[boost] [config] Rethinking feature macros?

Subject: [boost] [config] Rethinking feature macros?
From: Peter Dimov (lists_at_[hidden])
Date: 2017-11-06 01:15:16

Now that there are standard feature-testing macros 
(http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/p0096r5.html) that 
are being implemented by at least g++ and clang++, would it perhaps make 
sense for us to reevaluate our decision to provide negative macros in 
Boost.Config and start defining the standard feature macros instead on the 
compilers that don't support them?

This would seem to require less maintenance, and the feature macro can be 
used without waiting for Boost.Config to add it.

For a concrete example, let's take noexcept in function types. This is 
__cpp_noexcept_function_type, and is implemented by g++ 7, clang 4, clang 5 
(in C++17 mode), and apparently in the latest VS2017 preview.

noexcept function pointers break Boost.Bind, and to fix it, I need to add 
overloads for them, but only if they are implemented, otherwise the 
overloads would be an error.

With the feature macro, I can just #ifdef __cpp_noexcept_function_type and 
it will immediately work on g++ and clang++ and all compilers that don't 
support noexcept function types will still work. Only msvc would need to be 
fixed in some way.

With our traditional approach, I would need to request the addition of 
BOOST_NO_CXX17_NOEXCEPT_FUNCTION_TYPE, wait for it to be added and to be 
present on every compiler except the latest ones (which requires changes 
throughout Boost.Config), and only then be able to use #ifndef 
BOOST_NO_CXX17_NOEXCEPT_FUNCTION_TYPE. (Then wait for it to be merged to 
master before merging my changes to master.)