[Bug]: flat_hash_map with incomplete variant value fails to compile in MSVC and C++20

[briangreenery]

Describe the issue

I built with ee47800 (the current commit as of opening this issue).

On MSVC 2022 17.14.18 (October 2025) and /std:c++20 the following code fails to compile with this output:
https://gist.github.com/briangreenery/08a64ec3a0d04d33d3d2eb292ad7456e

#include "absl/container/flat_hash_map.h"
#include "absl/types/variant.h"

struct A;
struct B;

using Any = absl::variant< A, B >;

struct B
{
    absl::flat_hash_map< int, Any > map;
};

This compiles successfully when building using C++17. Like #1930, the problem seems to be related to IsLifetimeBoundAssignmentFrom, since commenting that out allows it to build.

Steps to reproduce the problem

Try to build this code with /std:c++20

#include "absl/container/flat_hash_map.h"
#include "absl/types/variant.h"

struct A;
struct B;

using Any = absl::variant< A, B >;

struct B
{
    absl::flat_hash_map< int, Any > map;
};

What version of Abseil are you using?

ee47800

What operating system and version are you using?

Windows 22H2

What compiler and version are you using?

MSVC 2022 17.14.18 (October 2025)

What build system are you using?

cmake 3.26.6

Additional context

This is the compiler output:
https://gist.github.com/briangreenery/08a64ec3a0d04d33d3d2eb292ad7456e

[higher-performance]

Would you by any chance like to take the opportunity to propose a fix in a pull request, similar to the one you noticed in #1931 (comment) for std::pair?