fix py::cast<void *> (#1605)

Pybind11 provides a cast operator between opaque void* pointers on the
C++ side and capsules on the Python side. The py::cast<void *>
expression was not aware of this possibility and incorrectly triggered a
compile-time assertion ("Unable to cast type to reference: value is
local to type caster") that is now fixed.

include/pybind11/cast.h

@@ -1611,7 +1611,8 @@ template <typename T> using move_never = none_of<move_always<T>, move_if_unrefer
 // everything else returns a reference/pointer to a local variable.
 template <typename type> using cast_is_temporary_value_reference = bool_constant<
     (std::is_reference<type>::value || std::is_pointer<type>::value) &&
-    !std::is_base_of<type_caster_generic, make_caster<type>>::value
+    !std::is_base_of<type_caster_generic, make_caster<type>>::value &&
+    !std::is_same<intrinsic_t<type>, void>::value
 >;
 
 // When a value returned from a C++ function is being cast back to Python, we almost always want to

tests/test_builtin_casters.cpp

@@ -160,4 +160,11 @@ TEST_SUBMODULE(builtin_casters, m) {
     m.def("int_cast", []() {return (int) 42;});
     m.def("long_cast", []() {return (long) 42;});
     m.def("longlong_cast", []() {return  ULLONG_MAX;});
+
+    /// test void* cast operator
+    m.def("test_void_caster", []() -> bool {
+        void *v = (void *) 0xabcd;
+        py::object o = py::cast(v);
+        return py::cast<void *>(o) == v;
+    });
 }

tests/test_builtin_casters.py

@@ -336,3 +336,7 @@ def test_int_long():
     assert isinstance(m.int_cast(), int)
     assert isinstance(m.long_cast(), int)
     assert isinstance(m.longlong_cast(), must_be_long)
+
+
+def test_void_caster_2():
+    assert m.test_void_caster()