optional_caster::from_python: reset value on None (fixes #747) (#748)

include/nanobind/stl/detail/nb_optional.h

@@ -21,9 +21,10 @@ struct optional_caster {
     NB_TYPE_CASTER(Optional, optional_name(Caster::Name))
 
     bool from_python(handle src, uint8_t flags, cleanup_list* cleanup) noexcept {
-        if (src.is_none())
-            // default-constructed value is already empty
+        if (src.is_none()) {
+            value.reset();
             return true;
+        }
 
         Caster caster;
         if (!caster.from_python(src, flags_for_local_caster<T>(flags), cleanup) ||

tests/test_stl.cpp

@@ -464,6 +464,10 @@ NB_MODULE(test_stl_ext, m) {
         return x;
     });
 
+    m.def("vector_optional_str", [](const std::vector<std::optional<std::string>>& x) {
+        return x;
+    });
+
     m.def("pass_wstr", [](std::wstring ws) { return ws; });
 
     // uncomment to see compiler error:

tests/test_stl.py

@@ -782,6 +782,7 @@ def test69_complex_array():
 def test70_vec_char():
     assert isinstance(t.vector_str("123"), str)
     assert isinstance(t.vector_str(["123", "345"]), list)
+    assert t.vector_optional_str(["abc", None]) == ["abc", None]
 
 
 def test71_null_input():

tests/test_stl_ext.pyi.ref

@@ -233,6 +233,8 @@ def vec_return_copyable() -> list[Copyable]: ...
 
 def vec_return_movable() -> list[Movable]: ...
 
+def vector_optional_str(arg: Sequence[str | None], /) -> list[str | None]: ...
+
 @overload
 def vector_str(arg: Sequence[str], /) -> list[str]: ...