fix python bindings for spot::parsed_formula::f getter

* python/spot/impl.i: Add a typemap.
* tests/python/ltlsimple.py: Add a test case for an issue.
* NEWS: Mention the bug.

NEWS

@@ -21,6 +21,10 @@ New in spot 2.6.0.dev  (not yet released)
     documentation was also matching the code, so this is a backward
     incompatible change.
 
+  - The Python binding of the getter of spot::parsed_formula::f was
+    returning a reference instead of a copy, causing issues if the
+    reference outlasted the parsed_formula struct.
+
 
 New in spot 2.6  (2018-07-04)
 

python/spot/impl.i

@@ -384,6 +384,15 @@ namespace swig
                                          SWIG_POINTER_IMPLICIT_CONV));
 }
 
+// Access to stucture members, such as spot::parsed_formula::f are done via
+// the pointer typemap.  We want to enforce a copy.
+%typemap(out) spot::formula* {
+  if (!*$1)
+    $result = SWIG_Py_Void();
+  else
+    $result = SWIG_NewPointerObj(new spot::formula(*$1), $descriptor(spot::formula*), SWIG_POINTER_OWN);
+}
+
 %typemap(out) spot::formula {
   if (!$1)
     $result = SWIG_Py_Void();

tests/python/ltlsimple.py

@@ -134,3 +134,11 @@ for (input, output) in [('(a&b)<->b', 'b->(a&b)'),
     print(input, f, output)
     assert(f == output)
     assert(spot.are_equivalent(input, output))
+
+def myparse(input):
+    env = spot.default_environment.instance()
+    pf = spot.parse_infix_psl(input, env)
+    return pf.f
+# This used to fail, because myparse would return a pointer
+# to pf.f inside the destroyed pf.
+assert myparse('a U b') == spot.formula('a U b')