Communication of version isn't even worth thinking about until you solve the interoperability problem. That is I have two unrelated libraries, one using std::foov1 and one std::foov2. How do i, in a performant and not too ugly way move data between these two libraries.

So long as the above problem isn't solved I need to build everything against the same standard library and so a simple system wide flag is all we need. Or maybe some for of package config (currently there is no C++ package manager but many implementations have something. Cmake for example) my build system can throw an error if i'm mixing versiones.

I have some ideas for how to solve the problem, but they only work in specific circumstances and not general. Where they work I don't need any marker like you propose. Though i'm open to other solutions that do. However either way, start by. Proposing something that breaks ABI, then propose a work around to allow mixing old and new (where new is aware of old), and then only if a marker is needed do we add it.

-- 
  Henry Miller
  hank_at_[hidden]
On Mon, Sep 19, 2022, at 20:21, William Linkmeyer via Std-Proposals wrote:
> # Versioned Standard
>
> ### Document: <awaiting>
>
> ### Date: September 19, 2022
>
> ### Audience: LEWG
>
> ### Reply To: willhl_at_[hidden]
>
> ## Introduction
>
> ABI stability is a communication problem. Libraries, meant to be 
> shared, often version themselves. The obvious purpose is to communicate 
> change over time. 
>
> The standard library itself is often versioned. Some shared standard 
> library, `libc++.so.0`, may differ from some other shared standard 
> library, `libc++.so.9`. 
>
> There is no reliable channel of communication between between libraries 
> and their associated standards.
>
> ## Motivation
>
> ABI is a perennial issue. Tools must be provided to users who wish to 
> specify their intended standard library version. 
>
> ## Implementation
>
> Introduce the *language* template to specify the intended standard 
> version range to be used:
>
> ```c++
> using std = std::v<2017, 2023>;
> ```
>
> Which will embed an implementation-defined hint within the resulting binary. 
>
> WL
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