Assignment is supposed to perform a deep copy because otherwise you need to be aware of the implementation of the object you are copying, which may or may not a

Assignment is supposed to perform a deep copy because otherwise you need to be aware of the implementation of the object you are copying, which may or may not allocate data on the heap. By forcing a deep copy, all assignments have identical semantics – you obtain a new, independent copy of the resource. You could say, "well, developers can just adopt a uniform semantics for what parts of objects are considered 'shallow' vs. 'deep'", but now the compiler can generate correct default copy operations in many fewer cases, since the rule is arbitrary vs. simply "copy everything".