[Bug libstdc++/106808] New: std::string_view range concept requirement causes compile error with Boost.Filesystem

[Bug libstdc++/106808] New: std::string_view range concept requirement causes compile error with Boost.Filesystem

[andysem at mail dot ru]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106808

            Bug ID: 106808
           Summary: std::string_view range concept requirement causes
                    compile error with Boost.Filesystem
           Product: gcc
           Version: 11.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: andysem at mail dot ru
  Target Milestone: ---

Created attachment 53528
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=53528&action=edit
Preprocessed code of the reproducer.

The following code fails to compile in C++23 mode:

#include <boost/filesystem/path.hpp>

int main()
{
    boost::filesystem::path p = "foo";
    boost::filesystem::path p2 = p;
}

$ g++ -std=c++23 -I. -c fs_concepts_issue.cpp -o fs_concepts_issue.o

In file included from /usr/include/c++/11/bits/stl_iterator_base_types.h:71,
                 from /usr/include/c++/11/bits/stl_algobase.h:65,
                 from /usr/include/c++/11/bits/char_traits.h:39,
                 from /usr/include/c++/11/string:40,
                 from /usr/include/c++/11/bits/locale_classes.h:40,
                 from /usr/include/c++/11/locale:39,
                 from ./boost/filesystem/detail/path_traits.hpp:18,
                 from ./boost/filesystem/path.hpp:21,
                 from fs_concepts_issue.cpp:1:
/usr/include/c++/11/bits/ranges_base.h: In substitution of ‘template<class _Tp>
 requires (__maybe_borrowed_range<_Tp>) && ((is_array_v<typename
std::remove_reference<_Tp>::type>) || (__member_begin<_Tp>) ||
(__adl_begin<_Tp>)) constexpr auto
std::ranges::__cust_access::_Begin::operator()(_Tp&&) const [with _Tp = const
boost::filesystem::path&]’:
/usr/include/c++/11/bits/ranges_base.h:576:15:   required by substitution of
‘template<class _Range, class _DRange>  requires !(is_same_v<_DRange,
std::basic_string_view>) && (contiguous_range<_Range>) && (sized_range<_Range>)
&& (is_same_v<typename std::__detail::__iter_traits_impl<typename
std::remove_cvref<decltype(std::ranges::__cust_access::__begin((declval<_Range&>)()))>::type,
std::indirectly_readable_traits<typename
std::remove_cvref<decltype(std::ranges::__cust_access::__begin((declval<_Range&>)()))>::type>
>::__iter_traits<typename
std::remove_cvref<decltype(std::ranges::__cust_access::__begin((declval<_Range&>)()))>::type,
std::indirectly_readable_traits<typename
std::remove_cvref<decltype(std::ranges::__cust_access::__begin((declval<_Range&>)()))>::type>
>::value_type, _CharT>) && !(is_convertible_v<_Range, const _CharT*>) &&
!requires(_DRange& __d) {__d->__conv_op ();} && (!requires{typename
_DRange::traits_type;} || (is_same_v<typename _DRange::traits_type, _Traits>))
constexpr std::basic_string_view<wchar_t>::basic_string_view(_Range&&) [with
_Range = wchar_t; _DRange = std::char_traits<wchar_t>]’
./boost/filesystem/detail/path_traits.hpp:514:93:   required from ‘constexpr
const bool
boost::filesystem::detail::path_traits::is_convertible_to_std_string_view<boost::filesystem::path>::value’
./boost/type_traits/disjunction.hpp:31:27:   required from ‘struct
boost::disjunction<boost::filesystem::detail::path_traits::is_convertible_to_std_string_view<boost::filesystem::path>,
boost::filesystem::detail::path_traits::is_convertible_to_path_source_non_std_string_view<boost::filesystem::path>
>’
./boost/filesystem/detail/path_traits.hpp:537:8:   required from ‘struct
boost::filesystem::detail::path_traits::is_convertible_to_path_source<boost::filesystem::path>’
./boost/type_traits/disjunction.hpp:26:8:   required from ‘struct
boost::disjunction<boost::filesystem::detail::path_traits::is_convertible_to_path_source<boost::filesystem::path>
>’
./boost/type_traits/disjunction.hpp:30:8:   required from ‘struct
boost::disjunction<boost::filesystem::detail::path_traits::is_path_source<boost::filesystem::path>,
boost::filesystem::detail::path_traits::is_convertible_to_path_source<boost::filesystem::path>
>’
./boost/type_traits/conjunction.hpp:31:27:   required from ‘struct
boost::conjunction<boost::disjunction<boost::filesystem::detail::path_traits::is_path_source<boost::filesystem::path>,
boost::filesystem::detail::path_traits::is_convertible_to_path_source<boost::filesystem::path>
>,
boost::negation<boost::filesystem::detail::path_traits::is_native_path_source<boost::filesystem::path>
> >’
./boost/filesystem/path.hpp:249:9:   required by substitution of
‘template<class Source, class> boost::filesystem::path::path(const Source&)
[with Source = boost::filesystem::path; <template-parameter-1-2> = <missing>]’
fs_concepts_issue.cpp:6:34:   required from here
/usr/include/c++/11/bits/iterator_concepts.h:942:15:   required for the
satisfaction of ‘__member_begin<_Tp>’ [with _Tp = const
boost::filesystem::path&]
/usr/include/c++/11/bits/iterator_concepts.h:942:32:   in requirements with
‘_Tp& __t’ [with _Tp = const boost::filesystem::path&]
/usr/include/c++/11/bits/iterator_concepts.h:942:32: error: satisfaction value
of atomic constraint ‘requires(_Tp& __t)
{{std::ranges::__cust_access::__decay_copy(__t->begin())} -> decltype(auto)
[requires std::input_or_output_iterator<<placeholder>, >];} [with _Tp = const
boost::filesystem::path&]’ changed from ‘false’ to ‘true’
  942 |       concept __member_begin = requires(_Tp& __t)
      |                                ^~~~~~~~~~~~~~~~~~
  943 |         {
      |         ~                       
  944 |           { __decay_copy(__t.begin()) } -> input_or_output_iterator;
      |           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  945 |         };
      |         ~                       
In file included from /usr/include/c++/11/string_view:48,
                 from /usr/include/c++/11/bits/basic_string.h:48,
                 from /usr/include/c++/11/string:55,
                 from /usr/include/c++/11/bits/locale_classes.h:40,
                 from /usr/include/c++/11/locale:39,
                 from ./boost/filesystem/detail/path_traits.hpp:18,
                 from ./boost/filesystem/path.hpp:21,
                 from fs_concepts_issue.cpp:1:
/usr/include/c++/11/bits/ranges_base.h:576:22: note: satisfaction value first
evaluated to ‘false’ from here
  576 |         ranges::begin(__t);
      |         ~~~~~~~~~~~~~^~~~~

