On Sun, Jan 23, 2022 at 11:59 AM Avi Kivity <avi_at_[hidden]> wrote:

>
>
> On 1/19/22 19:25, Михаил Найденов via Std-Proposals wrote:
> > Hello,
> > I have written a small proposal, based on a recent discussion
> > (https://lists.isocpp.org/std-proposals/2022/01/3472.php)
> >
> > Thanks
> >
>
> >
> > This paper suggest to use the concept, constraining a type to query
> > what its members are.
> >
> > template<class T>
> > concept SomeClasses = requires(T t){
> > typename T::type; //< `type` MUST BE a type
> > t.template doGreatWork<std::string>(); //< `doGreatWork` MUST BE a
> > function template
> > }
> >
> > template<SomeClasses AnyClass>
> > void someGreatCode(AnyClass c)
> > {
> > AnyClass::type t; //< proposed `type` IS A type, no
> > disambiguation needed
> > // use t
> > c.doGreatWork<std::string>(); //< proposed `doGreatWork` IS A
> > function template, no disambiguation needed
> > ...
> > }
> >
>
> What about optional constraints?
>
>
> Say
>
>
> template <typename T>
> concept SomeOtherClasses = SomeClasses<T> || std::same_as<T, int>
>
>
> template<SomeOtherClasses AnyClass>
> void someOtherGreatCode(AnyClass c)
>
>
> How is the compiler supposed to react to AnyClass::type? I suppose it
> can fall back to requiring a typename. But then I think the proposal
> should list the conditions under which the code is allowed to drop the
> typename, so we can expect code that compiles under one compiler to
> compile under all.
>

I think, even in this case, typename should not be needed, because if in
one instantiation the name is a type, unless concexpr-if is used, it must
be in all.
In this example the int path will fail to compile anyway.

@Jason McKesson
To be honest, avoiding `template` is much more of value, because it has
real consequences in how we design APIs at the moment (the fact that we
must take into accumet, the user will have a massively more verbose
experience when the template is member)
As for typename, considering right now we have a hard-coded list when it is
not required, having a more general "solution" that covers all cases is
valuable I believe as well.