[tialaramex]

> For starters epochs could never work in ecosystems that value binary libraries

Not solving everybody's problem is not the same thing as not solving anybody's problem.

You present subtle considerations which, ignoring the fact that they assume Epochs don't exist in C++ 20 yet ask what Epochs should have done in C++ 20 - might have taken up committee time if Epochs had advanced, and which now can only be answered in the vaguest way, they should definitely have decided on a coherent strategy for resolving such problems.

It is true that for any conceivable change, Hyrum's Law applies, and so it would apply to Epochs just as it does for Rust's editions. Mara's "competition" for writing Rust that gives different results when re-formatted provides examples of the sort of stuff Hyrum's Law invariably breaks. Nobody should be writing non-toy programs like that and in Rust it seems like nobody is. It's a sad fact that too often C++ programs are written in a very fragile way and many of them can and do break with the least provocation.

You can't magically rewrite all those programs, but you can make fragile techniques like SFINAE unattractive to propagate into new programs, and I argue Epochs would have allowed C++ to begin the much harder part of Stroustrup's ever-evolving quest to ship a good programming language - not adding yet more kitchen sinks but removing parts of the language that in hindsight were a bad idea and revisiting old design decisions in the light of what has been learned.

[pjmlp]

That is the thing with ISO/ECMA languages, it has to work in every possible scenario there is a certified compiler, if not then as you know, we go down the path of nasal daemons.

For example, the Epoch semantics in xlCC would have to behave as in VC++, to avoid too many nasty surprises in cross platform code that is in production for decades.

[tialaramex]

First, it seems weird to suggest that "nasal demons" are a particular fear of WG21 at this point. As a reminder when C++ got an Optional type std::optional the committee insisted that it have Undefined Behaviour if you for any reason don't check whether it is empty before using the contents.

We're not talking about 1998 here. This is in 2017. By this point plenty of people have experience already using languages with Optional types that do what you actually want here, but the C++ committee decided no, C++ is the footgun language, it's what we're known for.

But OK, in an alternate universe where the committee doesn't introduce footguns on purpose to prove their Real Programmer credibility, and where we do get Epochs, what should happen for fraught situations so as to deliver consistency?

The committee should decide on a rule. I know that's often portrayed as too difficult for such problems, but it won't get easier in subsequent versions. That's why I think it would have made sense to delay long awaited work like Concepts if that was the only way to land Epochs. Concepts is already too late for the main act and being a little later barely makes a difference. Whereas Epochs gets harder to do every version, so the sooner the better.