[Bug libstdc++/103924] New: views::join combined with std::string cannot be used in constant expressions

[Bug libstdc++/103924] New: views::join combined with std::string cannot be used in constant expressions

[hewillk at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103924

            Bug ID: 103924
           Summary: views::join combined with std::string cannot be used
                    in constant expressions
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: hewillk at gmail dot com
  Target Milestone: ---

The following code cannot be used as a constant expression, but it should work
since constexpr non-propagating-cache and std::string have been implemented.

#include <string>
#include <ranges>

int main() {
  using namespace std::string_literals;
  static_assert(std::ranges::distance(
      std::views::single("42"s) | std::views::join));
}

https://godbolt.org/z/zPv4xnxnP

#include <string>
#include <ranges>

int main() {
  using namespace std::string_literals;
  static_assert(std::ranges::distance(
      std::views::iota(0, 1) 
      | std::views::transform([](auto) { return "42"s; })
      | std::views::join));
}

https://godbolt.org/z/d5TM4x53z

[Bug libstdc++/103924] views::join combined with std::string cannot be used in constant expressions

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103924

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
   Last reconfirmed|                            |2022-01-06
             Status|UNCONFIRMED                 |NEW
                 CC|                            |ppalka at gcc dot gnu.org

[Bug libstdc++/103924] views::join combined with std::string cannot be used in constant expressions

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103924

--- Comment #1 from Patrick Palka <ppalka at gcc dot gnu.org> ---
I encountered a similar problem when testing views::join_with:

#include <string>
#include <ranges>
constexpr std::string_view rs[] = {"42"};
static_assert(!std::ranges::empty(rs |
std::views::join_with(std::string(""))));

error: accessing ‘std::__cxx11::basic_string<char>::<unnamed
union>::_M_allocated_capacity’ member instead of initialized
‘std::__cxx11::basic_string<char>::<unnamed union>::_M_local_buf’ member in
constant expression

[Bug libstdc++/103924] views::join combined with std::string cannot be used in constant expressions

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103924

--- Comment #2 from Patrick Palka <ppalka at gcc dot gnu.org> ---
*** Bug 114530 has been marked as a duplicate of this bug. ***

[Bug libstdc++/103924] views::join combined with std::string cannot be used in constant expressions

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103924

--- Comment #3 from Patrick Palka <ppalka at gcc dot gnu.org> ---
From PR114530:

#include <ranges>
#include <vector>

static_assert(
  std::ranges::distance(
      std::views::single(std::views::cartesian_product(std::string{}))
    | std::views::join
  ) == 0
);

[Bug libstdc++/103924] views::join combined with std::string cannot be used in constant expressions

[hewillk at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103924

--- Comment #4 from 康桓瑋 <hewillk at gmail dot com> ---
(In reply to Patrick Palka from comment #2)
> *** Bug 114530 has been marked as a duplicate of this bug. ***

That's my lost memory.