Beast: example doesn't compile with -fconcepts

[jed1]

Hi, I'm trying to integrate beast into my project witch uses concepts. But when I tried to compile it, it burried me in errors. So I just tried to compile one of the examples which was closest to my usecase but that doesn't compile either. Is that a bug or is it intended that way? In the latter case: what would I have to change in the example to make it work?

Version of Beast

BOOST_BEAST_VERSION 277

Steps necessary to reproduce the problem

• clone complete boost repository into lib/boost
• checkout tag: boost-1.72.0
• g++ -std=c++2a -fconcepts -Ilib/boost -Ilib/boost/libs/beast/ lib/boost/libs/beast/example/advanced/server-flex/advanced_server_flex.cpp -lpthread -lssl -lcrypto

All relevant compiler information

gcc version 9.2.1 20200123 (Debian 9.2.1-25)
error log: https://gist.github.com/jed1/555abce54f87fd20acfaf2e56749cf1d

[madmongo1]

Hi @jed1, thanks for reporting this and sorry for the delayed response.

I can confirm that I have replicated the bug and am now investigating:

a) whether it's a coding error or a compiler bug
b) whether there is a fix or workaround.

I'll keep you posted.

[madmongo1]

OK, it turns out to be a little more interesting that just a compiler/code bug.

I've tracked down the problem, and it's actually in the Boost.Asio library on which we depend.

Background

In Concepts.TS, the syntax for writing a concept was : concept bool ConceptName = Criteria;

However, in c++20 the accepted syntax is: concept ConceptName = Critera;. i.e. the bool is implied and you must not spell it out.

Clang accepts both forms of the syntax (but should utter a warning if it sees concept bool), but GCC's implementation came much later and is based on the accepted c++20 spec.

There is also a further problem that the method for constraining concepts has evolved since Concepts.TS (but I am by no means an expert on this).

Reference

https://stackoverflow.com/questions/60684179/visual-studio-2019-rejects-bool-concept-while-gcc-8-doesnt-compile-concepts-w/60684473#60684473

Workarounds

The recommended workaround (answer offered by Barry Revzin in the page above) is:

#if __cpp_concepts >= 201707
  // working paper, C++20 concepts
  #define CONCEPT concept
#else
  // TS
  #define CONCEPT concept bool
#endif

template <typename T>
CONCEPT C = true;

Unfortunately this doesn't help us because of the following lines in boost/asio/detail/config.hpp:348:

// Support concepts on compilers known to allow them.
#if !defined(BOOST_ASIO_HAS_CONCEPTS)
# if !defined(BOOST_ASIO_DISABLE_CONCEPTS)
#  if __cpp_concepts
#   define BOOST_ASIO_HAS_CONCEPTS 1
#   define BOOST_ASIO_CONCEPT concept bool      // <<==== HERE
#  endif // __cpp_concepts
# endif // !defined(BOOST_ASIO_DISABLE_CONCEPTS)
#endif // !defined(BOOST_ASIO_HAS_CONCEPTS)

Workaround 1

At the present time, the only non-invasive solution is to disable concepts in compilation units that involve Asio (which will presumably be the majority of them in your program) when you compile with gcc.

Workaround 2

Modify the ASIO source code as detailed in boostorg/asio#312 - see this comment

Sorry I can't be of more help.

[madmongo1]

Great news. Chris has indicated that a fix has been committed to master branch of asio and will be included in the next Boost release.

https://github.com/chriskohlhoff/asio/commit/00157db09efc9ce65e01ea37346a06b3d82d4f65

[jed1]

I can confirm that Boost-1.73 solved this issue, no workaround needed