degen: do not mark initial trivial SCCs as accepting

Doing so is not wrong, but it's superfluous, and the extra accepting
state will cause additional work in emptiness checks based on NDFS.
Report from Jan Strejček.

* src/tgbaalgos/degen.cc: Here.
* src/tgbatest/degenid.test: Add a test case.

src/tgbaalgos/degen.cc

@@ -72,6 +72,7 @@ namespace spot
 	acc_cond::mark_t common = a_->acc().all_sets();
         acc_cond::mark_t union_ = 0U;
 	bool has_acc_self_loop = false;
+	bool seen = false;
 	for (auto& t: a_->out(s))
           {
 	    // Ignore transitions that leave the SCC of s.
@@ -85,7 +86,10 @@ namespace spot
 
 	    // an accepting self-loop?
 	    has_acc_self_loop |= (t.dst == s) && a_->acc().accepting(t.acc);
+	    seen = true;
           }
+	if (!seen)
+	  common = 0U;
         cache_[s] = std::make_tuple(common, union_, has_acc_self_loop);
       }
 

src/tgbatest/degenid.test

@@ -222,3 +222,35 @@ EOF
 
 run 0 ../../bin/dstar2tgba in.dra -BD --stats=%s > out.stat
 test 6 = "`cat out.stat`"
+
+# Only one state should be accepting.  In spot 1.2.x an initial state
+# in a trivial SCC was marked as accepting: this is superfluous.
+../../bin/ltl2tgba -BH 'a & GFb & GFc' > out
+cat out
+cat >expected<<EOF
+HOA: v1
+name: "a & G(Fb & Fc)"
+States: 4
+Start: 0
+AP: 3 "a" "b" "c"
+acc-name: Buchi
+Acceptance: 1 Inf(0)
+properties: trans-labels explicit-labels state-acc deterministic
+properties: stutter-invariant
+--BODY--
+State: 0
+[0] 1
+State: 1 {0}
+[1&2] 1
+[!1&2] 2
+[!2] 3
+State: 2
+[1] 1
+[!1] 2
+State: 3
+[1&2] 1
+[!1&2] 2
+[!2] 3
+--END--
+EOF
+diff out expected