Fix std::nullptr_t caster (#840)

* Fix compilation error with std::nullptr_t

* Enable conversion from None to std::nullptr_t and std::nullopt_t

Fixes #839.

include/pybind11/cast.h

@@ -602,7 +602,11 @@ public:
 
 template<typename T> struct void_caster {
 public:
-    bool load(handle, bool) { return false; }
+    bool load(handle src, bool) {
+        if (src && src.is_none())
+            return true;
+        return false;
+    }
     static handle cast(T, return_value_policy /* policy */, handle /* parent */) {
         return none().inc_ref();
     }
@@ -653,7 +657,7 @@ private:
     void *value = nullptr;
 };
 
-template <> class type_caster<std::nullptr_t> : public type_caster<void_type> { };
+template <> class type_caster<std::nullptr_t> : public void_caster<std::nullptr_t> { };
 
 template <> class type_caster<bool> {
 public:

tests/test_python_types.cpp

@@ -359,9 +359,10 @@ test_initializer python_types([](py::module &m) {
         const char *operator()(int) { return "int"; }
         const char *operator()(std::string) { return "std::string"; }
         const char *operator()(double) { return "double"; }
+        const char *operator()(std::nullptr_t) { return "std::nullptr_t"; }
     };
 
-    m.def("load_variant", [](std::variant<int, std::string, double> v) {
+    m.def("load_variant", [](std::variant<int, std::string, double, std::nullptr_t> v) {
         return std::visit(visitor(), v);
     });
 
@@ -516,6 +517,9 @@ test_initializer python_types([](py::module &m) {
             });
         }
     );
+
+    m.def("load_nullptr_t", [](std::nullptr_t) {}); // not useful, but it should still compile
+    m.def("cast_nullptr_t", []() { return std::nullptr_t{}; });
 });
 
 #if defined(_MSC_VER)

tests/test_python_types.py

@@ -378,9 +378,10 @@ def test_variant(doc):
     assert load_variant(1) == "int"
     assert load_variant("1") == "std::string"
     assert load_variant(1.0) == "double"
+    assert load_variant(None) == "std::nullptr_t"
     assert cast_variant() == (5, "Hello")
 
-    assert doc(load_variant) == "load_variant(arg0: Union[int, str, float]) -> str"
+    assert doc(load_variant) == "load_variant(arg0: Union[int, str, float, None]) -> str"
 
 
 def test_constructors():
@@ -568,3 +569,10 @@ def test_capsule_with_destructor(capture):
         creating capsule
         destructing capsule: 1234
     """
+
+
+def test_void_caster():
+    import pybind11_tests as m
+
+    assert m.load_nullptr_t(None) is None
+    assert m.cast_nullptr_t() is None