Throw an exception when attempting to load an incompatible holder

Instead of a segfault. Fixes #751.

This covers the case of loading a custom holder from a default-holder
instance. Attempting to load one custom holder from a different custom
holder (i.e. not `std::unique_ptr`) yields undefined behavior, just as
#588 established for inheritance.

include/pybind11/cast.h

@@ -919,6 +919,9 @@ public:
             return true;
         }
 
+        if (typeinfo->default_holder)
+            throw cast_error("Unable to load a custom holder type from a default-holder instance");
+
         if (typeinfo->simple_type) { /* Case 1: no multiple inheritance etc. involved */
             /* Check if we can safely perform a reinterpret-style cast */
             if (PyType_IsSubtype(tobj, typeinfo->type))

tests/test_smart_ptr.cpp

@@ -265,4 +265,10 @@ test_initializer smart_ptr_and_references([](py::module &pm) {
 
     py::class_<C, CustomUniquePtr<C>>(m, "TypeWithMoveOnlyHolder")
         .def_static("make", []() { return CustomUniquePtr<C>(new C); });
+
+    struct HeldByDefaultHolder { };
+
+    py::class_<HeldByDefaultHolder>(m, "HeldByDefaultHolder")
+        .def(py::init<>())
+        .def_static("load_shared_ptr", [](std::shared_ptr<HeldByDefaultHolder>) {});
 });

tests/test_smart_ptr.py

@@ -211,3 +211,12 @@ def test_move_only_holder():
     assert stats.alive() == 1
     del a
     assert stats.alive() == 0
+
+
+def test_smart_ptr_from_default():
+    from pybind11_tests.smart_ptr import HeldByDefaultHolder
+
+    instance = HeldByDefaultHolder()
+    with pytest.raises(RuntimeError) as excinfo:
+        HeldByDefaultHolder.load_shared_ptr(instance)
+    assert "Unable to load a custom holder type from a default-holder instance" in str(excinfo)