[Bug libstdc++/96766] New: std::swap(std::variant, std::variant) triggers undefined behavior sanitizer

["kndevl at outlook dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96766

            Bug ID: 96766
           Summary: std::swap(std::variant, std::variant) triggers
                    undefined behavior sanitizer
           Product: gcc
           Version: 10.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: kndevl at outlook dot com
  Target Milestone: ---

Created attachment 49113
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=49113&action=edit
preprocessed source

This snippet 

```

#include <variant>

class Foo {
};
class Bar {
};

using T = std::variant<Foo, Bar>;

int main()
{
    T t1 { Foo {} };
    T t2 { Bar {} };
    std::swap(t1, t2);
    return 0;
}

```

triggers the following warning

```

/usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/10.2.0/../../../../include/c++/10.2.0/variant:444:20:
runtime error: implicit conversion from type 'std::size_t' (aka 'unsigned
long') of value 18446744073709551615 (64-bit, unsigned) to type
'std::__detail::__variant::_Variant_storage<true, Foo, Bar>::__index_type' (aka
'unsigned char') changed the value to 255 (8-bit, unsigned)
    #0 0x55c00ff464ea in std::__detail::__variant::_Variant_storage<true, Foo,
Bar>::_M_reset()
/usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/10.2.0/../../../../include/c++/10.2.0/variant:444
    #1 0x55c00ff476a4 in void std::__detail::__variant::_Move_ctor_base<true,
Foo, Bar>::_M_destructive_move<Bar>(unsigned short, Bar&&)
/usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/10.2.0/../../../../include/c++/10.2.0/variant:564
    #2 0x55c00ff472fb in auto std::variant<Foo, Bar>::swap(std::variant<Foo,
Bar>&)::{lambda(auto:1&&, auto:2)#1}::operator()<Bar&,
std::integral_constant<unsigned long, 1ul> >(Bar&,
std::integral_constant<unsigned long, 1ul>)
/usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/10.2.0/../../../../include/c++/10.2.0/variant:1588
    #3 0x55c00ff4701e in void std::__invoke_impl<void, std::variant<Foo,
Bar>::swap(std::variant<Foo, Bar>&)::{lambda(auto:1&&, auto:2)#1}, Bar&,
std::integral_constant<unsigned long, 1ul> >(std::__invoke_other,
std::variant<Foo, Bar>::swap(std::variant<Foo, Bar>&)::{lambda(auto:1&&,
auto:2)#1}&&, Bar&, std::integral_constant<unsigned long, 1ul>&&)
/usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/10.2.0/../../../../include/c++/10.2.0/bits/invoke.h:60
    #4 0x55c00ff46e3f in std::__invoke_result<std::variant<Foo,
Bar>::swap(std::variant<Foo, Bar>&)::{lambda(auto:1&&, auto:2)#1}, Bar&,
std::integral_constant<unsigned long, 1ul> >::type
std::__invoke<std::variant<Foo, Bar>::swap(std::variant<Foo,
Bar>&)::{lambda(auto:1&&, auto:2)#1}, Bar&, std::integral_constant<unsigned
long, 1ul> >(std::variant<Foo, Bar>::swap(std::variant<Foo,
Bar>&)::{lambda(auto:1&&, auto:2)#1}&&, (std::__invoke_result&&)...)
/usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/10.2.0/../../../../include/c++/10.2.0/bits/invoke.h:95
    #5 0x55c00ff458e3 in
std::__detail::__variant::__gen_vtable_impl<std::__detail::__variant::_Multi_array<std::__detail::__variant::__variant_idx_cookie
(*)(std::variant<Foo, Bar>::swap(std::variant<Foo, Bar>&)::{lambda(auto:1&&,
auto:2)#1}&&, std::variant<Foo, Bar>&)>, std::integer_sequence<unsigned long,
1ul> >::__visit_invoke(std::variant<Foo, Bar>::swap(std::variant<Foo,
Bar>&)::{lambda(auto:1&&, auto:2)#1}, std::variant<Foo, Bar>&)
/usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/10.2.0/../../../../include/c++/10.2.0/variant:1001
    #6 0x55c00ff4537c in decltype(auto)
std::__do_visit<std::__detail::__variant::__variant_idx_cookie,
std::variant<Foo, Bar>::swap(std::variant<Foo, Bar>&)::{lambda(auto:1&&,
auto:2)#1}, std::variant<Foo, Bar>&>(std::variant<Foo,
Bar>::swap(std::variant<Foo, Bar>&)::{lambda(auto:1&&, auto:2)#1}&&,
std::variant<Foo, Bar>&)
/usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/10.2.0/../../../../include/c++/10.2.0/variant:1694
    #7 0x55c00ff45188 in void
std::__detail::__variant::__raw_idx_visit<std::variant<Foo,
Bar>::swap(std::variant<Foo, Bar>&)::{lambda(auto:1&&, auto:2)#1},
std::variant<Foo, Bar>&>(std::variant<Foo, Bar>::swap(std::variant<Foo,
Bar>&)::{lambda(auto:1&&, auto:2)#1}&&, std::variant<Foo, Bar>&)
/usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/10.2.0/../../../../include/c++/10.2.0/variant:201
    #8 0x55c00ff4505a in std::variant<Foo, Bar>::swap(std::variant<Foo, Bar>&)
/usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/10.2.0/../../../../include/c++/10.2.0/variant:1570
    #9 0x55c00ff44451 in std::enable_if<(((is_move_constructible_v<Foo,
Bar>)&&...))&&(((is_swappable_v<Foo, Bar>)&&...)), void>::type std::swap<Foo,
Bar>(std::variant<Foo, Bar>&, std::variant<Foo, Bar>&)
/usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/10.2.0/../../../../include/c++/10.2.0/variant:1240
    #10 0x55c00ff44278 in main ../ubsan.cpp:14
    #11 0x7f7f64b19151 in __libc_start_main (/usr/lib/libc.so.6+0x28151)
    #12 0x55c00ff4414d in _start
(/home/user/main/cmake-build-debug-clang/ubsan-test+0x514d)

```

Is this a bug in libstdc++ or clang's sanitizer?