[Bug libstdc++/112607] New: <format>: _Normalize does not consider char_type for the basic_string_view case

[Bug libstdc++/112607] New: <format>: _Normalize does not consider char_type for the basic_string_view case

[hewillk at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112607

            Bug ID: 112607
           Summary: <format>: _Normalize does not consider char_type for
                    the basic_string_view case
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: hewillk at gmail dot com
  Target Milestone: ---

When T in basic_format_arg(T& v) is a specialization of basic_string_view or
basic_string, format#arg-6.8 indicates: 

otherwise, if TD is a specialization of basic_string_view or basic_string and
TD​::​value_type is char_type, initializes value with
basic_string_view<char_type>(v.data(), v.size());

We need to consider TD​::​value_type is char_type.

However, libstd++ only uses __is_specialization_of to detect whether T is a
specialization of basic_string_view or basic_string (format#L3118-L3121):

        else if constexpr (__is_specialization_of<_Td, basic_string_view>)
          return type_identity<basic_string_view<_CharT>>();
        else if constexpr (__is_specialization_of<_Td, basic_string>)
          return type_identity<basic_string_view<_CharT>>();

This causes basic_format_arg to incorrectly use wstring_view to initialize
string_view when customizing std::wstring.

https://godbolt.org/z/6Kd16z8qK

   #include <format>

   template<>
   struct std::formatter<std::wstring> : std::formatter<std::string> {
     auto format(const std::wstring& obj, auto& ctx) const {
     return std::formatter<std::string>::format(" ", ctx);
   }
  };

  int main(){
    std::wstring wstr;
    std::string str = std::format("{}", wstr);
  }

[Bug libstdc++/112607] <format>: _Normalize does not consider char_type for the basic_string_view case

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112607

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
That is not a valid specialization since it doesn't depend on a program-defined
type.

[Bug libstdc++/112607] <format>: _Normalize does not consider char_type for the basic_string_view case

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112607

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
basic_string<wchar_t, mytraits> would be a valid program-defined specialization
though.

[Bug libstdc++/112607] <format>: _Normalize does not consider char_type for the basic_string_view case

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112607

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
The standard tries quite hard to avoid this kind of specialization:

https://eel.is/c++draft/format.formatter.spec#note-1

But I suppose you can contrive this kind of custom formatter, or the inverse,
i.e. std::formatter<std::basic_string<char, mytraits>, wchar_t>

[Bug libstdc++/112607] <format>: _Normalize does not consider char_type for the basic_string_view case

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112607

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |ASSIGNED
           Assignee|unassigned at gcc dot gnu.org      |redi at gcc dot gnu.org
     Ever confirmed|0                           |1
   Last reconfirmed|                            |2023-11-18

[Bug libstdc++/112607] <format>: _Normalize does not consider char_type for the basic_string_view case

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112607

--- Comment #4 from CVS 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:279e407a06cc676d8e6e0bb5755b0a804e05377c

commit r14-5588-g279e407a06cc676d8e6e0bb5755b0a804e05377c
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Sat Nov 18 20:56:35 2023 +0000

    libstdc++: Check string value_type in std::make_format_args [PR112607]

    libstdc++-v3/ChangeLog:

            PR libstdc++/112607
            * include/std/format (basic_format_arg::_S_to_arg_type): Check
            value_type for basic_string_view and basic_string
            specializations.
            * testsuite/std/format/arguments/112607.cc: New test.

[Bug libstdc++/112607] <format>: _Normalize does not consider char_type for the basic_string_view case

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112607

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|---                         |13.3

--- Comment #5 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Fixed on trunk so far.

[Bug libstdc++/112607] <format>: _Normalize does not consider char_type for the basic_string_view case

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112607

--- Comment #6 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:c7c92d61d32f6fd7746e2844f68d1936e2b6f6f6

commit r13-8105-gc7c92d61d32f6fd7746e2844f68d1936e2b6f6f6
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Sat Nov 18 20:56:35 2023 +0000

    libstdc++: Check string value_type in std::make_format_args [PR112607]

    libstdc++-v3/ChangeLog:

            PR libstdc++/112607
            * include/std/format (basic_format_arg::_S_to_arg_type): Check
            value_type for basic_string_view and basic_string
            specializations.
            * testsuite/std/format/arguments/112607.cc: New test.

    (cherry picked from commit 279e407a06cc676d8e6e0bb5755b0a804e05377c)

[Bug libstdc++/112607] <format>: _Normalize does not consider char_type for the basic_string_view case

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112607

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

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

--- Comment #7 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Fixed for 13.3 as well, thanks for the report.