[Bug libstdc++/112832] New: [std::format] Broken non-SFINAE-friendly `set_debug_format()` for `const char *` formatter

[Bug libstdc++/112832] New: [std::format] Broken non-SFINAE-friendly `set_debug_format()` for `const char *` formatter

[iamsupermouse at mail dot ru]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112832

            Bug ID: 112832
           Summary: [std::format] Broken non-SFINAE-friendly
                    `set_debug_format()` for `const char *` formatter
           Product: gcc
           Version: 13.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: iamsupermouse at mail dot ru
  Target Milestone: ---

GCC doesn't seem to implement `{:?}` debug format for strings yet.

Yet for some reason `std::formatter<const char *>` has a `set_debug_format()`
function, which causes a hard error when used (isn't SFINAE-friendly).

`set_debug_format()` should be SFINAE-friendly, to allow the user to call it
conditionally when available.

Here's a code sample that fails to compile because of it:
https://gcc.godbolt.org/z/PbfnsxE9n (still fails in GCC 14 trunk)

    #include <format>
    #include <iostream>
    #include <string>
    #include <type_traits>
    #include <utility>

    template <typename T>
    struct PreferDebugFormat
    {
        T &target;
        constexpr PreferDebugFormat(T &target) : target(target) {}
    };
    template <typename T>
    PreferDebugFormat(T &&) -> PreferDebugFormat<std::remove_reference_t<T> &>;

    template <typename T, typename CharT>
    struct std::formatter<PreferDebugFormat<T>, CharT> :
std::formatter<std::remove_cvref_t<T>, CharT>
    {
        using base = std::formatter<std::remove_cvref_t<T>, CharT>;

        constexpr decltype(auto) parse(std::basic_format_parse_context<CharT>
&parse_ctx)
        {
            if constexpr (requires{this->set_debug_format();})
                this->set_debug_format();
            return base::parse(parse_ctx);
        }

        template <typename OutputIt>
        constexpr decltype(auto) format(const PreferDebugFormat<T> &arg,
std::basic_format_context<OutputIt, CharT> &format_ctx) const
        {
            return base::format(arg.target, format_ctx);
        }
    };

    template <typename T>
    std::string ToDebugString(const T &value)
    {
        return std::format("{}", PreferDebugFormat(value));
    }

    int main()
    {
        std::cout << ToDebugString("foo\nbar") << '\n'; // hard error == BUG
        std::cout << ToDebugString(std::string_view("foo\nbar")) << '\n'; //
ok, no debug format yet
        std::cout << ToDebugString(42) << '\n'; // ok, no debug format
    }

[Bug libstdc++/112832] [std::format] Broken non-SFINAE-friendly `set_debug_format()` for `const char *` formatter

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112832

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|                            |2023-12-04
     Ever confirmed|0                           |1
           Assignee|unassigned at gcc dot gnu.org      |redi at gcc dot gnu.org
             Status|UNCONFIRMED                 |ASSIGNED
   Target Milestone|---                         |13.3

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Egor from comment #0)
> GCC doesn't seem to implement `{:?}` debug format for strings yet.

Correct, see PR 109162

> Yet for some reason `std::formatter<const char *>` has a
> `set_debug_format()` function, which causes a hard error when used (isn't
> SFINAE-friendly).

The error has nothing to do with it being SFINAE-friendly or not.

> `set_debug_format()` should be SFINAE-friendly, to allow the user to call it
> conditionally when available.

No, it should just not exist yet.

[Bug libstdc++/112832] [std::format] Broken non-SFINAE-friendly `set_debug_format()` for `const char *` formatter

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112832

--- Comment #2 from GCC Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Jonathan Wakely <redi@gcc.gnu.org>:

https://gcc.gnu.org/g:3cd73543a1122d3c81409e3e9a26c3e94c3d324f

commit r14-6189-g3cd73543a1122d3c81409e3e9a26c3e94c3d324f
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Mon Dec 4 12:03:28 2023 +0000

    libstdc++: Disable std::formatter::set_debug_format [PR112832]

    All set_debug_format member functions should be guarded by the
    __cpp_lib_formatting_ranges macro (which is not defined yet).

    libstdc++-v3/ChangeLog:

            PR libstdc++/112832
            * include/std/format (formatter::set_debug_format): Ensure this
            member is defined conditionally for all specializations.
            * testsuite/std/format/formatter/112832.cc: New test.

[Bug libstdc++/112832] [std::format] Broken non-SFINAE-friendly `set_debug_format()` for `const char *` formatter

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112832

--- Comment #3 from GCC Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-13 branch has been updated by Jonathan Wakely
<redi@gcc.gnu.org>:

https://gcc.gnu.org/g:d40796e3813fdf646ca7b7ce9630d5cc121353b4

commit r13-8130-gd40796e3813fdf646ca7b7ce9630d5cc121353b4
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Mon Dec 4 12:03:28 2023 +0000

    libstdc++: Disable std::formatter::set_debug_format [PR112832]

    All set_debug_format member functions should be guarded by the
    __cpp_lib_formatting_ranges macro (which is not defined yet).

    libstdc++-v3/ChangeLog:

            PR libstdc++/112832
            * include/std/format (formatter::set_debug_format): Ensure this
            member is defined conditionally for all specializations.
            * testsuite/std/format/formatter/112832.cc: New test.

    (cherry picked from commit 3cd73543a1122d3c81409e3e9a26c3e94c3d324f)

[Bug libstdc++/112832] [std::format] Broken non-SFINAE-friendly `set_debug_format()` for `const char *` formatter

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112832

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|ASSIGNED                    |RESOLVED
         Resolution|---                         |FIXED

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Fixed for 13.3, thanks for the report.