>Like how struct designated initialisers have to be in order of member declaration due to object lifetime rules or something like that.
It matters in which order sub-objects are initialized - if you have a class with the members A and B, and B takes pointer or reference to A in its constructor and does something with A, A better be already initialized. Sub-objects are initialized in the order of their declaration and having different order of designed initializers would be confusing. In fact, exactly that problem we have in C++ with the list of initializers of base classes and data members in constructors.
It matters in which order sub-objects are initialized - if you have a class with the members A and B, and B takes pointer or reference to A in its constructor and does something with A, A better be already initialized. Sub-objects are initialized in the order of their declaration and having different order of designed initializers would be confusing. In fact, exactly that problem we have in C++ with the list of initializers of base classes and data members in constructors.