Note that I’m not talking about if std::optional itself allocates more than it needs to at construction (which I know it doesn’t, as cppreference.com says so), but if the compiler might generate an additional copy constructor call if it is value-returned from a function (which I’m worried about since NRVO isn’t in the spec and you can’t rely on it).

It’s more specific to the example code the original commenter posted (and about general out-parameter usage) than the flaws of std::optional itself. Maybe this didn’t get communicated properly from my comment.