The return value of some forms is implementation dependent, but all forms return a value in CL:

    CL-USER(1): (defparameter *foo* (declaim (special x)))    
    *FOO*
    CL-USER(2): *foo*
    (X)
    CL-USER(3): (defparameter *bar* (declaim (type integer x)))
    *BAR*
    CL-USER(4): *bar*    
    (X)

In other cases, e.g. `(values)` which is an expression that returns no values, the value is nil