Fix private enable_shared_from_this issue (with a lot of help from ChatGPT).

Author: rwgk

include/pybind11/detail/struct_smart_holder.h

@@ -62,13 +62,23 @@ High-level aspects:
 namespace pybindit {
 namespace memory {
 
+// Default fallback.
 static constexpr bool type_has_shared_from_this(...) { return false; }
 
+// This overload (generated by ChatGPT) uses SFINAE to skip enable_shared_from_this checks when the
+// base is inaccessible (e.g. private inheritance).
 template <typename T>
-static constexpr bool type_has_shared_from_this(const std::enable_shared_from_this<T> *) {
+static auto type_has_shared_from_this(const T *ptr)
+    -> decltype(static_cast<const std::enable_shared_from_this<T> *>(ptr), true) {
     return true;
 }
 
+// Inaccessible base → substitution failure → fallback overload selected
+template <typename T>
+static constexpr bool type_has_shared_from_this(const void *) {
+    return false;
+}
+
 struct guarded_delete {
     std::weak_ptr<void> released_ptr;    // Trick to keep the smart_holder memory footprint small.
     std::function<void(void *)> del_fun; // Rare case.

tests/pure_cpp/smart_holder_poc.h

@@ -10,6 +10,10 @@ namespace pybindit {
 namespace memory {
 namespace smart_holder_poc { // Proof-of-Concept implementations.
 
+struct PrivateESFT : private std::enable_shared_from_this<PrivateESFT> {};
+static_assert(!pybindit::memory::type_has_shared_from_this(static_cast<PrivateESFT *>(nullptr)),
+              "should detect inaccessible base");
+
 template <typename T>
 T &as_lvalue_ref(const smart_holder &hld) {
     static const char *context = "as_lvalue_ref";