[Bug c++/85707] New: -Wclass-memaccess should excempt safe usage inside of a class and its friends

[redbeard0531 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85707

            Bug ID: 85707
           Summary: -Wclass-memaccess should excempt safe usage inside of
                    a class and its friends
           Product: gcc
           Version: 8.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: redbeard0531 at gmail dot com
  Target Milestone: ---

I get the point of the warning and would like to be able to turn it on. However
we have some types that have been specifically designed to be memset and
memcpy/memmove-able as long as certain rules are followed. This is an
implementation detail of the type so non-friend users are expected to use the
default/copy/move constructors, rather than directly manipulating the bytes.
However classes (and their friends) are always allowed to violate their own
invariants, even if external users aren't. That is why I think the warning
should be suppressed* inside of contexts that have access to internals.

*Ideally it would still trip if the class's members were not mem-accessible by
it, but it seems more important (to me) to avoid the false-positives than to
avoid these kinds of false negatives if only one is possible.

I already know about the void*-cast to suppress the warning. I tried doing that
in our code base and it was required in too many places, all of which were
correct (as in no actual misuse was found). Additionally, this trips in
third-party code that we'd rather not alter unless there is an actual bug.

As a potential alternative, adding an attribute like [[gnu::memaccessable]]
that can be put on a type to suppress the warning for uses of that type might
also work.