On Sat, Mar 20, 2021 at 12:53 AM Phil Bouchard via Std-Proposals
<std-proposals@lists.isocpp.org> wrote:

Why is that disallowed when you can simply workaround the problem?

That's not a good enough reason to add any feature to the language.

I don't know if you're talking about allowing `auto`-deduction of
members of a class or CTAD-based deduction of members of a class. But
the reason not to do it is the same regardless.

Yes I was implying that.

Namely: that initializer does not *have to be* used to initialize that
member. With normal `auto` or CTAD deduction, you're talking about an
object which is definitely going to be initialized by a given
initializer. But in your example, you don't know what's going to
initialize the member. Yes, `A::l` has a default member initializer,
but this is only the *default* initializer. Since `A` is an aggregate,
it can be overridden as easily as: `A{{5.4f, 8.4f}};`

Should that mean that `A::l` is a `list<float>` in this case? Some
might assume that it should be. *Especially* since `auto` now is more
easily used to create template functions, some might think that you
can use `auto` or similar mechanisms to create template *classes*
without an explicit template header.

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...

--

Phil Bouchard
Founder
C.: (819) 328-4743