bit_cast memcpy warnings with -Wall -Wextra

[fkallen]

format.h: In instantiation of ‘To fmt::v9::detail::bit_cast(const From&) [with To = fmt::v9::detail::uint128_fallback; From = long double; typename std::enable_if<(sizeof (To) == sizeof (From)), int>::type = 0]’:
...

format.h:290:12: warning: ‘void* memcpy(void*, const void*, size_t)’ copying an object of non-trivial type ‘class fmt::v9::detail::uint128_fallback’ from an array of ‘const long double’ [-Wclass-memaccess]
290 | std::memcpy(&to, &from, sizeof(to));

[vitaut]

Could you provide a godbolt repro?

[fkallen]

The error does not appear on godbolt.

The reproducer is

#define FMT_HEADER_ONLY
#include <fmt/core.h>

When it is compiled with either
nvcc -std=c++14 -c -Xcompiler "-Wall" -Ifmt-master/include test1.cpp -o test1
or
g++ -std=c++14 -c -D__NVCC__ -Wall -I"fmt-master/include" test1.cpp -o test1
the warning occurs.
There is no warning with g++ -std=c++14 -c -Wall -I"fmt-master/include" test1.cpp -o test1.

nvcc version is V11.5.119 , g++ version is 9.4.0

The warning disappears if in format.h line https://github.com/fmtlib/fmt/blob/master/include/fmt/format.h#L290 is changed to std::memcpy((void*)&to, &from, sizeof(to)); From the gcc documentation:

Explicitly casting the pointer to the class object to void * or to a type that can be safely accessed by the raw memory function suppresses the warning.

[vitaut]

This looks like a bug in gcc because it should check whether the type is trivially copyable, not trivial (same as bit_cast). But I suppressed the warning anyway in e9ca7ea. Thanks for reporting.

[SoapGentoo]

Likely due to https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104855#c5