> Here I'm way out of my depth; I'm a C++ language implementer, not a user.
> Maybe SFINAE makes many kinds of library feature testing unnecessary.
>
> But at the very least, I know that there's no way to use SFINAE to see
whether
> an implementation has a specific library header. Would there be value in
> enabling people to write code that will adapt at that level to
implementations
> that have made different degrees of progress toward implementing standard
> library features?
>
> Or would it make more sense just to limit our scope to language features?

Can I ask a stupid question?

What are we defining as a "feature"?

And should we be able to detect it at compile time, or runtime, or both?

Niall