Re: Interest for C++20 modules support of boost officially

From: Ruben Perez (rubenperez038_at_[hidden])
Date: 2025-01-13 19:41:11

On Mon, 13 Jan 2025 at 20:37, Peter Dimov <pdimov_at_[hidden]> wrote:
>
> Ruben Perez wrote:
> > I think this applies for non module units, too. I've just tested that,
> > under MSVC, the following errors:
> >
> > // File: main.cpp
> > import std;
> > #include <cstddef>
> > int main() {}
> >
> > This does not seem to happen with all the headers, though.
> > * version, cstdint, climits, cassert, cfloat, cstdio, cstdlib, cerrno
> > seem to work fine
> > * cmath and cstddef seem to error.
> >
> > So if we have in a program:
> >
> > // File: main.cpp
> > #include <boost/mp11.hpp>
> > #include <boost/charconv.hpp>
> > int main() {}
>
> Yeah, that's going to be a problem.
>
> Interestingly, though,
>
> import std;
> #include <math.h>
> #include <stddef.h>
> int main() {}
>
> works fine.
>
> If we only need these for the macros, this might well work.
>
> (It's probably std::byte from cstddef that breaks things.)
>
>

I think it has to do with where entities are declared. Your snippet
errors if you import std.compat (which can safely be avoided
altogether in all our code without problems):

import std.compat;
#include <math.h>
#include <stddef.h>
int main() {}

[build] C:\Program Files (x86)\Windows
Kits\10\include\10.0.19041.0\ucrt\corecrt_math.h(399): error C2995:
'bool isfinite(_Ty) noexcept': function template has already been
defined
[build] C:\Program Files (x86)\Windows
Kits\10\include\10.0.19041.0\ucrt\corecrt_math.h(399): note: see
declaration of 'isfinite'
[build] C:\Program Files (x86)\Windows
Kits\10\include\10.0.19041.0\ucrt\corecrt_math.h(405): error C2995:
'bool isinf(_Ty) noexcept': function template has already been defined
[build] C:\Program Files (x86)\Windows
Kits\10\include\10.0.19041.0\ucrt\corecrt_math.h(405): note: see
declaration of 'isinf'
[build] C:\Program Files (x86)\Windows
Kits\10\include\10.0.19041.0\ucrt\corecrt_math.h(411): error C2995:
'bool isnan(_Ty) noexcept': function template has already been defined
[build] C:\Program Files (x86)\Windows
Kits\10\include\10.0.19041.0\ucrt\corecrt_math.h(411): note: see
declaration of 'isnan'
[build] C:\Program Files (x86)\Windows
Kits\10\include\10.0.19041.0\ucrt\corecrt_math.h(417): error C2995:
'bool isnormal(_Ty) noexcept': function template has already been
defined
[build] C:\Program Files (x86)\Windows
Kits\10\include\10.0.19041.0\ucrt\corecrt_math.h(417): note: see
declaration of 'isnormal'
[build] C:\Program Files (x86)\Windows
Kits\10\include\10.0.19041.0\ucrt\corecrt_math.h(423): error C2995:
'bool isgreater(_Ty1,_Ty2) noexcept': function template has already
been defined
[build] C:\Program Files (x86)\Windows
Kits\10\include\10.0.19041.0\ucrt\corecrt_math.h(423): note: see
declaration of 'isgreater'
[build] C:\Program Files (x86)\Windows
Kits\10\include\10.0.19041.0\ucrt\corecrt_math.h(429): error C2995:
'bool isgreaterequal(_Ty1,_Ty2) noexcept': function template has
already been defined
[build] C:\Program Files (x86)\Windows
Kits\10\include\10.0.19041.0\ucrt\corecrt_math.h(429): note: see
declaration of 'isgreaterequal'
[build] C:\Program Files (x86)\Windows
Kits\10\include\10.0.19041.0\ucrt\corecrt_math.h(435): error C2995:
'bool isless(_Ty1,_Ty2) noexcept': function template has already been
defined
[build] C:\Program Files (x86)\Windows
Kits\10\include\10.0.19041.0\ucrt\corecrt_math.h(435): note: see
declaration of 'isless'
[build] C:\Program Files (x86)\Windows
Kits\10\include\10.0.19041.0\ucrt\corecrt_math.h(441): error C2995:
'bool islessequal(_Ty1,_Ty2) noexcept': function template has already
been defined
[build] C:\Program Files (x86)\Windows
Kits\10\include\10.0.19041.0\ucrt\corecrt_math.h(441): note: see
declaration of 'islessequal'
[build] C:\Program Files (x86)\Windows
Kits\10\include\10.0.19041.0\ucrt\corecrt_math.h(447): error C2995:
'bool islessgreater(_Ty1,_Ty2) noexcept': function template has
already been defined
[build] C:\Program Files (x86)\Windows
Kits\10\include\10.0.19041.0\ucrt\corecrt_math.h(447): note: see
declaration of 'islessgreater'
[build] C:\Program Files (x86)\Windows
Kits\10\include\10.0.19041.0\ucrt\corecrt_math.h(453): error C2995:
'bool isunordered(_Ty1,_Ty2) noexcept': function template has already
been defined
[build] C:\Program Files (x86)\Windows
Kits\10\include\10.0.19041.0\ucrt\corecrt_math.h(453): note: see
declaration of 'isunordered'