> If we try to call this function as add 2 1, where the first argument is larger than the second, then the compiler will reject the program at compile time

This is not entirely true in general. auto is only a best bet. It might not be able to prove something right and will fail. For example, if it has to recurse deeper than 100 constructors. In this case, the user of the function has to provide a proof himself.

See it as a bloom filter. if it doesn't fail, you're sure your function is correctly called. if it does fail, it might still be correctly called but the compiler just didn't have enough time to prove it. In that case a user needs to convince the compiler that he is right.