Copy-paste-and-specialize copyable_holder_caster<shared_ptr>, move_only_holder_caster<unique_ptr>. No functional changes.

include/pybind11/cast.h

@@ -828,6 +828,84 @@ protected:
     holder_type holder;
 };
 
+// BAKEIN_WIP
+template <typename type>
+struct copyable_holder_caster<type, std::shared_ptr<type>> : public type_caster_base<type> {
+public:
+    using base = type_caster_base<type>;
+    static_assert(std::is_base_of<base, type_caster<type>>::value,
+                  "Holder classes are only supported for custom types");
+    using base::base;
+    using base::cast;
+    using base::typeinfo;
+    using base::value;
+
+    bool load(handle src, bool convert) {
+        return base::template load_impl<copyable_holder_caster<type, std::shared_ptr<type>>>(
+            src, convert);
+    }
+
+    explicit operator type *() { return this->value; }
+    // static_cast works around compiler error with MSVC 17 and CUDA 10.2
+    // see issue #2180
+    explicit operator type &() { return *(static_cast<type *>(this->value)); }
+    explicit operator std::shared_ptr<type> *() { return std::addressof(holder); }
+    explicit operator std::shared_ptr<type> &() { return holder; }
+
+    static handle cast(const std::shared_ptr<type> &src, return_value_policy, handle) {
+        const auto *ptr = holder_helper<std::shared_ptr<type>>::get(src);
+        return type_caster_base<type>::cast_holder(ptr, &src);
+    }
+
+protected:
+    friend class type_caster_generic;
+    void check_holder_compat() {
+        if (typeinfo->default_holder) {
+            throw cast_error("Unable to load a custom holder type from a default-holder instance");
+        }
+    }
+
+    bool load_value(value_and_holder &&v_h) {
+        if (v_h.holder_constructed()) {
+            value = v_h.value_ptr();
+            holder = v_h.template holder<std::shared_ptr<type>>();
+            return true;
+        }
+        throw cast_error("Unable to cast from non-held to held instance (T& to Holder<T>) "
+#if !defined(PYBIND11_DETAILED_ERROR_MESSAGES)
+                         "(#define PYBIND11_DETAILED_ERROR_MESSAGES or compile in debug mode for "
+                         "type information)");
+#else
+                         "of type '"
+                         + type_id<std::shared_ptr<type>>() + "''");
+#endif
+    }
+
+    template <typename T = std::shared_ptr<type>,
+              detail::enable_if_t<!std::is_constructible<T, const T &, type *>::value, int> = 0>
+    bool try_implicit_casts(handle, bool) {
+        return false;
+    }
+
+    template <typename T = std::shared_ptr<type>,
+              detail::enable_if_t<std::is_constructible<T, const T &, type *>::value, int> = 0>
+    bool try_implicit_casts(handle src, bool convert) {
+        for (auto &cast : typeinfo->implicit_casts) {
+            copyable_holder_caster sub_caster(*cast.first);
+            if (sub_caster.load(src, convert)) {
+                value = cast.second(sub_caster.value);
+                holder = std::shared_ptr<type>(sub_caster.holder, (type *) value);
+                return true;
+            }
+        }
+        return false;
+    }
+
+    static bool try_direct_conversions(handle) { return false; }
+
+    std::shared_ptr<type> holder;
+};
+
 /// Specialize for the common std::shared_ptr, so users don't need to
 template <typename T>
 class type_caster<std::shared_ptr<T>> : public copyable_holder_caster<T, std::shared_ptr<T>> {};
@@ -847,6 +925,19 @@ struct move_only_holder_caster {
     static constexpr auto name = type_caster_base<type>::name;
 };
 
+// BAKEIN_WIP
+template <typename type, typename deleter>
+struct move_only_holder_caster<type, std::unique_ptr<type, deleter>> {
+    static_assert(std::is_base_of<type_caster_base<type>, type_caster<type>>::value,
+                  "Holder classes are only supported for custom types");
+
+    static handle cast(std::unique_ptr<type, deleter> &&src, return_value_policy, handle) {
+        auto *ptr = src.get();
+        return type_caster_base<type>::cast_holder(ptr, std::addressof(src));
+    }
+    static constexpr auto name = type_caster_base<type>::name;
+};
+
 template <typename type, typename deleter>
 class type_caster<std::unique_ptr<type, deleter>>
     : public move_only_holder_caster<type, std::unique_ptr<type, deleter>> {};