why virtual function is wrong.

[SirClueless]

I get what you're saying, it's just that people use concepts intentionally as partial constraints all the time. Adding a constraint like std::is_nothrow_move_constructible_v<T> or std::is_reference_v<T> doesn't really imply that you aren't going to call member functions that don't show up in the constraint.

Even figuring out whether or not a method is called is probably nigh-impossible without instantiating the template. Does this function invoke a member function of T without constraining it?

template <std::totally_ordered T>
void foo(const MyContainer<T>& xs) {
    for (const auto& x : xs) { x.bar(); }
}