[Bug libstdc++/101091] New: std::views::take and std::views::drop are overconstrained

[Bug libstdc++/101091] New: std::views::take and std::views::drop are overconstrained

[j.galecki11 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101091

            Bug ID: 101091
           Summary: std::views::take and std::views::drop are
                    overconstrained
           Product: gcc
           Version: 11.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: j.galecki11 at gmail dot com
  Target Milestone: ---

[Bug libstdc++/101091] std::views::take and std::views::drop are overconstrained

[j.galecki11 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101091

--- Comment #1 from Jakub Gałecki <j.galecki11 at gmail dot com> ---
Consider the following example: https://godbolt.org/z/YYzqWaM9G

#include <algorithm>
#include <ranges>
#include <vector>

void copy_drop_n1(const std::vector<double>& src, std::vector<double>& dest,
                  std::size_t n) {  // size_t - narrowing conversion
  std::ranges::copy(src | std::views::drop(n), begin(dest) + n);
}

void copy_drop_n2(const std::vector<double>& src, std::vector<double>& dest,
                  std::ptrdiff_t n) {  // ptrdiff_t
  std::ranges::copy(src | std::views::drop(n), begin(dest) + n);
}

As per the standard [range.take.overview]:
The name views​::​take denotes a range adaptor object ([range.adaptor.object]).
Let E and F be expressions, let T be remove_­cvref_­t<decltype((E))>, and let D
be range_­difference_­t<decltype((E))>. If decltype((F)) does not model
convertible_­to<D>, views​::​take(E, F) is ill-formed. Otherwise, the
expression views​::​take(E, F) is expression-equivalent to [...]

The example above should therefore compile for size_t as well as ptrdiff_t,
since size_t is convertible to ptrdiff_t. This is a regression from libstdc++
version 10.x, where both functions compile.

[Bug libstdc++/101091] std::views::take and std::views::drop are overconstrained

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101091

Patrick Palka <ppalka at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |DUPLICATE
                 CC|                            |ppalka at gcc dot gnu.org
             Status|UNCONFIRMED                 |RESOLVED

--- Comment #2 from Patrick Palka <ppalka at gcc dot gnu.org> ---
Thanks for the bug report.  This regression is already fixed for GCC 11.2 and
12, see PR100558.

*** This bug has been marked as a duplicate of bug 100558 ***