[wwwdocs] Document C++ DR 1579 in /gcc-5/porting_to.html

[Sandra Loosemore]

On 03/10/2015 11:43 AM, Jonathan Wakely wrote:
>
> +<h3>Return by converting move constructor</h3>
> +
> +<p>GCC 5 implements
> +<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#1579">DR 1579</a>
> +which means that in a function like:</p>
> +
> +<pre><code>
> +  X
> +  foo()
> +  {
> +    Y y;
> +    return y;
> +  }
> +</code></pre>
> +
> +<p>GCC will first attempt to construct the return value as if <code>y</code>
> +were an rvalue, and if that fails then it will try again for an lvalue
> +(all C++11 compilers already do this when returning a variable of the
> +same type as the function returns, but now they are required to do it
> +when the types are not the same).
> +This will change the constructor that gets called in some cases,
> +for example it might now call <code>X(Y&&)</code> instead of
> +<code>X(const Y&)</code>.
> +</p>
> +<p>
> +In most cases the only observable difference will be code that runs faster
> +(by moving instead of copying) but if it causes a problem the new behavior
> +can be prevented by ensuring the compiler treats <code>y</code> as an lvalue,
> +using <code>return X(y);</code> or
> +<code>return static_cast<Y&>(y);</code>.
> +</p>
> +

Errrmmmm.  If GCC 5 implements (present tense) this new feature, why is 
the description written mostly in the future tense?  :-(

-Sandra the grammar geek