[gumby]

> but the complaint should be about the lack of epochs

Rather than something as coarse as epochs, C++ includes a lot of fine grained feature test preprocessor definitions such as (to pick one at random) '__cpp_lib_constexpr_algorithms'.

It's harder to make this work with breaking API changes of course. I assume non-preprocessor versions that work with modules will be available in C++23...and perhaps they will support API changes?

[ncmncm]

Epochs would be more usable. Nobody can afford to litter their code with hundreds of feature tests.

API changes that change semantics are forbidden in Standard interfaces, although APIs can be extended, backward-compatibly. To make an actual change, we need to introduce a new name. Thus, when we get around to modernizing std::vector, the fixed version will have a different name, maybe std::vec, but conceivably std2::vector.

I don't know of any plans for feature test macro analogs that would integrate with modules.

To be usable, epochs would have to apply locally to a file, not to anything #included into that file, nor to any file #including it. And, it should be possible to tie them to modules, to say "you can use this module only from code in epoch C++XX or later".

[gumby]

> Thus, when we get around to modernizing std::vector, the fixed version will have a different name, maybe std::vec, but conceivably std2::vector.

Let us hope that if such a name transition ever happens that the hideous plague of multi-case identifiers does not infect the standard. Leave that to (a subset of) user code.

Either std26::vector or std::vec would work for me. `std26::vec` would probably be better because of possible confusion (as to which vector implementation is desired) due to use of `using` directives.

[gumby]

> I don't know of any plans for feature test macro analogs that would integrate with modules.

What is the plan, if any, for deciding at compile time whether the format module is available or if I would still have to use fmtlib/fmt?

[ncmncm]

Sorry, I haven't paid any attention at all to fmt.