sbcl not returning a fixnum in this scenario without specifying the explicit type of the return value breaks the model of type signatures as compiler promises

lispexample.lisp

(locally
    (declaim
     (optimize (speed 3)
               (space 3)
               (safety 0)
               (debug 0)
               (compilation-speed 0))
     (ftype (function (fixnum) fixnum)
            dec-fixnum
            dec-fixnum-fixnum))
  (defun dec-fixnum (n)
    (1- n))
  (defun dec-fixnum-fixnum (n)
    (the fixnum (1- n))))

; CL-USER> (disassemble #'dec-fixnum-fixnum)
; disassembly for DEC-FIXNUM-FIXNUM                                                                                                  
; Size: 13 bytes. Origin: #x10042EDCF2                                                                                               
; 2:       4883E902         SUB RCX, 2                        ; no-arg-parsing entry point                                           
; 6:       488BD1           MOV RDX, RCX                                                                                             
; 9:       488BE5           MOV RSP, RBP                                                                                             
; C:       F8               CLC                                                                                                      
; D:       5D               POP RBP                                                                                                  
; E:       C3               RET                                                                                                      
; NIL                                                                                                                                  
; CL-USER> (disassemble #'dec-fixnum)
; disassembly for DEC-FIXNUM                                                                                                         
; Size: 30 bytes. Origin: #x10042ED795                                                                                               
; 95:       4883E901         SUB RCX, 1                       ; no-arg-parsing entry point                                           
; 99:       488BD1           MOV RDX, RCX                                                                                            
; 9C:       48D1E2           SHL RDX, 1                                                                                              
; 9F:       710C             JNO L0                                                                                                  
; A1:       488BD1           MOV RDX, RCX                                                                                            
; A4:       41BB60060020     MOV R11D, 536872544              ; ALLOC-SIGNED-BIGNUM-IN-RDX                                           
; AA:       41FFD3           CALL R11                                                                                                
; AD: L0:   488BE5           MOV RSP, RBP                                                                                            
; B0:       F8               CLC                                                                                                     
; B1:       5D               POP RBP                                                                                                 
; B2:       C3               RET                                                                                                     
; NIL