$z3release bug.smt2                                                             
unsat                                                                           
$cvc4 -q --strings-exp bug.smt2                                                 
sat                                                                             
$cat bug.smt2                                                                   
(declare-fun x () String)                                                       
(assert (> (- (str.to_int (str.++ (str.from_int 0) x))) 0))                     
(check-sat)