C++20 module support

[anarthal]

Adds support for import boost.charconv. This pull request is intended to get feedback and support discussion in the ML. It depends on changes in Boost.CMake, Boost.Core, Boost.ThrowException and Boost.Assert that aren't in develop. Any feedback is welcome.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 94.11765% with 1 line in your changes missing coverage. Please review.

Project coverage is 94.85%. Comparing base (dacb62a) to head (65e5356).

Files with missing lines	Patch %	Lines
...st/charconv/detail/fast_float/digit_comparison.hpp	50.00%	1 Missing :warning:

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #255   +/-   ##
========================================
  Coverage    94.85%   94.85%           
========================================
  Files           69       69           
  Lines         9077     9077           
========================================
  Hits          8610     8610           
  Misses         467      467           

Files with missing lines	Coverage Δ
include/boost/charconv/detail/apply_sign.hpp	100.00% <ø> (ø)
include/boost/charconv/detail/buffer_sizing.hpp	100.00% <ø> (ø)
include/boost/charconv/detail/compute_float32.hpp	100.00% <ø> (ø)
include/boost/charconv/detail/compute_float64.hpp	74.07% <ø> (ø)
include/boost/charconv/detail/compute_float80.hpp	91.30% <ø> (ø)
...lude/boost/charconv/detail/dragonbox/dragonbox.hpp	97.68% <ø> (ø)
...ost/charconv/detail/dragonbox/dragonbox_common.hpp	100.00% <ø> (ø)
include/boost/charconv/detail/dragonbox/floff.hpp	87.10% <ø> (ø)
include/boost/charconv/detail/emulated128.hpp	100.00% <100.00%> (ø)
...nclude/boost/charconv/detail/fallback_routines.hpp	84.61% <ø> (ø)
... and 51 more

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[mborland]

This is cool progress. Thanks for continuing to plug away on it.

[Lastique]

I think, instead of adding a bunch of headers in include/boost/config/std, a different approach would be better.

Add a new macro in Boost.Config:

#if defined(BOOST_USE_MODULES)
#define BOOST_STD_HEADER(x) <boost/config/detail/import_std.hpp>
#else
#define BOOST_STD_HEADER(x) <x>
#endif

and a new boost/config/detail/import_std.hpp header:

#ifndef BOOST_CONFIG_DETAIL_IMPORT_STD_HPP_INCLUDED_
#define BOOST_CONFIG_DETAIL_IMPORT_STD_HPP_INCLUDED_

import std;

#endif // BOOST_CONFIG_DETAIL_IMPORT_STD_HPP_INCLUDED_

Then in libraries standard headers can be included as such:

#include <boost/config.hpp> // for BOOST_STD_HEADER

#include BOOST_STD_HEADER(utility)

This approach requires less maintenance and is forward compatible with std headers that might be added in the future.

The above assumes that BOOST_STD_HEADER and boost/config/detail/import_std.hpp are added to Boost.Config, but this doesn't have to be the case. It can be a separate Boost library, if needed.

[pdimov]

I think I prefer the normal #include, for aesthetic reasons if nothing else.

[anarthal]

I'm good with any of the two approaches.