The operator += expands into + and assignment. The assignment is not overloaded so the required code is
customers += newCustomer;
This expands into
customers = customers + newCustomer;
The overloaded + operator is called for the right expression. This returns a `CustomerList`, which is then assigned through the = operator to `customers`.
I believe the answer is a syntax error. For example, in java if a semicolon is missed, a syntax error will be produced because the code cannot compile.
