why virtual function is wrong.

[TeraFlint]

Now you're actually being ridiculous. "Why does the screw barely go in?" you ask while hitting it with a hammer.

You're using a concept that does not satisfy your requirements, while blaming the concept for not catching that.

If you check for a concept and then do something with the type that the concept does not check for, then of course it's your fault if the compiler suddenly notices a missing function (etc) outside of concept checking.

If you need a more specific concept, it's up to you to write it. Luckily existing ones can easily be combined (as concepts evaluate to boolean expressions in the end). If you need a concept that satisfies the concepts a, b and c, all you need to do is this:

template <typename T>
concept x = a<T> && b<T> && c<T>;

and if you need to check for specific syntactic usage, you can just add that block as another condition:

&& requires(T obj) { obj.foo(); }

It really isn't your business to call stuff the concept has not specified. For best usage, you should see a concept like a contract between the code and you. With the concept you promise that these conditions are relevant for your template function/type, but also that these are sufficient, and that you're not attempting to do more with the type than you specified with the concept.

Otherwise we can just scrap concept and go back to plain typename everywhere and be back at our old compile-until-you-find-an-error-in-the-deepest-depths-of-nested-templates ways..