ANSI Common Lisp 15 Arrays 15.1 Array Concepts 15.1.2 Specialized Arrays
220.127.116.11 Array Upgrading
The upgraded array element type of a type T1 is a type T2 that is a supertype of T1 and that is used instead of T1 whenever T1 is used as an array element type for object creation or type discrimination.
During creation of an array, the element type that was requested is called the expressed array element type. The upgraded array element type of the expressed array element type becomes the actual array element type of the array that is created.
Type upgrading implies a movement upwards in the type hierarchy lattice. A type is always a subtype of its upgraded array element type. Also, if a type Tx is a subtype of another type Ty, then the upgraded array element type of Tx must be a subtype of the upgraded array element type of Ty. Two disjoint types can be upgraded to the same type.
The upgraded array element type T2 of a type T1 is a function only of T1 itself; that is, it is independent of any other property of the array for which T2 will be used, such as rank, adjustability, fill pointers, or displacement. The function upgraded-array-element-type can be used by conforming programs to predict how the implementation will upgrade a given type.