The declaration of a class type which is an aggregate containing a member of reference type causes warning C4610 ("struct/class '<name>' can never be instantiated - user defined constructor required").
This is false. Such a type _can_ be instantiated by means of the standard aggregate initialization syntax. (See example code given in "Steps to Reproduce", which does exactly that.)
Note that the code in question also issues warning C4510; this is perhaps unhelpful, since there is no attempt made to _use_ a default constructor, but it is not incorrect.