[Bug libstdc++/111102] New: illegal pointer arithmetic invoked by std::format("L{:65536}",1)

[gcc at pauldreik dot se via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111102

            Bug ID: 111102
           Summary: illegal pointer arithmetic invoked by
                    std::format("L{:65536}",1)
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: gcc at pauldreik dot se
  Target Milestone: ---

The following program:

#include <format>

constexpr auto blah = std::format(L"{:65536}", 1);

is problematic when compiled with gcc 13 as well as the current trunk.

It triggers a pointer arithmetic error inside
https://gcc.gnu.org/git/?p=gcc.git;a=blame;f=libstdc%2B%2B-v3/include/std/format;hb=1d9454aba615eadd0d85c93713dd848227345f67#l295

it seems like the width is parsed into a unsigned short, but the return value
is not checked in the case the format string was not a char string (as above,
which is a wide char string).

The following patch fixes the problem:
commit 78ac41590432f4f01036797fd9d661f6ed80cf37 (HEAD -> master)
Author: Paul Dreik <gccpatc...@pauldreik.se>
Date:   Tue Aug 22 19:16:57 2023 +0200

    libstdc++: fix illegal pointer arithmetic in format

    when parsing a format string, the width is parsed into an unsigned short
    but the result is not checked in the case the format string is not a
    char string (such as a wide string). in case the parse fails,
    a null pointer is returned which is used for pointer arithmetic
    which is undefined behaviour.

    Signed-off-by: Paul Dreik <gccpatc...@pauldreik.se>

diff --git a/libstdc++-v3/include/std/format b/libstdc++-v3/include/std/format
index f3d9ae152f..fe2caa5868 100644
--- a/libstdc++-v3/include/std/format
+++ b/libstdc++-v3/include/std/format
@@ -285,7 +285,8 @@ namespace __format
          for (int __i = 0; __i < __n && (__first + __i) != __last; ++__i)
            __buf[__i] = __first[__i];
          auto [__v, __ptr] = __format::__parse_integer(__buf, __buf + __n);
-         return {__v, __first + (__ptr - __buf)};
+         if (__ptr) [[likely]]
+           return {__v, __first + (__ptr - __buf)};
        }
       return {0, nullptr};
     }