[Bug libstdc++/119469] iter_rvalue_reference_t is wrong for function types

[redi at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=119469

--- Comment #6 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Jonathan Wakely from comment #4)
> So we can still avoid the cost of doing overload resolution for
> std::move(*E) like so:
> 
>           using __lval = __iter_ref_t<_Tp>;
>           using __rval = remove_reference_t<__lval>&&;
>           //  N.B. this is not just __rval, because casting to an rvalue
>           //  reference to function type produces an lvalue not an rvalue.
>           // C++23 [expr.static.cast]/1, [expr.type.conv]/1 etc.
>           using type = decltype(static_cast<__rval>(std::declval<__lval>()));

Actually we don't even need the cast, just decltype(declval<__rval>()) gives
the right answer. And declval always adds && so
decltype(declval<remove_reference_t<__lval>>()) is the same type, and so:

            using type
              =
decltype(std::declval<remove_reference_t<__iter_ref_t<_Tp>>>());