When introducing exceptions and destroy order, this feature gets really complicated with all the special cases from P2025R2.

Let us simplify to get rid of lots of the special cases:

 - A function cannot combine guaranteed NRVO and guaranteed URVO.

 - A function with guaranteed NRVO always constructs the return value at the start of the function.

// uses guaranteed URVO

mutex Func()

{

    if (cond)

        return unlocked_mutex();

    else

        return locked_mutex();

}

// uses guaranteed NRVO

auto locked_mutex() -> mutex m{}

{

    m.lock();

    return m;

}

// uses guaranteed NRVO

auto unlocked_mutex() -> mutex m{}

{

    return m;

}

Now exception handling and destruction order should be simple.

-----Ursprüngliche Nachricht-----
Von: Frederick Virchanza Gotham via Std-Proposals <std-proposals@lists.isocpp.org>
Gesendet: Mo 20.05.2024 00:00
Betreff: Re: [std-proposals] std::elide
An: std-proposals@lists.isocpp.org;
CC: Frederick Virchanza Gotham <cauldwell.thomas@gmail.com>;
On Sun, May 19, 2024 at 10:49 PM Sebastian Wittmeier wrote:
>
> auto Func() -> mutex m
> {
>     if (cond)
>         return Func2();
>     else
>     {
>         std::construct(m);
>         m.lock();
>         return m;
>      }
> }


Or maybe like this:

mutex Func(void)
{
    try
    {
         if ( cond ) return Func2();
         else
         {
             auto &m = std::construct( /* ctor arguments go here */ );
             m.lock();
             return m;
          }
   }
   catch(...)
   {
         if ( std::is_constructed_already() ) return std::constructed_object();
         std::construct( /* ctor arguments go here */ );
         std::constructed().lock();
         return std::constructed();
   }
}
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