happen with class members
> like that. It's not a matter of whether it's possible to specify in
> the standard; it's a matter of whether it's a good idea to permit it.
>
> And there are good arguments why it shouldn't be.
>
> So it's better to be stricter than stuck down the road with an allowance you can't revert...

See http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n3897.html