Defer None loading to second pass

Many of our `is_none()` checks in type caster loading return true, but
this should really be considered a deferral so that, for example, an
overload with a `py::none` argument would win over one that takes
`py::none` as a null option.

This keeps None-accepting for the `!convert` pass only for std::optional
and void casters.  (The `char` caster already deferred None; this just
extends that behaviour to other casters).

include/pybind11/cast.h

@@ -218,6 +218,8 @@ public:
         if (!src || !typeinfo)
             return false;
         if (src.is_none()) {
+            // Defer accepting None to other overloads (if we aren't in convert mode):
+            if (!convert) return false;
             value = nullptr;
             return true;
         }
@@ -982,6 +984,8 @@ public:
         if (!src || !typeinfo)
             return false;
         if (src.is_none()) {
+            // Defer accepting None to other overloads (if we aren't in convert mode):
+            if (!convert) return false;
             value = nullptr;
             return true;
         }

include/pybind11/functional.h

@@ -22,9 +22,12 @@ struct type_caster<std::function<Return(Args...)>> {
     using function_type = Return (*) (Args...);
 
 public:
-    bool load(handle src, bool) {
-        if (src.is_none())
+    bool load(handle src, bool convert) {
+        if (src.is_none()) {
+            // Defer accepting None to other overloads (if we aren't in convert mode):
+            if (!convert) return false;
             return true;
+        }
 
         if (!isinstance<function>(src))
             return false;

tests/test_python_types.cpp

@@ -500,6 +500,18 @@ test_initializer python_types([](py::module &m) {
     m.def("return_none_int",    []() -> int *         { return nullptr; });
     m.def("return_none_float",  []() -> float *       { return nullptr; });
 
+    m.def("defer_none_cstring", [](char *) { return false; });
+    m.def("defer_none_cstring", [](py::none) { return true; });
+    m.def("defer_none_custom", [](ExamplePythonTypes *) { return false; });
+    m.def("defer_none_custom", [](py::none) { return true; });
+    // void and optional, however, don't defer:
+    m.def("nodefer_none_void", [](void *) { return true; });
+    m.def("nodefer_none_void", [](py::none) { return false; });
+#ifdef PYBIND11_HAS_OPTIONAL
+    m.def("nodefer_none_optional", [](std::optional<int>) { return true; });
+    m.def("nodefer_none_optional", [](py::none) { return false; });
+#endif
+
     m.def("return_capsule_with_destructor",
         []() {
             py::print("creating capsule");

tests/test_python_types.py

@@ -550,8 +550,22 @@ def test_builtins_cast_return_none():
     assert m.return_none_float() is None
 
 
+def test_none_deferred():
+    """None passed as various argument types should defer to other overloads"""
+    import pybind11_tests as m
+
+    assert not m.defer_none_cstring("abc")
+    assert m.defer_none_cstring(None)
+    assert not m.defer_none_custom(m.ExamplePythonTypes.new_instance())
+    assert m.defer_none_custom(None)
+    assert m.nodefer_none_void(None)
+    if has_optional:
+        assert m.nodefer_none_optional(None)
+
+
 def test_capsule_with_destructor(capture):
     import pybind11_tests as m
+    pytest.gc_collect()
     with capture:
         a = m.return_capsule_with_destructor()
         del a