don't try to cast 'None' into a C++ lvalue reference

example/issues.cpp

@@ -21,7 +21,7 @@ void init_issues(py::module &m) {
     // #70 compilation issue if operator new is not public
     class NonConstructible { private: void *operator new(size_t bytes) throw(); };
     py::class_<NonConstructible>(m, "Foo");
-    m.def("getstmt", []() -> NonConstructible * { return nullptr; },
+    m2.def("getstmt", []() -> NonConstructible * { return nullptr; },
         py::return_value_policy::reference);
 #endif
 
@@ -101,4 +101,7 @@ void init_issues(py::module &m) {
                 list.append(py::cast(e));
             return list;
         });
-};
+
+    // (no id): should not be able to pass 'None' to a reference argument
+    m2.def("print_element", [](ElementA &el) { std::cout << el.value() << std::endl; });
+}

example/issues.py

@@ -7,7 +7,7 @@ from example.issues import print_cchar, print_char
 from example.issues import DispatchIssue, dispatch_issue_go
 from example.issues import Placeholder, return_vec_of_reference_wrapper
 from example.issues import iterator_passthrough
-from example.issues import ElementList, ElementA
+from example.issues import ElementList, ElementA, print_element
 import gc
 
 print_cchar("const char *")
@@ -42,3 +42,8 @@ gc.collect()
 for i, v in enumerate(el.get()):
     print("%i==%i, " % (i, v.value()), end='')
 print()
+
+try:
+    print_element(None)
+except Exception as e:
+    print("Failed as expected: " + str(e))

example/issues.ref

@@ -5,3 +5,6 @@ Yay..
 [Placeholder[1], Placeholder[2], Placeholder[3], Placeholder[4]]
 [3, 5, 7, 9, 11, 13, 15]
 0==0, 1==1, 2==2, 3==3, 4==4, 5==5, 6==6, 7==7, 8==8, 9==9, 
+Failed as expected: Incompatible function arguments. The following argument types are supported:
+    1. (example.issues.ElementA) -> NoneType
+