Problems with typechecking enumerated types

[tallman at dspo.UUCP]

In the process of writing a new C compiler for the NS32032,
I have been trying some unusual expressions using enums on the
Berkley 4.2 C compiler on a Vax 11/780.

For example:

enum color {red,orange,yellow,green,blue,indigo,violet};
enum color crayon,rainbow;

main() {
	int x[5];

	x[yellow] = 1;			/* causes "illegal type for +" error */
	crayon = red*blue;		/* no error or warning */
	crayon = red + blue;		/* no error or warning */
	rainbow = red + 1;		/* "enumeration type clash" warning */
	rainbow = orange << 1;		/* no error or warning */
	rainbow = orange & 1;		/* no error or warning */
	if (rainbow > orange) x[1]=2;	/* "illegal comparison of enums" */
	if (rainbow == orange) x[1]=2;	/* legal */
}

These results show the inconsistency in type checking enums - some 
bizzare operations are allowed without any warning (like enum * enum)
and some reasonable operations are not allowed (like array indexing).

Has a standard been developed for what operations should and should not
be allowed on enums?