I'd guess it is neither by design nor a bug. It is simply the result of
current rules, and is potentially a design flaw (or an ease-of-use bug). It
occurs not only with uniform_random_bit_generator, but all kinds of other
concepts when used with auto&&.

The current easy way of achieving your purpose is probably avoiding the
abbreviated syntax:

template <typename G>
requires std::uniform_random_bit_generator<std::remove_cvref_t<G>>
void bar(G&& i)
…

Please also notice that using remove_reference_t is probably not good
enough.

On Mon, 30 Sept 2024 at 00:49, M Gurschi via Std-Discussion <
std-discussion_at_[hidden]> wrote:

> Hello,
>
> Can someone please confirm if it is by design that the
> std::uniform_random_bit_generator concept expects static members on
> (template type) G as opposed to on (std::remove_reference_t<G>) ?
>
> I have a use case that fails below. Passing an lvalue to bar fails, but
> succeeds with foo. Foo - strips any reference from G.
>
> https://godbolt.org/z/46WdMqhqE
> ```
> #include <concepts>
> #include <memory>
> #include <random>
>
> template< class G, class T = std::remove_reference_t<G>>
> concept remove_cv_uniform_random_bit_generator =
> std::invocable<G> && std::unsigned_integral<std::invoke_result_t<G>> &&
> requires {
> { T::min() } -> std::same_as<std::invoke_result_t<G>>;
> { T::max() } -> std::same_as<std::invoke_result_t<G>>;
> requires std::bool_constant<(T::min() < T::max())>::value;
> };
>
> template<remove_cv_uniform_random_bit_generator F>
> void foo(F&& i) {
> }
>
> template<std::uniform_random_bit_generator F>
> void bar(F&& i) {
> }
>
> int main()
> {
> static_assert(remove_cv_uniform_random_bit_generator<std::mt19937>);
> static_assert(std::uniform_random_bit_generator<std::mt19937>);
> std::mt19937 mt;
> foo(mt);
> bar(mt);//Offending line.
> return 0;
> } ```
>
> If it is in fact an omission, what do you recommend to do?
> Kind Regards,
> Maxim Gurschi
> --
> Std-Discussion mailing list
> Std-Discussion_at_[hidden]
> https://lists.isocpp.org/mailman/listinfo.cgi/std-discussion
>

-- 
Yongwei Wu
URL: http://wyw.dcweb.cn/