$ g++ -v
Using built-in specs.
COLLECT_GCC=g++
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/11/lto-wrapper
OFFLOAD_TARGET_NAMES=nvptx-none:amdgcn-amdhsa
OFFLOAD_TARGET_DEFAULT=1
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu
11.2.0-19ubuntu1' --with-bugurl=file:///usr/share/doc/gcc-11/README.Bugs
--enable-languages=c,ada,c++,go,brig,d,fortran,objc,obj-c++,m2 --prefix=/usr
--with-gcc-major-version-only --program-suffix=-11
--program-prefix=x86_64-linux-gnu- --enable-shared --enable-linker-build-id
--libexecdir=/usr/lib --without-included-gettext --enable-threads=posix
--libdir=/usr/lib --enable-nls --enable-bootstrap --enable-clocale=gnu
--enable-libstdcxx-debug --enable-libstdcxx-time=yes
--with-default-libstdcxx-abi=new --enable-gnu-unique-object
--disable-vtable-verify --enable-plugin --enable-default-pie --with-system-zlib
--enable-libphobos-checking=release --with-target-system-zlib=auto
--enable-objc-gc=auto --enable-multiarch --disable-werror --enable-cet
--with-arch-32=i686 --with-abi=m64 --with-multilib-list=m32,m64,mx32
--enable-multilib --with-tune=generic
--enable-offload-targets=nvptx-none=/build/gcc-11-gBFGDP/gcc-11-11.2.0/debian/tmp-nvptx/usr,amdgcn-amdhsa=/build/gcc-11-gBFGDP/gcc-11-11.2.0/debian/tmp-gcn/usr
--without-cuda-driver --enable-checking=release --build=x86_64-linux-gnu
--host=x86_64-linux-gnu --target=x86_64-linux-gnu
--with-build-config=bootstrap-lto-lean --enable-link-serialization=2
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 11.2.0 (Ubuntu 11.2.0-19ubuntu1)

