In C an enum type is an integer type large enough to hold all the enum constants:
(C11, 6.7.2.2p4) "Each enumerated type shall be compatible with char, a signed integer type, or an unsigned integer type. The choice of type is implementation-defined,110) but shall be capable of representing the values of all the members of the enumeration".
Let's say the selected type for enum E is _Bool. A _Bool object can only store the values 0 and 1. It's not possible to have a _Bool object storing a value different than 0 or 1 without invoking undefined behavior.
In that case the compiler is allowed to assume that an object of the enum E type can only hold 0 or 1 in a strictly conforming program and is so allowed to optimize out the default switch case.
