[boost] [type_traits] Rewrite and dependency free version

Subject: [boost] [type_traits] Rewrite and dependency free version
From: John Maddock (boost.regex_at_[hidden])
Date: 2015-01-14 14:58:06

I'm going to throw this out there to see what folks think:

In a branch called "Version2", see 
https://github.com/boostorg/type_traits/tree/Version2 is a partially 
rewritten type_traits lib: it doesn't yet contain all the traits of the 
old one, but there's a pretty good selection to test.

The main pros for this version are:

1) Zero dependencies other than Boost.Config.
2) No horrible macro-obfuscation - all "real code", making it easier to 
read and maintain.
3) Substantially reduced template instantiations where practical - often 
half as many as before.
4) Substantially reduced #includes - the aim is that all files should 
include only what they need and no more - with what's needed determined 
by the actual implementation variant when appropriate.  There were also 
a lot of spurious #includes left over from old workarounds no longer used.

The main cons:

1) I don't know what I've broken, but surely something, you can help by 
testing but I would guess we would never be 100% sure it's all right 
until it's out in the field.  This is the main reason for avoiding this 
until now.
2) MPL interoperability should still work, but files which expect 
type_traits headers to have included mpl ones will be broken unless they 
explicitly include what they need.
3) Current MPL inter-operability is fragile see 
https://github.com/boostorg/type_traits/blob/Version2/include/boost/type_traits/integral_constant.hpp
4) Function dispatching to an mpl::bool_ involves an (inline) function 
call where as the old inheritance scheme did not.  This should mostly 
disappear once optimizations are turned on, but is still a potential 
issue for some folks I suspect.
5) I haven't got to the "difficult" headers like common_type.hpp yet.
6) Whatever I've forgotten ;)

Of course (2), (3) and (4) could be solved by making mpl's bool.hpp some 
kind of "core" header.

So... what I need from you kind folks is:

* Is this the right way forward?
* Can you test with your favourite compilers and compare with results 
for develop - So far I've tested with msvc-10,11,12,14  Intel 14,15, 
GCC-4.5.4...4.9.1, plus a reasonably recent clang.
* If anyone would like to help converting further traits all help would 
be much appreciated.... Oh and the compiler requirements in the docs are 
all wildly out of date too...

Regards, John.