stringop-overflow warning in 8.1.1 on g++ (GCC) 10.2.1 20200804

[andrewkcorcoran]

In function ‘copy2’,
    inlined from ‘format_decimal’ at /local/1/home/build/gcc/_deps/fmt-src/include/fmt/format.h:1124:8,
    inlined from ‘format_decimal’ at /local/1/home/build/gcc/_deps/fmt-src/include/fmt/format.h:1134:28,
    inlined from ‘write.constprop’ at /local/1/home/build/gcc/_deps/fmt-src/include/fmt/chrono.h:1663:36:
/local/1/home/build/gcc/_deps/fmt-src/include/fmt/format.h:1090:11: error: writing 2 bytes into a region of size 0 [-Werror=stringop-overflow=]
 1090 |     memcpy(dst, src, 2);
      |           ^
/local/1/home/build/gcc/_deps/fmt-src/include/fmt/chrono.h: In member function ‘write.constprop’:
/local/1/home/build/gcc/_deps/fmt-src/include/fmt/format.h:1133:8: note: at offset -2 to object ‘buffer’ with size 10 declared here
 1133 |   Char buffer[digits10<UInt>() + 1];
      |        ^
In function ‘format_decimal’,
    inlined from ‘format_decimal’ at /local/1/home/build/gcc/_deps/fmt-src/include/fmt/format.h:1134:28,
    inlined from ‘write.constprop’ at /local/1/home/build/gcc/_deps/fmt-src/include/fmt/chrono.h:1663:36:
/local/1/home/build/gcc/_deps/fmt-src/include/fmt/format.h:1120:12: error: writing 1 byte into a region of size 0 [-Werror=stringop-overflow=]
 1120 |     *--out = static_cast<Char>('0' + value);
      |            ^
/local/1/home/build/gcc/_deps/fmt-src/include/fmt/chrono.h: In member function ‘write.constprop’:
/local/1/home/build/gcc/_deps/fmt-src/include/fmt/format.h:1133:8: note: at offset -1 to object ‘buffer’ with size 10 declared here
 1133 |   Char buffer[digits10<UInt>() + 1];

[vitaut]

Please provide a repro (preferably on godbolt).

[andrewkcorcoran]

A simple fmt::format("{}", std::chrono::nanoseconds{0}); reproduces it on my system but not on godbolt. Possibly a bug in GCC10?

/opt/rh/devtoolset-10/root/usr/bin/c++ -DFMT_LOCALE -I/local/1/home/build/gcc/_deps/fmt-src/include  -g -march=native -fno-var-tracking-assignments -Werror -Wall -Wextra -Wpedantic -Wshadow -Wnon-virtual-dtor     -Wold-style-cast -Wunused -Woverloaded-virtual -Wdouble-promotion -O3 -DNDEBUG   -std=c++2a -o CMakeFiles/fmt_test.dir/fmt.cpp.o -c /local/1/home/source/bin/fmt.cpp

[phprus]

This is a bug in GCC: #2442 (comment)

[andrewkcorcoran]

Ah ok :/. Is there a known workaround when using fmt that doesn't involve globally disabling this warning for the project including fmt?

[vitaut]

You can set the FMT_SYSTEM_HEADERS cmake variable as a workaround.

[vitaut]

The false positive has been worked around in 7f157dc.