This is with Boost.Filesystem revision
5d4c1caaab65012f939ffa118cc1f36d34198c0b.

The code compiles with -std=c++20. It also compiles with MSVC 14.3
/std:c++latest

Sorry, I have little knowledge about concepts and I couldn't reduce the repro
further. I've attached the preprocessed code.

[Bug libstdc++/106808] std::string_view range concept requirement causes compile error with Boost.Filesystem

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106808

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
C++23 mode in gcc 11 is still in progress due it not even being 2023 yet. Can
you try the trunk?

[Bug libstdc++/106808] std::string_view range concept requirement causes compile error with Boost.Filesystem

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106808

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
             Status|UNCONFIRMED                 |WAITING
   Last reconfirmed|                            |2022-09-01

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Oh and gcc 11 was released over 1.5 years ago and as mentioned c++23 support is
just barely got started then.

[Bug libstdc++/106808] std::string_view range concept requirement causes compile error with Boost.Filesystem

[andysem at mail dot ru]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106808

--- Comment #3 from andysem at mail dot ru ---
I don't have the environment to build gcc locally, so I can't readily test
trunk. But I have been told the same issue reproduces with gcc-12
20220319-1ubuntu1 from Ubuntu 22.04:

https://github.com/boostorg/multiprecision/runs/8117686288?check_suite_focus=true#step:17:248

[Bug libstdc++/106808] std::string_view range concept requirement causes compile error with Boost.Filesystem

[andysem at mail dot ru]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106808

--- Comment #4 from andysem at mail dot ru ---
It fails the same way with 12.2 and trunk on godbolt:

https://gcc.godbolt.org/z/rT6TWhhvP

[Bug libstdc++/106808] std::string_view range concept requirement causes compile error with Boost.Filesystem

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106808

--- Comment #5 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Looks related to the problem I fixed in
g:1e690757d30775ed340a368b9a9463b2ad68de01

[Bug libstdc++/106808] std::string_view range concept requirement causes compile error with Boost.Filesystem

[andysem at mail dot ru]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106808

--- Comment #6 from andysem at mail dot ru ---
So do you think this is a problem in Boost.Filesystem?

I would say this is a regression in string_view as the code is valid in
pre-C++23, and I would expect it to stay valid in C++23 onwards. Shouldn't the
range constructor be simply disabled when fs::path::iterator is not defined?

[Bug libstdc++/106808] std::string_view range concept requirement causes compile error with Boost.Filesystem

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106808

--- Comment #7 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to andysem from comment #6)
> So do you think this is a problem in Boost.Filesystem?

I don't know yet, I can't reproduce it with the Boost in Fedora 36, and I
haven't looked further.

> I would say this is a regression in string_view as the code is valid in
> pre-C++23, and I would expect it to stay valid in C++23 onwards.


That range constructor is explicit in the C++23 draft now, but that change
hasn't been backordered to GCC 12 yet, and apparently doesn't change anything
if you still see this in trunk.

> Shouldn't
> the range constructor be simply disabled when fs::path::iterator is not
> defined?

