23.2.3.2 String types at compile time

The compiler always does type inferencing for simple-string as if *default-character-element-type* was set to character .

For example, when you declare something to be of type simple-string , the compiler will never treat it as simple-base-string . Therefore calls like

will work whether x is a simple-base-string , simple-text-string or sys:simple-augmented-string .