autfilt: fix incorrect diagnostic

* bin/autfilt.cc (OPT_KEEP_STATES): Mention the correct option.
* tests/core/maskkeep.test: Test for it.
* NEWS: Mention the bug.

NEWS

@@ -189,6 +189,9 @@ New in spot 2.9.6.dev (not yet released)
   - Some formulas using ->, <->, or xor were not properly detected as
     purely universal or pure eventualities.
 
+  - autfilt --keep-states=... could incorrectly mention --mask-acc
+    when diagnosing errors.
+
   Deprecation notices:
 
   - twa_graph::defrag_states() and digraph::defrag_states() now take

bin/autfilt.cc

@@ -1,5 +1,5 @@
 // -*- coding: utf-8 -*-
-// Copyright (C) 2013-2020 Laboratoire de Recherche et Développement
+// Copyright (C) 2013-2021 Laboratoire de Recherche et Développement
 // de l'Epita (LRDE).
 //
 // This file is part of Spot, a model checking library.
@@ -1011,7 +1011,7 @@ parse_opt(int key, char* arg, struct argp_state*)
           {
             if (res < 0)
               error(2, 0, "state ids should be non-negative:"
-                    " --mask-acc=%ld", res);
+                    " --keep-states=%ld", res);
             // We don't know yet how many states the automata contain.
             if (opt_keep_states.size() <= static_cast<unsigned long>(res))
               opt_keep_states.resize(res + 1, false);

tests/core/maskkeep.test

@@ -1,6 +1,6 @@
 #!/bin/sh
 # -*- coding: utf-8 -*-
-# Copyright (C) 2015, 2016 Laboratoire de Recherche et Développement
+# Copyright (C) 2015, 2016, 2021 Laboratoire de Recherche et Développement
 # de l'Epita (LRDE).
 #
 # This file is part of Spot, a model checking library.
@@ -109,5 +109,9 @@ run 0 autfilt --keep-states=1,2,0 input1 -H >output
 diff output expect4
 
 # Errors
-run 2 autfilt --keep-states=a3 input1
-run 2 autfilt --keep-states=3-2 input1
+run 2 autfilt --keep-states=a3 input1 2> error
+cat error
+grep 'failed to parse' error
+run 2 autfilt --keep-states=3-2 input1 2> error
+cat error
+grep 'non-negative: --keep-states' error