It seems that the following example should be valid since C++20 (patched by P2564R3).

```
#include <variant>

consteval auto func(const std::variant<int>& v1, const std::variant<int>& v2) {
    return std::visit([](auto x, auto y) consteval { return x + y; }, v1, v2);
}

static_assert(func(std::variant<int>{42}, std::variant<int>{1729}) == 1771);
```

But libstdc++ currently rejects it (https://godbolt.org/z/h59hWK9G3), because the there's a hand-made vtable which will contain pointers to immediate (immediate-escalated) functions while being constexpr.

I guess we can make the vtable a non-constexpr variable of automatic storage duration when the vtable can't be constexpr. It seems that we can determine this by testing that `__some_alias_template<__gen_vtable_impl<...>::_S_apply>()>` is not a valid type.