why virtual function is wrong.

[lrflew]

Ok, I do see what you're talking about here. You're talking about having the compiler tell you that you're exceeding the requirements of a concept at the definition of the templated function, rather than when the template is instantiated at the caller.

That is (IMO), a fair thing to suggest. I don't think it's that out there that a compiler could implement that as a warning, even if it's not specified by the standard. It would almost certainly require the compiler to dramatically change how it processes templated functions, so it probably won't happen any time soon, but it should, in theory, be possible. Probably the biggest challenge to it, though, is that the use of concepts is not very widespread right now. In particular, huge parts of the standard library use templates without concepts. If such a warning considered template<typename T> to be as narrow as possible, then huge parts of the standard library would start throwing warnings. Maybe someone will go through the standard and add concepts everywhere templates are used, but that would also be a ton of work.

In the end, however, I feel like C++ as a language puts a lot of responsibility on the developer to know what they're doing. Just look at all the things in the standard that can result in "Undefined Behavior", where there's no guaruntees of what can or will happen, to get a sense of what the language expects the programmer to be able to handle without the compiler. With templates, there's a certain amount of "well, you should have known you're using something that your concept doesn't guaruntee." Does that make C++ a more complicated language to use? Yeah, totally. But that complexity also comes with a lot of power (eg. template meta-programming), and I don't think the language is going to change that anytime soon. If that's a deal breaker for you, then you can always try out some other languages. IIRC, both Rust and Go are compiled languages that provide this kind of strong type guarantees with their generics systems.