I would think, only in unevaluated contexts, no?

Of course direct recursion should be banned.

On Sun, Mar 21, 2021 at 9:18 PM Marcin Jaczewski <
marcinjaczewski86_at_[hidden]> wrote:

> niedz., 21 mar 2021 o 20:05 Omer Rosler <omer.rosler_at_[hidden]>
> napisał(a):
> >
> > It does work, Thank you.
> > I wonder if there is a use case for forward declaration of concepts
> which can't be worked around like so.
> >
>
> There are, if you would like to have:
>
> ```
> template<typename T>
> concept tag_type = other_concept<T> && different_concept<T>;
> ```
> There is a subtle difference when you try to order specializations
> based on concepts.
> If you use a class template then there is no ordering between concepts.
>
> If we have this forward declared concepts, the question is when you
> could use them?
> Because case like this should be forbidden:
>
> ```
> template<typename T>
> concept tag_type;
>
> template<typename T>
> concept tag_type = other_concept<T> && tag_type<T>; //direct recursion
> ```
>
> > On Sun, Mar 21, 2021 at 4:54 PM Marcin Jaczewski <
> marcinjaczewski86_at_[hidden]> wrote:
> >>
> >> But this is not a mery forward declaration but circular definition
> (like CRTP).
> >> This means even if we allow forward declaration it still could not
> >> allow recursion there.
> >>
> >> Edward in other mail said that you can forward `is_tag_impl`, if this
> >> will not work (compiler will throw some errors)
> >> then the same will be with your example.
> >>
> >> niedz., 21 mar 2021 o 15:41 Omer Rosler via Std-Proposals
> >> <std-proposals_at_[hidden]> napisał(a):
> >> >
> >> > Is there a reason forward declaration of concepts are not allowed?
> >> >
> >> > I wanted such feature in the following snippet:
> >> >
> >> > ```c++
> >> > template<typename T>
> >> > concept tag_type;
> >> > //Use the concept in a template declaration, when instantiated, the
> concept definition will be avilable
> >> > template<template<tag_type> typename ArgsType, template<ArgsType...>
> typename Templ>
> >> > struct template_ {};
> >> > template<typename T>
> >> > struct is_tag_impl : std::false_type {};
> >> > template<template<tag_type> typename ArgsType, template<ArgsType...>
> typename Templ>
> >> > struct is_tag_impl<template_<ArgsType, Templ>> : std::true_type {};
> >> >
> >> > template<typename T>
> >> > concept tag_type = is_tag_impl<T>::value;
> >> >
> >> >
> >> >
> >> > ```
> >> > --
> >> > Std-Proposals mailing list
> >> > Std-Proposals_at_[hidden]
> >> > https://lists.isocpp.org/mailman/listinfo.cgi/std-proposals
>