Avoid mrb_check_string_type() in raising exception; fix #3506

The change may reduce flexibility, but I believe no one wants
that level of flexibility here.

src/error.c

@@ -435,8 +435,8 @@ mrb_bug(mrb_state *mrb, const char *fmt, ...)
   exit(EXIT_FAILURE);
 }
 
-static mrb_value
-make_exception(mrb_state *mrb, int argc, const mrb_value *argv, mrb_bool isstr)
+MRB_API mrb_value
+mrb_make_exception(mrb_state *mrb, int argc, const mrb_value *argv)
 {
   mrb_value mesg;
   int n;
@@ -448,12 +448,9 @@ make_exception(mrb_state *mrb, int argc, const mrb_value *argv, mrb_bool isstr)
     case 1:
       if (mrb_nil_p(argv[0]))
         break;
-      if (isstr) {
-        mesg = mrb_check_string_type(mrb, argv[0]);
-        if (!mrb_nil_p(mesg)) {
-          mesg = mrb_exc_new_str(mrb, E_RUNTIME_ERROR, mesg);
-          break;
-        }
+      if (mrb_string_p(argv[0])) {
+        mesg = mrb_exc_new_str(mrb, E_RUNTIME_ERROR, argv[0]);
+        break;
       }
       n = 0;
       goto exception_call;
@@ -488,12 +485,6 @@ exception_call:
   return mesg;
 }
 
-MRB_API mrb_value
-mrb_make_exception(mrb_state *mrb, int argc, const mrb_value *argv)
-{
-  return make_exception(mrb, argc, argv, TRUE);
-}
-
 MRB_API void
 mrb_sys_fail(mrb_state *mrb, const char *mesg)
 {

src/kernel.c

@@ -859,8 +859,8 @@ mrb_f_raise(mrb_state *mrb, mrb_value self)
     mrb_raise(mrb, E_RUNTIME_ERROR, "");
     break;
   case 1:
-    a[1] = mrb_check_string_type(mrb, a[0]);
-    if (!mrb_nil_p(a[1])) {
+    if (mrb_string_p(a[0])) {
+      a[1] = a[0];
       argc = 2;
       a[0] = mrb_obj_value(E_RUNTIME_ERROR);
     }