https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99433
--- Comment #6 from Patrick Palka <ppalka at gcc dot gnu.org> ---
(In reply to gcc-bugs from comment #5)
> Thank you for the fix, but the following code does not compile any more:
>
> ```c++
> #include <list>
> #include <ranges>
>
> int main()
> {
> std::list<char> list;
>
> constexpr auto drop = []<std::ranges::viewable_range urng_t>(urng_t &&
> urange, size_t drop_size)
> {
> // does not work:
> return std::forward<urng_t>(urange) | std::views::drop(drop_size);
>
> // does work:
> // return std::forward<urng_t>(urange) | std::views::drop(0);
> };
> drop(list, 0);
> }
> ```
>
> Should I open a new issue?
Reduced:
#include <list>
#include <ranges>
int main() {
std::list<char> list;
std::views::drop(list, 0ul);
}
The constraint satisfaction failure diagnostic says:
libstdc++-v3/include/std/ranges:2100:10: required for the satisfaction of
‘__can_drop_view<_Range, _Tp>’ [with _Range = std::__cxx11::list<char,
std::allocator<char> >&; _Tp = lon
g unsigned int]
libstdc++-v3/include/std/ranges:2101:6: in requirements [with _Tp = long
unsigned int; _Range = std::__cxx11::list<char, std::allocator<char> >&]
libstdc++-v3/include/std/ranges:2101:24: note: the required expression
‘drop_view<...auto...>{declval<_Range>(), declval<_Tp>()}’ is invalid, because
2101 | = requires { drop_view{std::declval<_Range>(),
std::declval<_Tp>()}; };
|
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
but it strangely doesn't explain why the expression is invalid. Turns out if
we pass -Wsystem-headers to the command line, then we do get an explanation in
the form of a warning:
<snip>
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]
So I suppose we're correct to reject the testcase, since narrowing conversions
aren't permitted in braced init lists (and views::drop(E, F) is specified to
be expression-equivalent to the braced init ranges::drop_view{E, F}).
But it's perhaps a frontend bug that we need to pass -Wsystem-headers to get
the warning here in the first place; I'll file a PR for this issue.
