On Mon, 15 Jun 2020 at 19:47, Gabriel Dos Reis <gdr_at_[hidden]> wrote:
> | I don't have a shade of illusion of portability here; it's all about
> | selectively enabling superior
> | functionality when it's available, and it's not yet available
> | ubiquitously. Early adopters can adopt
> | early, without waiting for everything to synchronize into a brave new world.
>
> As a user depending on that superior functionality, how do I use it in practice when I am compiling with two different compilers with differing levels of support? Or, I am not supposed to see any difference?

That depends on the API/implementation choice of the library. There's
three options:

1) provide the full API everywhere with one implementation
2) provide the full API everywhere with two implementations, with the
intent to get rid of the pre-C++17 implementation
eventually
3) provide the full API for C++17 only, so as to keep its
implementation clean of pre-C++17 work-arounds

The macro enables (2) and (3). Option (1) can be done even without the macro.