[Bug libstdc++/114325] New: std::format gives incorrect results for negative numbers

[Bug libstdc++/114325] New: std::format gives incorrect results for negative numbers

[luigighiron at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114325

            Bug ID: 114325
           Summary: std::format gives incorrect results for negative
                    numbers
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: luigighiron at gmail dot com
  Target Milestone: ---

The following code generates an incorrect result with libstdc++:

std::format("{}",-100)

From testing on godbolt this seems to generate the string "-1\0000", then when
printed it looks like -10. This seems exclusive to GCC 14, and happens for any
numbers less than -99.

[Bug libstdc++/114325] std::format gives incorrect results for negative numbers

[mwd at md5i dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114325

Michael Duggan <mwd at md5i dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |mwd at md5i dot com

--- Comment #1 from Michael Duggan <mwd at md5i dot com> ---
I will note that, in experiments, this seems to solely happen with "{}".  If
anything else is in the format string, it works correctly.  This is probably a
bug in the fairly recent codepath that optimizes the "{}" case.

[Bug libstdc++/114325] [14 Regression] std::format gives incorrect results for negative numbers

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114325

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
      Known to work|                            |13.2.1
   Target Milestone|---                         |14.0
   Last reconfirmed|                            |2024-03-13
     Ever confirmed|0                           |1
             Status|UNCONFIRMED                 |ASSIGNED
            Summary|std::format gives incorrect |[14 Regression] std::format
                   |results for negative        |gives incorrect results for
                   |numbers                     |negative numbers
      Known to fail|                            |14.0
           Assignee|unassigned at gcc dot gnu.org      |redi at gcc dot gnu.org

[Bug libstdc++/114325] [14 Regression] std::format gives incorrect results for negative numbers

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114325

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Indeed. Here's the fix:

--- a/libstdc++-v3/include/std/format
+++ b/libstdc++-v3/include/std/format
@@ -4124,14 +4124,14 @@ namespace __format
                    __uval = make_unsigned_t<_Tp>(~__arg) + 1u;
                  else
                    __uval = __arg;
-                 const auto __n = __detail::__to_chars_len(__uval) + __neg;
-                 if (auto __res = __sink_out._M_reserve(__n))
+                 const auto __n = __detail::__to_chars_len(__uval);
+                 if (auto __res = __sink_out._M_reserve(__n + __neg))
                    {
                      auto __ptr = __res.get();
                      *__ptr = '-';
                      __detail::__to_chars_10_impl(__ptr + (int)__neg, __n,
                                                   __uval);
-                     __res._M_bump(__n);
+                     __res._M_bump(__n + __neg);
                      __done = true;
                    }
                }

[Bug libstdc++/114325] [14 Regression] std::format gives incorrect results for negative numbers

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114325

--- Comment #3 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:f89cfdb2f2e9b4fe517b1e00511c4d70a7014cbc

commit r14-9479-gf89cfdb2f2e9b4fe517b1e00511c4d70a7014cbc
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Wed Mar 13 21:19:54 2024 +0000

    libstdc++: Fix std::format("{}", negative_integer) [PR114325]

    The fast path for "{}" format strings has a bug for negative integers
    where the length passed to std::to_chars is too long.

    libstdc++-v3/ChangeLog:

            PR libstdc++/114325
            * include/std/format (_Scanner::_M_scan): Pass correct length to
            __to_chars_10_impl.
            * testsuite/std/format/functions/format.cc: Check negative
            integers with empty format-spec.

[Bug libstdc++/114325] [14 Regression] std::format gives incorrect results for negative numbers

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114325

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

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

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Fixed for GCC 14, thanks for the report.