[Bug c++/68901] New: UBSan triggers false -Wpadded warning

[MildShoe at mailinator dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68901

            Bug ID: 68901
           Summary: UBSan triggers false -Wpadded warning
           Product: gcc
           Version: 5.2.0
               URL: http://coliru.stacked-crooked.com/a/6aa2bc2bd5ef0e01
            Status: UNCONFIRMED
          Keywords: diagnostic
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: MildShoe at mailinator dot com
  Target Milestone: ---
              Host: x86_64
            Target: x86_64

When compiling with -fsanitize=undefined certain structs trigger the -Wpadded
warning while without the sanitizer they do not. Judging by the values returned
by sizeof/alignof the emitted warning is actually false.

The expected size of the sample struct is 16 bytes on x86_64, with 8 byte
alignment. The expected values are printed with and without UBSan, but the
-Wpadded warning is only emitted when UBSan is enabled.

See the attached Coliru link for the code snippet and compiler options used
during compilation.

Note that this behavior is also present in GCC 4.9.x and 5.1.x