warning: operator delete called on unallocated object - c++

Question

I'm trying to compile the following code (this is a minimal example), but I get a warning I can't figure out:
#include <string>
#include <variant>
#include <vector>
struct Bar {
std::wstring x = L"";
};
Bar Foo() {
std::variant<std::vector<int>, Bar> tmp = Bar();
if (std::holds_alternative<Bar>(tmp)) return std::move(std::get<Bar>(tmp));
return Bar();
}
I'm trying to build this with g++ -std=c++20 -Wall -Wextra -O2 /tmp/test.cc -c
I get the following warning:
In file included from /usr/include/x86_64-linux-gnu/c++/12/bits/c++allocator.h:33,
from /usr/include/c++/12/bits/allocator.h:46,
from /usr/include/c++/12/string:41,
from /tmp/test.cc:1:
In member function ‘void std::__new_allocator<_Tp>::deallocate(_Tp*, size_type) [with _Tp = int]’,
inlined from ‘constexpr void std::allocator< <template-parameter-1-1> >::deallocate(_Tp*, std::size_t) [with _Tp = int]’ at /usr/include/c++/12/bits/allocator.h:200:35,
inlined from ‘static constexpr void std::allocator_traits<std::allocator<_CharT> >::deallocate(allocator_type&, pointer, size_type) [with _Tp = int]’ at /usr/include/c++/12/bits/alloc_traits.h:496:23,
inlined from ‘constexpr void std::_Vector_base<_Tp, _Alloc>::_M_deallocate(pointer, std::size_t) [with _Tp = int; _Alloc = std::allocator<int>]’ at /usr/include/c++/12/bits/stl_vector.h:387:19,
inlined from ‘constexpr std::_Vector_base<_Tp, _Alloc>::~_Vector_base() [with _Tp = int; _Alloc = std::allocator<int>]’ at /usr/include/c++/12/bits/stl_vector.h:366:15,
inlined from ‘constexpr std::vector<_Tp, _Alloc>::~vector() [with _Tp = int; _Alloc = std::allocator<int>]’ at /usr/include/c++/12/bits/stl_vector.h:733:7,
inlined from ‘constexpr void std::destroy_at(_Tp*) [with _Tp = vector<int>]’ at /usr/include/c++/12/bits/stl_construct.h:88:18,
inlined from ‘constexpr void std::_Destroy(_Tp*) [with _Tp = vector<int>]’ at /usr/include/c++/12/bits/stl_construct.h:149:22,
inlined from ‘std::__detail::__variant::_Variant_storage<false, std::vector<int, std::allocator<int> >, Bar>::_M_reset()::<lambda(auto:11&&)> mutable [with auto:11 = std::vector<int>&]’ at /usr/include/c++/12/variant:472:19,
inlined from ‘constexpr _Res std::__invoke_impl(__invoke_other, _Fn&&, _Args&& ...) [with _Res = void; _Fn = __detail::__variant::_Variant_storage<false, vector<int, allocator<int> >, Bar>::_M_reset()::<lambda(auto:11&&)>; _Args = {vector<int, allocator<int> >&}]’ at /usr/include/c++/12/bits/invoke.h:61:36,
inlined from ‘constexpr std::enable_if_t<is_invocable_r_v<_Res, _Callable, _Args ...>, _Res> std::__invoke_r(_Callable&&, _Args&& ...) [with _Res = void; _Callable = __detail::__variant::_Variant_storage<false, vector<int, allocator<int> >, Bar>::_M_reset()::<lambda(auto:11&&)>; _Args = {vector<int, allocator<int> >&}]’ at /usr/include/c++/12/bits/invoke.h:111:28,
inlined from ‘static constexpr decltype(auto) std::__detail::__variant::__gen_vtable_impl<std::__detail::__variant::_Multi_array<_Result_type (*)(_Visitor, _Variants ...)>, std::integer_sequence<long unsigned int, __indices ...> >::__visit_invoke(_Visitor&&, _Variants ...) [with _Result_type = void; _Visitor = std::__detail::__variant::_Variant_storage<false, std::vector<int, std::allocator<int> >, Bar>::_M_reset()::<lambda(auto:11&&)>&&; _Variants = {std::variant<std::vector<int, std::allocator<int> >, Bar>&}; long unsigned int ...__indices = {0}]’ at /usr/include/c++/12/variant:1035:40,
