functional and type_traits cleanup

[Jonathan Wakely]

On 4 April 2013 21:16, François Dumont wrote:
> Hi
>
>     While checking the difference between implementation of
> std::is_copy_constructible and std::is_copy_assignable that I though was
> behaving differently I noticed that std::is_copy_constructible
> implementation was cleaner without using __sfinea_types. So I generalize it
> to 3 other cases and removed __sfinae_types. Tests show that it was used in
> std/functional but for types not used anymore, so I remove them too.
>
>     I also removed several explicit instantiations of integral_constant that
> were useless.

I think this is mostly very good, thanks for cleaning it up.

The indentiation of the closing brace for __is_assignable_helper looks wrong.

Is there a reason that __is_assignable_helper::__test uses a default
template argument but __is_convertible_helper::__test uses
decltype(expr, type) in the function return type?  I think the
decltype(__test_aux<_Tp1>(...)) expression would work as a default
template argument too, which I find easier to read because it doesn't
clutter up the return type.