Fix crash when trying to format an object created by a different frontend.

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>

src/frontend/pp.cpp

@@ -689,7 +689,13 @@ public:
         case object_kind::Postulate:        return pp_postulate(obj);
         case object_kind::Definition:       return pp_definition(obj);
         case object_kind::Neutral:
-            return pp_notation_decl(obj);
+            if (dynamic_cast<notation_declaration const *>(obj.cell())) {
+                // If the object is not notation, then the object was
+                // created in different frontend, and we ignore it.
+                return pp_notation_decl(obj);
+            } else {
+                return format("Unknown neutral object");
+            }
         }
         lean_unreachable();
         return format();

src/tests/frontend/frontend.cpp

@@ -90,11 +90,30 @@ static void tst5() {
     }
 }
 
+class alien_cell : public neutral_object_cell {
+    unsigned m_data[128];
+public:
+    alien_cell() {}
+    unsigned & get_data(unsigned i) { return m_data[i]; }
+    virtual char const * keyword() const { return "alien"; }
+};
+
+static void tst6() {
+    std::cout << "=================\n";
+    frontend f;
+    environment env;
+    env.add_neutral_object(new alien_cell());
+    std::shared_ptr<formatter> fmt_ptr = mk_pp_formatter(f);
+    formatter & fmt = *fmt_ptr;
+    std::cout << fmt(env) << "\n";
+}
+
 int main() {
     tst1();
     tst2();
     tst3();
     tst4();
     tst5();
+    tst6();
     return has_violations() ? 1 : 0;
 }