Annotating the variable itself is important because guaranteed NRVO
means that the lifetime of the object will be extended beyond the
scope of the function.
One of the biggest issues with guaranteed NRVO is that it
fundamentally changes the meaning of the code. A local variable is not
*local* anymore; it *will* persist beyond its evident scope.
[...]
Your rules now require that this code works. The user is following the
rules of guaranteed NRVO, so `retvar` *must* persist beyond the scope
of the function call. As such, `ptr` *must* point to `var`.
That's one very reasonable model for "guaranteed NRVO." I think (but I'm not sure off the top of my head) that that's the model used by P2025.
But it's not the only possible model.
Another possible model is that the lifetime of `var` really truly has ended, and a new object of the same type has begun its lifetime at the same address. Pointers and references to the old object `var` are invalidated at this point. However — by core-language magic — this has occurred without calling any constructor or destructor.
This gets you your UB (and optimization/escape-analysis opportunity), and also your runtime efficiency, at the same time.
But it requires wording to get there. Which P2025 attempted, and which this std-proposals thread certainly isn't attempting (and shouldn't be: if you have a real idea, put it in a paper or something).
Relevant reading:
and probably several other CWG issues.
my $.02,
–Arthur