Rule 8.10

A method that writes its object (assigns to this or to a field, a component or an element of it, applies ++ or -- to one, or calls such a method or reads such a getter on this, on a field, a component or an element of it whatever class that field is, or through super) must take the object by reference, and may return a value like any other method; a base’s body that a class calls through super and that writes its object takes it by reference too, and so does the copy of a method that takes a function (Rule 8.18) when a function handed over writes the variable the call is on. Its receiver must be a place a function may write: a let local, a const local whose initializer built its value (Rule 6.10), a module variable, a storage element, or this inside a constructor or another such method, or a field or an element of one of those. A receiver that is a parameter, a const whose value something else may hold, or a value nothing holds must be refused with the remedy, as must a call of such a method that returns nothing where a value is expected. A method whose every return is return this returns its object, and a chain of calls on what it returns that is the whole of a call statement, of a declaration’s initializer or of a return (v.setX(1.).setY(2.)) must run each call but the last as a statement of its own, in source order, on the place the chain starts from, found once before the first call (an index it is reached through is read into a let), a new at the root being held in a temporary; inside a larger expression what such a method returns is a copy, and a call on it that writes its object must be refused with the remedy.