Büchi translation should not go through fg_safety_to_dca_maybe()

Fixes #366, reported by Simon Jantsch.

* spot/twaalgos/translate.cc: type_&Generic will also match if
type_==BA... use type_==Generic instead.
* tests/core/unambig.test: Add a test corresponding to Simon's report.
* NEWS: Describe the bug.

NEWS

@@ -64,6 +64,13 @@ New in spot 2.6.2.dev (not yet released)
     with arbitrary acceptance condition into a parity automaton,
     based on a last-appearance record (LAR) construction.
 
+  Bugs fixed:
+
+  - Running "ltl2tgba -B" on formulas of the type FG(safety) would
+    unexpectedly use a co-Büchi automaton as an intermediate step.
+    This in turn caused "ltl2tgba -U -B" to not produce unambiguous
+    automata.
+
 New in spot 2.6.2 (2018-09-28)
 
   Build:

spot/twaalgos/translate.cc

@@ -337,7 +337,8 @@ namespace spot
                 if (aut2 && ((type_ == BA) || (type_ & Parity))
                     && (pref_ & Deterministic))
                   return finalize(aut2);
-                if (!aut2 && (type_ & (Generic | Parity | CoBuchi)))
+                if (!aut2 && (type_ == Generic
+                              || type_ & (Parity | CoBuchi)))
                   {
                     aut2 = fg_safety_to_dca_maybe(r, simpl_->get_dict(), sba);
                     if (aut2

tests/core/unambig.test

@@ -26,6 +26,7 @@ for f in 'Ga' \
          'Ga | Gb' \
          'Ga | GFb' \
          'Ga | FGb' \
+         'F(Ga | Gb)' \
          'XFGa | GFb | Gc' \
          '(Ga -> Gb) W c' \
 	 'F(a & !b & (!c W b))' \
@@ -33,6 +34,8 @@ for f in 'Ga' \
 do
   ltl2tgba -UH "$f" | autfilt -q --is-unambiguous
   ltl2tgba -UH "!($f)" | autfilt -q --is-unambiguous
+  ltl2tgba -BUH "$f" | autfilt -q --is-unambiguous
+  ltl2tgba -BUH "!($f)" | autfilt -q --is-unambiguous
   ltl2tgba -UH "$f" | autfilt --check |
       grep -E 'properties:.* (unambiguous|deterministic)'
   ltl2tgba -UH "!($f)" | autfilt --check |