9.4.2 - Why C-style enums are exempted from requiring a default clause?

[cgpzs]

Rule 9.4.2 requires that all switch blocks have a "default" case, for defensive programming.

This is however not required when using C-style enums, i.e. unscoped and without underlying type. Presumably because it's Undefined Behavior to cast an int out-of-range to a C-style enum, whereas it's well-defined behavior for "C++ enums". Since UB is assumed to not happen, one can assume that a C-style enum has a valid value.

However, there's a subtlety. The "range" of a C-style enum is not just the list of valid enumerators, but the smallest bitset that contains all enumerators.

So for example:

Code:

enum Foo
{
   a = 0,
   b = 1,
   c = 2,
   d = 3
};

The following code is **not** Undefined Behavior, because the valid range of this enum is 0-7 (2^3 - 1).

Code:

enum Foo x = static_cast<Foo>(7);

So I'd argue that defensive programming still applies here and a "default" case would still be needed.

What do you think?