support None for const char* arg, with minimal overhead

Since pybind11 also accepts None for const char* as an exception among
built-in types, remove a paragraph about differences in porting.rst.

docs/api_core.rst

@@ -1622,9 +1622,11 @@ parameter of :cpp:func:`module_::def`, :cpp:func:`class_::def`,
 
       Set a flag noting that the function argument accepts ``None``. Can only
       be used for python wrapper types (e.g. :cpp:class:`handle`,
-      :cpp:class:`int_`) and types that have been bound using
-      :cpp:class:`class_`. You cannot use this to implement functions that
-      accept null pointers to builtin C++ types like ``int *i = nullptr``.
+      :cpp:class:`int_`), for types that have been bound using
+      :cpp:class:`class_`, for :cpp:class:`ndarray`, ``std::optional``,
+      ``std::variant``, and ``const char*``.
+      You cannot use this to implement functions that
+      accept null pointers to other builtin C++ types like ``int *i = nullptr``.
 
    .. cpp:function:: arg &noconvert(bool value = true)
 

docs/porting.rst

@@ -86,12 +86,6 @@ It is also possible to set a ``None`` default value, in which case the
 
    m.def("func", &func, "arg"_a = nb::none());
 
-``None``-valued arguments are only supported by two of the three parameter
-passing styles described in the section on :ref:`information exchange
-<exchange>`. In particular, they are supported by :ref:`bindings <bindings>`
-and :ref:`wrappers <wrappers>`, *but not* by :ref:`type casters
-<type_casters>`.
-
 Shared pointers and holders
 ---------------------------
 

include/nanobind/nb_cast.h

@@ -286,7 +286,8 @@ template <> struct type_caster<char> {
         value = PyUnicode_AsUTF8AndSize(src.ptr(), &size);
         if (!value) {
             PyErr_Clear();
-            return false;
+            // optimize for src being a string, check for None afterwards
+            return src.is_none();
         }
         return true;
     }

tests/test_functions.cpp

@@ -161,6 +161,7 @@ NB_MODULE(test_functions_ext, m) {
 
     // Test string caster
     m.def("test_12", [](const char *c) { return nb::str(c); });
+    m.def("test_12_n", [](const char *c) { return nb::str(c ? c : "n/a"); }, nb::arg().none());
     m.def("test_13", []() -> const char * { return "test"; });
     m.def("test_14", [](nb::object o) -> const char * { return nb::cast<const char *>(o); });
 

tests/test_functions.py

@@ -236,6 +236,8 @@ def test21_numpy_overloads():
 
 def test22_string_return():
     assert t.test_12("hello") == "hello"
+    assert t.test_12_n("hello") == "hello"
+    assert t.test_12_n(None) == "n/a"
     assert t.test_13() == "test"
     assert t.test_14("abc") == "abc"