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src/nb_enum.cpp

@@ -95,6 +95,8 @@ PyObject *enum_create(enum_init_data *ed) noexcept {
         #endif
     }
 
+    maybe_make_immortal(result.ptr());
+
     result.attr("__nb_enum__") = capsule(t, [](void *p) noexcept {
         type_init_data *t = (type_init_data *) p;
         delete (enum_map *) t->enum_tbl.fwd;

src/nb_func.cpp

@@ -321,13 +321,9 @@ PyObject *nb_func_new(const void *in_) noexcept {
 
 #if !defined(NB_FREE_THREADED)
     // Register the function
-    {
-        nb_ptr_map::iterator it;
-        bool success;
-        std::tie(it, success) = internals_->funcs.try_emplace(func, nullptr);
-        check(success,
-              "nanobind::detail::nb_func_new(): internal update failed (2)!");
-    }
+    auto [it, success] = internals_->funcs.try_emplace(func, nullptr);
+    check(success,
+          "nanobind::detail::nb_func_new(): internal update failed (2)!");
 #endif
 
     func_data *fc = nb_func_data(func) + to_copy;

src/nb_type.cpp

@@ -379,7 +379,23 @@ void nb_type_unregister(type_data *t) noexcept {
 
 #if defined(NB_FREE_THREADED)
     // In free-threaded mode, stale type information remains in the
-    // 'type_c2p_fast' TLS. It is freed when the thread terminates.
+    // 'type_c2p_fast' TLS. This data structure is eventually deallocated
+    // when the thread terminates.
+    //
+    // In principle, this is dangerous because the user could delete a type
+    // binding from a module at runtime, causing the associated
+    // Python type object to be freed. If a function then attempts to return
+    // a value with such a de-registered type, nanobind should raise an
+    // exception, which requires knowing that the entry in 'type_c2p_fast'
+    // has become invalid in the meantime.
+    //
+    // Right now, this problem is avoided because we immortalize type objects in
+    // ``nb_type_new()`` and ``enum_create()``. However, we may not always
+    // want to stick with immortalization, which is just a workaround.
+    //
+    // In the future, a global version counter modified with acquire/release
+    // semantics (see https://github.com/wjakob/nanobind/pull/695#discussion_r1761600010)
+    // might prove to be a similarly efficient but more general solution.
     bool fail = n_del_slow != 1;
 #else
     nb_type_map_fast &type_c2p_fast = internals_->type_c2p_fast;