Re: On Boost and modules: second part

From: Stephan T. Lavavej (stl_at_[hidden])
Date: 2024-04-19 20:39:58

[John Maddock]
> 5) Using the regex is an all or nothing affair, with msvc this works:
> import boost.regex;
> [...]
> as does:
> import std;
> import boost.regex;
> [...]
> But this blows up:
> #include <iostream>
> import boost.regex;

[Boris Kolpackov]
> We have tried this with 17.10 preview4 and there is definitely an
> improvement.

Yep, this was recorded in our Changelog. See https://github.com/microsoft/STL/wiki/Changelog#vs-2022-1710-preview-1 (or https://github.com/microsoft/STL/wiki/Changelog#vs-2022-1710 if you're reading this in the near future after 17.10 has released for production):

[STL Changelog]
> * Wrapped the STL in extern "C++" as a temporary workaround to allow
> #include <meow> to coexist with import std; in the same translation unit,
> in that order. #4154
>   + The other order, import std; before #include <meow>, will still cause
>   compiler errors. We're working on a long-term solution.

[Boris Kolpackov]
> Having said that, when we tried to enable `import std;` in a more
> realistic project, things did fall apart pretty quickly due to
> what looks like hard to pin-point interactions between include
> and import.

If you can reduce this to something self-contained that rigorously follows our current implementation restriction of include-before-import, we would be happy to investigate.

I know the experience is messy, but it's improving, and the only way it can improve is for eager early adopters (including me; the STL is the compiler's first and closest user) to report the problems they encounter so that "it doesn't work" can be resolved into fine-grained issues that can be fixed - see https://github.com/microsoft/STL/issues/1694 for the laundry list of bugs affecting the STL (which is a small subset of all modules bugs in the compiler that have been fixed) that we've been hammering away at since VS 2019 16.8 when I started looking into getting header units and then named modules working.

Thanks,
STL