src/HOL/Tools/SMT/smt_config.ML

         warning ("The SMT solver " ^ quote name ^ " is not installed")
       else ()
   | warn_solver _ _ = ()
 
 fun select_solver name context =
-  if not (member (op =) (all_solvers_of' context) name) then
+  if not (defined_solvers context name) then
     error ("Trying to select unknown solver: " ^ quote name)
   else if not (is_available' context name) then
     (warn_solver context name; put_solver name context)
   else put_solver name context;