Avoid a crash in nb::try_cast<T>(nb::none())

docs/changelog.rst

@@ -18,7 +18,12 @@ below inherit that of the preceding release.
 Version 1.9.0 (TBA)
 -------------------
 
-* Nothing yet
+* :cpp:func:`nb::try_cast() <try_cast>` no longer crashes the interpreter
+  when attempting to cast a Python ``None`` to a C++ type that was bound
+  using ``nb::class_<>``. Previously this would raise an exception from the
+  cast operator, which would result in a call to ``std::terminate()``
+  because ``try_cast()`` is declared ``noexcept``. (PR `#386
+  <https://github.com/wjakob/nanobind/pull/386>`__.)
 
 Version 1.8.0 (Nov 2, 2023)
 ---------------------------

include/nanobind/nb_cast.h

@@ -393,8 +393,12 @@ bool try_cast(const detail::api<Derived> &value, T &out, bool convert = true) no
     if (caster.from_python(value.derived().ptr(),
                            convert ? (uint8_t) detail::cast_flags::convert
                                    : (uint8_t) 0, nullptr)) {
-        out = caster.operator detail::cast_t<T>();
-        return true;
+        try {
+            out = caster.operator detail::cast_t<T>();
+            return true;
+        } catch (...) {
+            return false;
+        }
     }
 
     return false;

tests/test_classes.cpp

@@ -500,20 +500,20 @@ NB_MODULE(test_classes_ext, m) {
         Struct s;
         bool rv = nb::try_cast<Struct>(h, s);
         return std::make_pair(rv, std::move(s));
-    });
+    }, nb::arg().none());
 
     m.def("try_cast_2", [](nb::handle h) {
         Struct s;
         Struct &s2 = s;
         bool rv = nb::try_cast<Struct &>(h, s2);
         return std::make_pair(rv, std::move(s2));
-    });
+    }, nb::arg().none());
 
     m.def("try_cast_3", [](nb::handle h) {
         Struct *sp = nullptr;
         bool rv = nb::try_cast<Struct *>(h, sp);
         return std::make_pair(rv, sp);
-    }, nb::rv_policy::none);
+    }, nb::arg().none(), nb::rv_policy::none);
 
     m.def("try_cast_4", [](nb::handle h) {
         int i = 0;

tests/test_classes.py

@@ -684,18 +684,36 @@ def test39_try_cast(clean):
     assert_stats(default_constructed=1, move_constructed=2, copy_assigned=1, destructed=3)
     t.reset()
 
+    rv, s2 = t.try_cast_1(None)
+    assert rv is False and s2 is not s and s2.value() == 5
+    del s2
+    assert_stats(default_constructed=1, move_constructed=2, copy_assigned=0, destructed=3)
+    t.reset()
+
     rv, s2 = t.try_cast_2(s)
     assert rv is True and s2 is not s and s.value() == 123 and s2.value() == 123
     del s2
     assert_stats(default_constructed=1, move_constructed=2, copy_assigned=1, destructed=3)
     t.reset()
 
+    rv, s2 = t.try_cast_2(None)
+    assert rv is False and s2 is not s and s2.value() == 5
+    del s2
+    assert_stats(default_constructed=1, move_constructed=2, copy_assigned=0, destructed=3)
+    t.reset()
+
     rv, s2 = t.try_cast_3(s)
     assert rv is True and s2 is s and s.value() == 123
     del s2
     assert_stats()
     t.reset()
 
+    rv, s2 = t.try_cast_3(None)
+    assert rv is True and s2 is None
+    del s2
+    assert_stats(default_constructed=0, move_constructed=0, copy_assigned=0, destructed=0)
+    t.reset()
+
     rv, s2 = t.try_cast_2(1)
     assert rv is False
     del s2