[Bug c++/83780] New: False positive alignment error with -fsanitize=undefined with virtual base

["securesneakers at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83780

            Bug ID: 83780
           Summary: False positive alignment error with
                    -fsanitize=undefined with virtual base
           Product: gcc
           Version: 7.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: securesneakers at gmail dot com
  Target Milestone: ---

Created attachment 43091
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=43091&action=edit
Minimal example that reproduces the issue

Attached program generates false misalignment errors when compiled with
-fsanitize=undefined

$ g++ --version
g++ (GCC) 7.2.1 20171224

$ uname -s -m
Linux x86_64

$ g++ -std=c++11 -O2 -fsanitize=undefined minimal.cpp && ./a.out
minimal.cpp:9:8: runtime error: constructor call on misaligned address
0x7ffdd1e1e658 for type 'struct Base2', which requires 16 byte alignment

Attached example contains following hierarchy:

struct alignas(16) Base1 { };
struct Base2 : virtual Base1 { };
struct Base3 : virtual Base2 { };

alignof(Base2) is set to 16 due to alignment of its base class. But when Base3
is instantiated, Base2 is placed with alignment of 8 as it should be according
to Itanium C++ ABI (due to its non-virtual alignment being equal 8):
https://refspecs.linuxfoundation.org/cxxabi-1.75.html#class-types. Yet
sanitizer complains about alignment not being 16.

Seems that sanitizer checks address using "normal" alignment when "non-virtual
alignment" should be used.