A4-5-1 with overloading operator<<

[Jakob Klein]

Hi

A4-5-1 states:

Expressions with type enum or enum class shall not be used as operands to built-in and overloaded operators other than the subscript operator [ ], the assignment operator =, the equality operators == and ! =, the unary & operator, and the relational operators <, <=, >, >=.

My understanding of the rule is that confusion should be avoided when using the underlying type arithmetically.
My use-case is overloading operator<< for convenient logging and printing of an unscoped enum:

Code:

inline std::ostream& operator<<(std::ostream& os, MyEnum const& enum_value)
{
    switch (enum_value)
    {
        case MyEnum_Unknown:
        {
            return os << "Unknown";
        }
        case MyEnum_Val1:
        {
            return os << "Descriptive message... (MyEnum_Val1)";
        }//...
     }
}

Usage, where the warning is shown:

Code:

LOG_WARNING << "Could not perform action with " << enum_value;

So as long as the enum is not used in arithmetic context ie. bitwise operations (the enum is the left-hand operand) but with the streeam insertion operator I can not follow the rationale of the rule. Is this an oversight in the rule or is there a more specific reason behind this?