[boost] [type_traits] Modularization proposal

Subject: [boost] [type_traits] Modularization proposal
From: Andrey Semashev (andrey.semashev_at_[hidden])
Date: 2014-09-16 13:30:07

Hi,

Boost.TypeTraits is used in lots of other Boost libraries, and it currently 
unnecessarily pulls MPL and TypeOf. The dependencies are introduced by just 
these two public headers: floating_point_promotion.hpp and common_type.hpp. 
I'd like to avoid these dependencies.

I propose to extract common_type.hpp (and its implementation and tests) into a 
sublib within type_traits (e.g. type_traits/common_type).

As for floating_point_promotion.hpp, it is used in promote.hpp, which is using 
integral_promotion.hpp. I can see two ways of tackling it:

1. Extract all promotion traits into a sublib (e.g. type_traits/promotion). 
This includes floating_point_promotion.hpp, integral_promotion.hpp and 
promote.hpp, as well as their tests.

2. Change floating_point_promotion.hpp to not use MPL. MPL is used only to 
workaround BOOST_NO_CV_SPECIALIZATIONS, which is defined for Borland 5.51 and 
older and MPW 8.90 and older. The simplest way is to drop the workaround, but 
at a quick glance it should be possible to retain it.

I'm currently leaning towards the option 2, although we obviously don't test 
those compilers and there is no way of knowing if the code will still work 
after the change. So I'm interested to know others' opinion, both with regard 
to the general idea and the way of proceeding with 
floating_point_promotion.hpp in particular.

PS: TypeTraits dependencies can be seen in the report Peter graciously 
uploaded:

http://www.pdimov.com/tmp/report-d5ca5be/type_traits.html

After the modularization efforts TypeTraits should only depend on Config, 
Core, Preprocessor, MPL.Core and StaticAssert. Don't mind that MPL.Core 
depends on MPL - the pull request fixing it has already been created.