5.2.10 Type Restrictions

Avoid use of the and, not and satisfies types in declarations, since type inference has problems with them. When these types do appear in a declaration, they are still checked precisely, but the type information is of limited use to the compiler. and types are effective as long as the intersection can be canonicalized to a type that doesn’t use and. For example:

(and fixnum unsigned-byte)

is fine, since it is the same as:

(integer 0 most-positive-fixnum)

but this type:

(and symbol (not (member :end)))

will not be fully understood by type interference since the and can’t be removed by canonicalization.

Using any of these type specifiers in a type test with typep or typecase is fine, since as tests, these types can be translated into the and macro, the not function or a call to the satisfies predicate.