basic_result &operator=(const basic_result &)
error_type both implement copy assignment.
Complexity: If the
value_type for both is present, uses
value_type’s copy assignment operator, else either destructs or copy constructs
value_type as appropriate.
error_type’s copy assignment operator is always used. Constexpr, triviality and noexcept of underlying operations is propagated.
Guarantees: If an exception is thrown during the operation, the object is left in a partially completed state, as per the normal rules for the same operation on a