C++0x to save the day

C++ appeals to serious writers of useful libraries, and users of those libraries.  

There's nothing freaky here. The compiler can't know, by itself, that a template argument must
model a numeric type.  Concepts allow the library writer to say so. Then, when a user tries to
use the template on something that isn't numeric, the compiler can say so right there, instead
of complaining that some expression down in the function body is ill-formed.

As noted above, macros in C have the same problem, but hardly anybody tries to do anything
ambitious with C macros; far fewer do it successfully, in part because of precisely this sort
of problem.  In essence, a C++ library can extend the compiler, and compiler extensions need
their own error messages.  Concepts gives library writers a practical way to provide them, to
describe the error in terms of the library interface, not in terms of details of the library
implementation, which is all a compiler could conceivably do by itself.