[Bug c++/94923] New: False positive -Wclass-memaccess with trivially copyable std::optional

["dawid_jurek at vp dot pl" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94923

            Bug ID: 94923
           Summary: False positive -Wclass-memaccess with trivially
                    copyable std::optional
           Product: gcc
           Version: 9.3.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: dawid_jurek at vp dot pl
  Target Milestone: ---

Consider following C++ snippet:

static_assert(std::is_trivially_copyable_v<std::optional<char>>);
static void not_ok() {
  std::optional<char> value;
  std::byte buf[128;
  std::memcpy(&buf[0], &value, sizeof value);
  std::memcpy(&value, &buf[0], sizeof value);
}

With above snippet fresh x86-64 gcc trunk reports following warning:

source>: In function 'void not_ok()':

<source>:27:46: warning: 'void* memcpy(void*, const void*, size_t)' copying an
object of non-trivial type 'class std::optional<char>' from an array of 'enum
class std::byte' [-Wclass-memaccess]

   27 |     std::memcpy(&value, &buf[0], sizeof value);

[basic.types]/2 explicitly permits for such round trip for every trivially
copyable T via std::memcpy even if T is not trivial.
Also clang accept such code without complains (take a look at link below).
Changing std::byte buf[128; to char buf[128]; suppress the warning so seems
that std::byte is relevant.
You can find full source code here https://godbolt.org/z/JrcnS5

Regards,
Dawid