Reverting commit 63495313 by un-commenting...

Reverting commit 63495313 by un-commenting `const` in `def_buffer(...)`. To make this possible, `operator T const&` and `operator T const*` in `smart_holder_type_caster` need to be marked as `const` member functions.

include/pybind11/cast.h

@@ -1384,12 +1384,11 @@ struct smart_holder_type_caster : smart_holder_type_caster_load<T>,
         return cast(const_cast<T const *>(src), policy, parent); // Mutbl2Const
     }
 
-    // clang-format off
-
 #if defined(_MSC_VER) && _MSC_VER < 1910
-    // Working around MSVC 2015 bug. `const` sensitivity is lost.
+    // Working around MSVC 2015 bug. const-correctness is lost.
     // SMART_HOLDER_WIP: IMPROVABLE: make common code work with MSVC 2015.
-    template <typename T_> using cast_op_type = detail::cast_op_type<T_>;
+    template <typename T_>
+    using cast_op_type = detail::cast_op_type<T_>;
 #else
     template <typename T_>
     using cast_op_type = conditional_t<
@@ -1397,17 +1396,20 @@ struct smart_holder_type_caster : smart_holder_type_caster_load<T>,
         T const *,
         conditional_t<std::is_same<remove_reference_t<T_>, T *>::value,
                       T *,
-                      conditional_t<std::is_same<T_, T const &>::value,
-                                    T const &,
-                                    T &>>>;
+                      conditional_t<std::is_same<T_, T const &>::value, T const &, T &>>>;
 #endif
 
-    operator T const&() { return this->loaded_as_lvalue_ref(); }
-    operator T&()       { return this->loaded_as_lvalue_ref(); }
-    operator T const*() { return this->loaded_as_raw_ptr_unowned(); }
-    operator T*()       { return this->loaded_as_raw_ptr_unowned(); }
-
-    // clang-format on
+    // The const operators here prove that the existing type_caster mechanism already supports
+    // const-correctness. However, fully implementing const-correctness inside this type_caster
+    // is still a major project.
+    operator T const &() const {
+        return const_cast<smart_holder_type_caster *>(this)->loaded_as_lvalue_ref();
+    }
+    operator T const *() const {
+        return const_cast<smart_holder_type_caster *>(this)->loaded_as_raw_ptr_unowned();
+    }
+    operator T &() { return this->loaded_as_lvalue_ref(); }
+    operator T *() { return this->loaded_as_raw_ptr_unowned(); }
 
     // Originally type_caster_generic::cast.
     PYBIND11_NOINLINE static handle cast_const_raw_ptr(const void *_src,

include/pybind11/pybind11.h

@@ -1430,8 +1430,7 @@ public:
 
     template <typename Return, typename Class, typename... Args>
     class_ &def_buffer(Return (Class::*func)(Args...) const) {
-        // NEEDED: Explanation why the const needs to be removed, or fix elsewhere.
-        return def_buffer([func] (/*const*/ type &obj) { return (obj.*func)(); });
+        return def_buffer([func] (const type &obj) { return (obj.*func)(); });
     }
 
     template <typename C, typename D, typename... Extra>