Asio, gcc and -fconcepts

From: Richard Hodges (hodges.r_at_[hidden])
Date: 2020-04-06 17:01:18

Dear all,

With the upcoming Boost 1.73 release imminent, I felt it was worthwhile
pointing out that latest gcc has a problem with Asio if the `-fconcepts`
command line flag is set.

There is an issue in the Boost.Asio repo here relating to the manifestation
of the bug in MSVC:

https://github.com/boostorg/asio/issues/312

and an issue here in the Boost.Beast repo:

https://github.com/boostorg/beast/issues/1884

It seems that the file boost/asio/detail/config.hpp was written when
concepts support was not widespread and still in the concepts.ts stage.

The syntax and capabilities of concepts were subtly changed during
standardisation, with gcc and MSVC only supporting the C++20 dialect.

Thus, compiling an Asio-based program with concepts enabled in either of
gcc or MSVC fails unconditionally.

I'm wondering if anyone has Chris' ear and might ask him to indulge us with
a fix prior to the 1.73 release.

I fear that by the time Boost 1.73 is in mainstream use, C++20 will be upon
us we'll see an increase in issues and a loss of reputation as a result.

Thanks for your time,

Sincerely,

-- 
Richard Hodges
hodges.r_at_[hidden]
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