Replace bool cast by call to bool builtin

Some types (e.g. std::tuple) don't support bool cast, let's use the
builtin to do it.

pythran/backend.py

@@ -907,7 +907,7 @@ def visit_IfExp(self, node):
         body = self.visit(node.body)
         orelse = self.visit(node.orelse)
         return (
-            "(((bool){0}) "
+            "(pythonic::builtins::functor::bool_{{}}({0}) "
             "? typename __combined<decltype({1}), decltype({2})>::type({1}) "
             ": typename __combined<decltype({1}), decltype({2})>::type({2}))"
         ).format(test, body, orelse)

pythran/tests/test_base.py

@@ -247,6 +247,11 @@ def test_multidef4(self):
     def test_tuple(self):
         self.run_test("def tuple_(t): return t[0]+t[1]", (0,1), tuple_=[Tuple[int, int]])
 
+    def test_tuple_implicit_empty(self):
+        self.run_test("def tuple_implicit_empty_(t): return 1 if t else 0",
+                      (0,1),
+                      tuple_implicit_empty_=[Tuple[int, int]])
+
     def test_nested_list_comprehension(self):
         self.run_test("def nested_list_comprehension(): return [ [ x+y for x in range(10) ] for y in range(20) ]", nested_list_comprehension=[])