Suggestions for convenience aliases in <type_traits>

[Jonathan Wakely]

I find myself wanting these quite frequently:

template<bool _Val>
  using __boolean_constant = integral_constant<bool, _Val>;

template<typename _Cond, typename _Tp = void>
  using __enable_if_t_ = typename enable_if<_Cond::value, _Tp>::type;

template<typename _If, typename _Then, typename _Else>
  using __conditional_t_
    = typename conditional<_If::value, _Then, _Else>::type;

One of the reasons I've avoided adding the last two is that I'm not
sure how to name them appropriately.

In Boost.MPL the trait that takes a type parameter as its first
template parameter is named enable_if, and the version taking a
non-type bool parameter is named enable_if_c, but in the std::lib we
dropped the _c suffix.

The alias templates above are named to be consistent with the similar
C++14 aliases, but with an additional _ suffix, which is consistent
with the invaluable __and_, __or_ and __not_ aliases we already have
in <type_traits> (which also have type parameters not bools).

Is that reasonable?

Another naming scheme I've considered is using _T or _T_ as the
suffix.

Does anyone think these should/shouldn't be added?

Anyone have any arguments for or against a particular naming scheme?