https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99433
--- Comment #8 from Patrick Palka <ppalka at gcc dot gnu.org> ---
(In reply to gcc-bugs from comment #7)
> Thank you for the quick analysis!
>
> > views::drop(E, F) is specified to be expression-equivalent to the braced
> > init ranges::drop_view{E, F}
>
> Is not completely true, right? As the narrowing warning shows:
>
> ```
> libstdc++-v3/include/std/ranges:2101:24: warning: narrowing conversion of
> ‘std::declval<long unsigned int>()’ from ‘long unsigned int’ to
> ‘std::ranges::range_difference_t<std::ranges::ref_view<std::__cxx11::
> list<char> > >’ {aka ‘long int’} [-Wnarrowing]
> ```
>
> There is some `std::views::all` involved.
Yeah, that comes from drop_view's deduction guide
template<typename _Range>
drop_view(_Range&&, range_difference_t<_Range>)
-> drop_view<views::all_t<_Range>>;
>
> But the following expressions
>
> ```
> #include <list>
> #include <ranges>
>
> int main()
> {
> std::list<char> list;
> // std::views::drop(list, 0ull); // does not compile
> std::ranges::drop_view{list, 0ull}; // does compile without warnings
> std::ranges::drop_view{std::views::all(list), 0ull}; // does compile
> without warnings
> }
> ```
>
> do compile without any warnings when using `g++-11 -std=c++2a -pedantic
> -Wall -Wextra`!
>
> Even when adding `-Wsystem-headers` there is no "narrowing" warning found in
> those expressions.
Ah, I think that's because 0ull is a constant expression, and a narrowing
conversion in a braced init list _is_ allowed if the value is a constant
expression that's representable in the target type. If you replace 0ull with
some non-constant expression of the same type, the narrowing warning reappears.
