ignored fwrite results trigger a gcc-8.2 -Wunused-result

[BillyDonahue]

We don't use cmake, so I'm doing a naive compile of a libfmt subtree in my own toolchain.
(using fmt @ e979c78)

I could easily add the -Wno-unused-result to my build, but maybe you'd want to tackle it upstream. This is gcc-8.2 on an Ubuntu 18.04 workstation, btw.
These could perhaps be mitigated with some (void) casts or pragmas or something.

 [2019/03/28 17:32:10.092] /opt/mongodbtoolchain/v3/bin/g++ \
    -o build/cached/third_party/fmt/dist/src/format.o \
    -c -Woverloaded-virtual -Wno-maybe-uninitialized -std=c++17 \
    -fno-omit-frame-pointer -fno-strict-aliasing -ggdb -pthread \
    -Wall -Wsign-compare -Wno-unknown-pragmas -Winvalid-pch -Werror \
    -O2 \
    -Wno-unused-local-typedefs -Wno-unused-function -Wno-deprecated-declarations -Wno-unused-const-variable -Wno-unused-but-set-variable -Wno-missing-braces \
    -fstack-protector-strong -fno-builtin-memcmp -fPIE \
    -DPCRE_STATIC -DNDEBUG -D_XOPEN_SOURCE=700 -D_GNU_SOURCE -D_FORTIFY_SOURCE=2 -DBOOST_SYSTEM_NO_DEPRECATED -DBOOST_MATH_NO_LONG_DOUBLE_MATH_FUNCTIONS -DABSL_FORCE_ALIGNED_ACCESS \
    -Isrc/third_party/fmt/dist/include -Ibuild/cached -Isrc \
    src/third_party/fmt/dist/src/format.cc


 [2019/03/28 17:32:11.072] In file included from src/third_party/fmt/dist/src/format.cc:8:
 [2019/03/28 17:32:11.072] src/third_party/fmt/dist/include/fmt/format-inl.h: In function 'void fmt::v5::{anonymous}::report_error(fmt::v5::{anonymous}::FormatFunc, int, fmt::v5::string_view)':
 [2019/03/28 17:32:11.072] src/third_party/fmt/dist/include/fmt/format-inl.h:190:14: error: ignoring return value of 'size_t fwrite(const void*, size_t, size_t, FILE*)', declared with attribute warn_unused_result [-Werror=unused-result]
 [2019/03/28 17:32:11.072]    std::fwrite(full_message.data(), full_message.size(), 1, stderr);
 [2019/03/28 17:32:11.072]    ~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 [2019/03/28 17:32:11.072] src/third_party/fmt/dist/include/fmt/format-inl.h: In function 'void fmt::v5::vprint(FILE*, fmt::v5::string_view, fmt::v5::format_args)':
 [2019/03/28 17:32:11.072] src/third_party/fmt/dist/include/fmt/format-inl.h:910:14: error: ignoring return value of 'size_t fwrite(const void*, size_t, size_t, FILE*)', declared with attribute warn_unused_result [-Werror=unused-result]
 [2019/03/28 17:32:11.072]    std::fwrite(buffer.data(), 1, buffer.size(), f);
 [2019/03/28 17:32:11.072]    ~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 [2019/03/28 17:32:11.072] src/third_party/fmt/dist/include/fmt/format-inl.h: In function 'void fmt::v5::vprint(FILE*, fmt::v5::wstring_view, fmt::v5::wformat_args)':
 [2019/03/28 17:32:11.072] src/third_party/fmt/dist/include/fmt/format-inl.h:916:14: error: ignoring return value of 'size_t fwrite(const void*, size_t, size_t, FILE*)', declared with attribute warn_unused_result [-Werror=unused-result]
 [2019/03/28 17:32:11.072]    std::fwrite(buffer.data(), sizeof(wchar_t), buffer.size(), f);
 [2019/03/28 17:32:11.072]    ~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

[vitaut]

We should check the result of fwrite and throw fmt::system_error in case of an error as it is done in

fmt/src/posix.cc

Line 161 in e979c78

if (result < 0) FMT_THROW(system_error(errno, "cannot write to file"));

. Could you by any chance submit a PR?