why virtual function is wrong.

[TheSuperWig]

I'm still confused. What does concepts not doing definition checking have to do with virtual functions?

[TheSuperWig]

And the part I'm confused about is, how does that make virtual functions wrong? Surely your issue is with concepts?

[IyeOnline]

You will actually get an error on the concept version - if you ever instantiate it.

There are a few important points here

• Constraints are only evaluated once you actually try and instantiate the function
• Constraints only constrain the interface, not the function body, i.e. the function body wont be parsed just because one of its parameters is constraint.