[boost] [variant] Breaking change introduced by variadic support

Subject: [boost] [variant] Breaking change introduced by variadic support
From: Agustín K-ballo Bergé (kaballo86_at_[hidden])
Date: 2014-01-18 11:45:46

The recent changes to introduce variadic support to `variant` introduce 
breaking changes in the documented macros. When variadic support is 
detected, `BOOST_VARIANT_LIMIT_TYPES` is not defined and 
`BOOST_VARIANT_ENUM_[SHIFTED_]PARAMS` uses variadic templates instead of 
enumerating parameters.

This is a breaking change in the documented interface, and one that 
requires all but the simplest use cases to be rewritten. For instance, 
this breaks _Boost.Spirit_ support of `variant` 
(https://svn.boost.org/trac/boost/ticket/9238), requiring two different 
implementations to be provided.

Is this the preferred path to take? I would suggest instead to keep the 
macros as documented at all times, and let the user handle the variadic 
case manually. This would allow users to support both variadic and 
non-variadic `variant`s up to a certain limit with a single code case 
(without breaking the documented interface), and result in cleaner code 
for those cases that only target compilers with variadic support. 
There's no point in using a macro if it won't work for both cases.

Regards,
-- 
Agustín K-ballo Bergé.-
http://talesofcpp.fusionfenix.com