[Bug c/106904] New: Incorrect -Wstringop-overflow with partial memcpy() into a nested structure

["zfigura at codeweavers dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106904

            Bug ID: 106904
           Summary: Incorrect -Wstringop-overflow with partial memcpy()
                    into a nested structure
           Product: gcc
           Version: 12.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: zfigura at codeweavers dot com
  Target Milestone: ---

Created attachment 53562
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=53562&action=edit
minimal test case

I encountered a warning while trying to compile 32-bit wine 7.17 with gcc 12.2,
specificially at this line here:

https://source.winehq.org/git/wine.git/blob/wine-7.17:/dlls/win32u/message.c#l359

The relevant code copies a smaller structure into a larger one of a different
type. (This may be a violation of aliasing rules, but adding
-fno-strict-aliasing doesn't change anything.)

I was able to reproduce this with a minimal test case. This is a very weird set
of conditions, but I couldn't seem to reduce this test case any further.
Changing the type of "ps" to "struct packed_windowpos" makes the error go away;
so does changing the first argument of the memcpy to "ps".

leslie@terabithia:~$ gcc --version
gcc (Debian 12.2.0-1) 12.2.0
Copyright (C) 2022 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

leslie@terabithia:~$ gcc -m32 test.c -c -o test.o -Wall -O2
test.c: In function ‘func’:
test.c:26:5: warning: writing 8 bytes into a region of size 4
[-Wstringop-overflow=]
   26 |     __builtin_memcpy(&ps->wp, &wp, sizeof(wp));
      |     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
test.c:9:9: note: destination object ‘hwnd’ of size 4
    9 |     int hwnd;
      |         ^~~~