Looks good, except a small nitpick and a question.

Given that the vast majority of data going through this function will be ascii, all three of str, wide and out will usually have the same size and it might be a good idea to call reserve on wide and out with the input size to prevent reallocations. With reserve worst case we overallocate a bit (on wide) or have to reallocate once (for out).

The other, bigger thing I am worried about is whether its really a good idea to replace the old/deprecated wstring_convert with the even older mbrtowc C api (which by the way is defined in cwchar, not in cstring. While I agree that we shouldn't use deprecated functions if it can be avoided, the wstring_convert code is just so much cleaner and I just can't see them removing it before there's an usable replacement.

EDIT: Oh and linting. you can just apply the patch command thats in the details of the failed ci run.