C++ style guide: rule for default case

> If not conditional on an enumerated value, switch statements should always have a `default case` (in the case of an enumerated value, the compiler will warn you if any values are not handled). If the default case should never execute, treat this as an error. For example:

This rule for `default case` in [Loops and Switch Statements](https://google.github.io/styleguide/cppguide.html#Loops_and_Switch_Statements) seems not like a [`Formatting`](https://google.github.io/styleguide/cppguide.html#Formatting) issue, how about shift it to some other place?