Define operations on enumerations for safe and simple use

Reason

Convenience of use and avoidance of errors.

Example

enum class Day { mon, tue, wed, thu, fri, sat, sun };

Day& operator++(Day& d)
{
    return d = (d == Day::sun) ? Day::mon : static_cast<Day>(static_cast<int>(d)+1);
}

Day today = Day::sat;
Day tomorrow = ++today;

The use of a static_cast is not pretty, but

Day& operator++(Day& d)
{
    return d = (d == Day::sun) ? Day::mon : Day{++d};    // error
}

is an infinite recursion, and writing it without a cast, using a switch on all cases is long-winded.

Enforcement

Flag repeated expressions cast back into an enumeration.