Fix nullptr to None conversion for builtin type casters

Fixes #731.

Generally, this applies to any caster made with PYBIND11_TYPE_CASTER().

include/pybind11/cast.h

@@ -469,6 +469,7 @@ public:
     public: \
         static PYBIND11_DESCR name() { return type_descr(py_name); } \
         static handle cast(const type *src, return_value_policy policy, handle parent) { \
+            if (!src) return none().release(); \
             return cast(*src, policy, parent); \
         } \
         operator type*() { return &value; } \

tests/test_python_types.cpp

@@ -464,6 +464,12 @@ test_initializer python_types([](py::module &m) {
     m.def("ord_char16", [](char16_t c) -> uint16_t { return c; });
     m.def("ord_char32", [](char32_t c) -> uint32_t { return c; });
     m.def("ord_wchar", [](wchar_t c) -> int { return c; });
+
+    m.def("return_none_string", []() -> std::string * { return nullptr; });
+    m.def("return_none_char",   []() -> const char *  { return nullptr; });
+    m.def("return_none_bool",   []() -> bool *        { return nullptr; });
+    m.def("return_none_int",    []() -> int *         { return nullptr; });
+    m.def("return_none_float",  []() -> float *       { return nullptr; });
 });
 
 #if defined(_MSC_VER)

tests/test_python_types.py

@@ -501,3 +501,14 @@ def test_single_char_arguments():
     with pytest.raises(ValueError) as excinfo:
         assert ord_wchar(u'aa')
     assert str(excinfo.value) == toolong_message
+
+
+def test_builtins_cast_return_none():
+    """Casters produced with PYBIND11_TYPE_CASTER() should convert nullptr to None"""
+    import pybind11_tests as m
+
+    assert m.return_none_string() is None
+    assert m.return_none_char() is None
+    assert m.return_none_bool() is None
+    assert m.return_none_int() is None
+    assert m.return_none_float() is None