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

[Paolo Carlini]

On 04/14/2011 10:32 AM, Magnus Fromreide wrote:
> This is just a nit, but why do you remove the /// is_function comments?
Because was on the forward declaration, not on the implementation 
itself, where I wanted it.

Paolo.