[v3] Re-implement is_constructible, add is_default_constructible, is_destructible, fix PR 48526

[Magnus Fromreide]

On Thu, 2011-04-14 at 00:48 +0200, Paolo Carlini wrote:
> Hi!
> 
> over the last 2 days or so, Daniel Krugler - having filed in due course 
> his Copyright Assignment - finally kindly contributed to the project 
> shiny new std::is_constructible, std::is_default_constructible and 
> std::is_destructible. Being his first contribution, I helped integrating 
> the work in our existing <type_traits> infrastructure. As you can see, I 
> also took the occasion to use the new logical helper traits elsewhere, 
> add a few more tweaks (remove a rather redundant macro, at this point), 
> and also add tests in areas where the testsuite was weak (particularly 
> because for some time we could assume shared code was at least exercised 
> by the tr1/ testsuite).

This is just a nit, but why do you remove the /// is_function comments?

/MF