inlined from ‘constexpr decltype(auto) std::__do_visit(_Visitor&&, _Variants&& ...) [with _Result_type = void; _Visitor = __detail::__variant::_Variant_storage<false, vector<int, allocator<int> >, Bar>::_M_reset()::<lambda(auto:11&&)>; _Variants = {variant<vector<int, allocator<int> >, Bar>&}]’ at /usr/include/c++/12/variant:1783:5,
inlined from ‘constexpr void std::__detail::__variant::_Variant_storage<false, _Types ...>::_M_reset() [with _Types = {std::vector<int, std::allocator<int> >, Bar}]’ at /usr/include/c++/12/variant:470:23,
inlined from ‘constexpr std::__detail::__variant::_Variant_storage<false, _Types ...>::~_Variant_storage() [with _Types = {std::vector<int, std::allocator<int> >, Bar}]’ at /usr/include/c++/12/variant:480:17,
inlined from ‘constexpr std::__detail::__variant::_Copy_ctor_base<false, std::vector<int, std::allocator<int> >, Bar>::~_Copy_ctor_base()’ at /usr/include/c++/12/variant:554:12,
inlined from ‘constexpr std::__detail::__variant::_Move_ctor_base<false, std::vector<int, std::allocator<int> >, Bar>::~_Move_ctor_base()’ at /usr/include/c++/12/variant:591:12,
inlined from ‘constexpr std::__detail::__variant::_Copy_assign_base<false, std::vector<int, std::allocator<int> >, Bar>::~_Copy_assign_base()’ at /usr/include/c++/12/variant:629:12,
inlined from ‘constexpr std::__detail::__variant::_Move_assign_base<false, std::vector<int, std::allocator<int> >, Bar>::~_Move_assign_base()’ at /usr/include/c++/12/variant:681:12,
inlined from ‘constexpr std::__detail::__variant::_Variant_base<std::vector<int, std::allocator<int> >, Bar>::~_Variant_base()’ at /usr/include/c++/12/variant:735:12,
inlined from ‘constexpr std::variant<_Types>::~variant() [with _Types = {std::vector<int, std::allocator<int> >, Bar}]’ at /usr/include/c++/12/variant:1407:28,
inlined from ‘Bar Foo()’ at /tmp/test.cc:13:1:
/usr/include/c++/12/bits/new_allocator.h:158:33: warning: ‘void operator delete(void*, std::size_t)’ called on unallocated object ‘tmp’ [-Wfree-nonheap-object]
158 | _GLIBCXX_OPERATOR_DELETE(_GLIBCXX_SIZED_DEALLOC(__p, __n));
| ^
/tmp/test.cc: In function ‘Bar Foo()’:
/tmp/test.cc:10:39: note: declared here
10 | std::variant<std::vector<int>, Bar> tmp = Bar();
| ^~~
g++ --version says g++ (Debian 12.2.0-10) 12.2.0
I know this code, as is, doesn't make that much sense; but it also shouldn't have this problem: this is a minimal example that exhibits this warning, of a much more complex function.
Strangely, the warning disappears if I do any of the following:
Change Bar::x to be a std::string.
Change tmp to be std::variant<int, Bar> (rather than having a std::vector<int> as the first type).
Remove g++ command-line flag -O2 (i.e., compile with g++ -std=c++20 -Wall -Wextra -O2 /tmp/test.cc -c).
Why would any of these changes make the warning go away!?
If I build this with --std=c++17, the problem remains.

Answer 1

The -Wfree-nonheap-object flag of gcc has a series of false positives bugs, among which bug 99098 is used to record these meta-bugs.
The bug in your example is similar to bug 108088 and has been listed, which issues a false positive warning message since gcc-12.