Fix write_significand buffer size to avoid GCC 16 LTO -Wstringop-overflow (#4767)

[MuskanKhatwani]

Description

• Increase the temporary buffer in write_significand to ensure there is enough headroom for backward writes performed by the digit-writing path under LTO.
• Change: digits10() + 2 -> digits10() + 3 in include/fmt/format.h.

Rationale

• GCC 16 with LTO reports -Wstringop-overflow in the floating-point formatting path because the backward-writing routine can touch significand_size + 1 bytes while the previous buffer allocation was tight in one edge case.
• Increasing the buffer by one element removes the off-by-one risk without changing observable behavior.

Verification

• Built on Windows (MSYS2 UCRT64) with:
  • gcc 16.2.0, g++ 16.2.0, cmake 4.4.3
  • cmake configured with -DCMAKE_INTERPROCEDURAL_OPTIMIZATION=ON
• Example link-time warning excerpt observed during build (for reviewer context):

warning: writing 2 bytes into a region of size 0 [-Wstringop-overflow=]
include/fmt/format.h:1375:11: memcpy(out, digits2(value), 2);
note: at offset [-13, -2] into destination object 'buffer' of size 12
include/fmt/format.h:2659:8: Char buffer[digits10() + 3];

Compatibility & Risk

• No API or behavioral changes.
• Minimal internal change to buffer sizing; low risk.

Suggested testing

• Build with GCC 16 + LTO (-flto) on CI (MSYS2/Ubuntu with GCC 16) to confirm no false-positive -Wstringop-overflow remains.
• Run unit tests with and without LTO to ensure no regressions.

Notes

• If maintainers prefer not to change allocation, an alternate approach is to gate a compiler warning suppression for affected toolchains. The buffer-size fix is simpler and safer.

[MuskanKhatwani]

Hi, attaching the full verbose LTO build log (lto-build-log.txt) I captured locally.
Summary:

• Toolchain: MSYS2 UCRT64, gcc/g++ 16.2.0, CMake 4.4.3
• Built with -DCMAKE_INTERPROCEDURAL_OPTIMIZATION=ON
• The log contains the link-time -Wstringop-overflow warnings observed during LTO.
  Thanks for taking a look. I can also:
• produce a tiny standalone repro (CMake + source) to reproduce the warning, or
• run additional builds/tests and attach their logs — tell me which and I’ll add it.
  lto-build-log.txt

[vitaut]

This looks like a false positive but please provide a godbolt repro.