Moving factored-out make_constructor to test_classh_wip.cpp, restoring previous version of cast.h. This is currently the most practical approach. See PR #2798 for background.

include/pybind11/cast.h

@@ -824,30 +824,6 @@ template <typename Container> struct is_copy_assignable<Container, enable_if_t<a
 template <typename T1, typename T2> struct is_copy_assignable<std::pair<T1, T2>>
     : all_of<is_copy_assignable<T1>, is_copy_assignable<T2>> {};
 
-// Helper for type_caster_base.
-struct make_constructor {
-    using Constructor = void *(*)(const void *);
-
-    /* Only enabled when the types are {copy,move}-constructible *and* when the type
-       does not have a private operator new implementation. */
-    template <typename T, typename = enable_if_t<is_copy_constructible<T>::value>>
-    static auto make_copy_constructor(const T *x) -> decltype(new T(*x), Constructor{}) {
-        return [](const void *arg) -> void * {
-            return new T(*reinterpret_cast<const T *>(arg));
-        };
-    }
-
-    template <typename T, typename = enable_if_t<std::is_move_constructible<T>::value>>
-    static auto make_move_constructor(const T *x) -> decltype(new T(std::move(*const_cast<T *>(x))), Constructor{}) {
-        return [](const void *arg) -> void * {
-            return new T(std::move(*const_cast<T *>(reinterpret_cast<const T *>(arg))));
-        };
-    }
-
-    static Constructor make_copy_constructor(...) { return nullptr; }
-    static Constructor make_move_constructor(...) { return nullptr; }
-};
-
 PYBIND11_NAMESPACE_END(detail)
 
 // polymorphic_type_hook<itype>::get(src, tinfo) determines whether the object pointed
@@ -890,8 +866,7 @@ struct polymorphic_type_hook : public polymorphic_type_hook_base<itype> {};
 PYBIND11_NAMESPACE_BEGIN(detail)
 
 /// Generic type caster for objects stored on the heap
-template <typename type> class type_caster_base : public type_caster_generic,
+template <typename type> class type_caster_base : public type_caster_generic {
-                                                  protected make_constructor {
     using itype = intrinsic_t<type>;
 
 public:
@@ -952,6 +927,28 @@ public:
 
     operator itype*() { return (type *) value; }
     operator itype&() { if (!value) throw reference_cast_error(); return *((itype *) value); }
+
+protected:
+    using Constructor = void *(*)(const void *);
+
+    /* Only enabled when the types are {copy,move}-constructible *and* when the type
+       does not have a private operator new implementation. */
+    template <typename T, typename = enable_if_t<is_copy_constructible<T>::value>>
+    static auto make_copy_constructor(const T *x) -> decltype(new T(*x), Constructor{}) {
+        return [](const void *arg) -> void * {
+            return new T(*reinterpret_cast<const T *>(arg));
+        };
+    }
+
+    template <typename T, typename = enable_if_t<std::is_move_constructible<T>::value>>
+    static auto make_move_constructor(const T *x) -> decltype(new T(std::move(*const_cast<T *>(x))), Constructor{}) {
+        return [](const void *arg) -> void * {
+            return new T(std::move(*const_cast<T *>(reinterpret_cast<const T *>(arg))));
+        };
+    }
+
+    static Constructor make_copy_constructor(...) { return nullptr; }
+    static Constructor make_move_constructor(...) { return nullptr; }
 };
 
 template <typename type, typename SFINAE = void> class type_caster : public type_caster_base<type> { };

tests/test_classh_wip.cpp

@@ -102,6 +102,34 @@ protected:
     holder_type *loaded_smhldr_ptr = nullptr;
 };
 
+// type_caster_base BEGIN
+// clang-format off
+// Helper factored out of type_caster_base.
+struct make_constructor {
+    using Constructor = void *(*)(const void *);
+
+    /* Only enabled when the types are {copy,move}-constructible *and* when the type
+       does not have a private operator new implementation. */
+    template <typename T, typename = enable_if_t<is_copy_constructible<T>::value>>
+    static auto make_copy_constructor(const T *x) -> decltype(new T(*x), Constructor{}) {
+        return [](const void *arg) -> void * {
+            return new T(*reinterpret_cast<const T *>(arg));
+        };
+    }
+
+    template <typename T, typename = enable_if_t<std::is_move_constructible<T>::value>>
+    static auto make_move_constructor(const T *x) -> decltype(new T(std::move(*const_cast<T *>(x))), Constructor{}) {
+        return [](const void *arg) -> void * {
+            return new T(std::move(*const_cast<T *>(reinterpret_cast<const T *>(arg))));
+        };
+    }
+
+    static Constructor make_copy_constructor(...) { return nullptr; }
+    static Constructor make_move_constructor(...) { return nullptr; }
+};
+// clang-format on
+// type_caster_base END
+
 template <>
 struct type_caster<mpty> : smart_holder_type_caster_load<mpty> {
     static constexpr auto name = _<mpty>();