Do you want ODR violations? Because that's how you get ODR violations.

[Bug libstdc++/106808] std::string_view range concept requirement causes compile error with Boost.Filesystem

[andysem at mail dot ru]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106808

--- Comment #8 from andysem at mail dot ru ---
(In reply to Jonathan Wakely from comment #7)
> (In reply to andysem from comment #6)
> > So do you think this is a problem in Boost.Filesystem?
> 
> I don't know yet, I can't reproduce it with the Boost in Fedora 36, and I
> haven't looked further.

I have added a workaround to the current Boost.Filesystem develop and master,
and Boost 1.80 (the latest official release) didn't support std::string_view,
so is not affected either. In order to reproduce you would need the revision I
posted in the original problem description.

> > I would say this is a regression in string_view as the code is valid in
> > pre-C++23, and I would expect it to stay valid in C++23 onwards.
> 
> That range constructor is explicit in the C++23 draft now, but that change
> hasn't been backordered to GCC 12 yet, and apparently doesn't change
> anything if you still see this in trunk.
> 
> > Shouldn't
> > the range constructor be simply disabled when fs::path::iterator is not
> > defined?
> 
> Do you want ODR violations? Because that's how you get ODR violations.

I understand this, but my point is that this is a breaking change, apparently
even with the constructor being marked explicit, and it breaks in a rather
surprising way. Also, apparently MSVC is able to not break somehow as it
doesn't have this issue. So maybe there is a solution that does not introduce
ODR violations and yet still works. I think it would be preferable to fix
std::string_view rather than all its uses like in std/Boost.Filesystem.

[Bug libstdc++/106808] std::string_view range concept requirement causes compile error with Boost.Filesystem

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106808

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|WAITING                     |NEW

--- Comment #9 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Of course there's a solution, but disabling the constructor is not it. The
problem is not that the constructor exists, it's that the constraints on it
have a problem.

[Bug libstdc++/106808] std::string_view range concept requirement causes compile error with Boost.Filesystem

[andysem at mail dot ru]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106808

--- Comment #10 from andysem at mail dot ru ---
(In reply to andysem from comment #8)
> (In reply to Jonathan Wakely from comment #7)
> > 
> > Do you want ODR violations? Because that's how you get ODR violations.
> 
> I understand this, but my point is that this is a breaking change,
> apparently even with the constructor being marked explicit...

I take it back, with the constructor marked explicit it doesn't break. Before,
I tested the preprocessed source from gcc 11.2, which didn't have explicit.
Adding the explicit fixes compilation. Sorry for the confusion.

[Bug libstdc++/106808] std::string_view range concept requirement causes compile error with Boost.Filesystem

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106808

--- Comment #11 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Oh good, I hoped that would be the case. I was already planning to backport
that change to gcc-11 and gcc-12.

[Bug libstdc++/106808] std::string_view range concept requirement causes compile error with Boost.Filesystem

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106808

--- Comment #12 from Jonathan Wakely <redi at gcc dot gnu.org> ---
In fact, it's already backported to gcc 12.2, as
g:61076545cb3c3cbc79036eff8bc46b0c2083730c

[Bug libstdc++/106808] std::string_view range concept requirement causes compile error with Boost.Filesystem

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106808

--- Comment #13 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to andysem from comment #3)
> I don't have the environment to build gcc locally, so I can't readily test
> trunk. But I have been told the same issue reproduces with gcc-12
> 20220319-1ubuntu1 from Ubuntu 22.04:

That version is "ancient" (it's not even the final 12.1 release).

[Bug libstdc++/106808] std::string_view range concept requirement causes compile error with Boost.Filesystem

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106808

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|---                         |11.4
             Status|NEW                         |RESOLVED
         Resolution|---                         |FIXED

--- Comment #14 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Backported as r11-10236-g9c6141bfec48c2 so I think this is now fixed on all
branches, meaning the C++23 range constructor is explicit in 12.2 already, and
will be explicit in 11.4 when that is released.