Inserting const_cast for std::get_deleter return, to keep Ubuntu 20 GCC 6.3.0 happy.

Author: rwgk

include/pybind11/detail/smart_holder_poc.h

@@ -59,12 +59,8 @@ namespace memory {
 struct guarded_operator_call {
     bool armed_flag;
     explicit guarded_operator_call(bool armed_flag) : armed_flag{armed_flag} {}
-    virtual void operator()(void *)                 = 0;
-    virtual ~guarded_operator_call()                = default;
-    guarded_operator_call(guarded_operator_call &&) = default;
-    guarded_operator_call(guarded_operator_call &)  = default;
-    guarded_operator_call &operator=(guarded_operator_call &&) = delete;
-    guarded_operator_call &operator=(const guarded_operator_call &) = delete;
+    virtual void operator()(void *)  = 0;
+    virtual ~guarded_operator_call() = default;
 };
 
 template <typename T>
@@ -214,20 +210,25 @@ struct smart_holder {
         }
     }
 
+    template <typename T, typename D>
+    void vptr_reset(T *raw_ptr, const D &del) {
+        vptr.reset(raw_ptr, del); // Copies del.
+        // The const_cast is only for certain compilers (Ubuntu 20 GCC 6.3.0 being one).
+        vptr_del = const_cast<D *>(std::get_deleter<D>(vptr)); // Pointer to copy of del.
+    }
+
     void reset_vptr_deleter_armed_flag(bool armed_flag) const {
-        auto vptr_del_ptr = std::get_deleter<guarded_delete>(vptr);
-        if (vptr_del_ptr == nullptr) {
+        if (vptr_del == nullptr) {
             throw std::runtime_error(
                 "smart_holder::reset_vptr_deleter_armed_flag() called in an invalid context.");
         }
-        vptr_del_ptr->armed_flag = armed_flag;
+        vptr_del->armed_flag = armed_flag;
     }
 
     template <typename T>
     static smart_holder from_raw_ptr_unowned(T *raw_ptr) {
         smart_holder hld;
-        hld.vptr.reset(raw_ptr, guarded_builtin_delete<T>(false));
-        hld.vptr_del                   = std::get_deleter<guarded_builtin_delete<T>>(hld.vptr);
+        hld.vptr_reset(raw_ptr, guarded_builtin_delete<T>(false));
         hld.vptr_is_using_noop_deleter = true;
         hld.is_populated               = true;
         return hld;
@@ -258,21 +259,12 @@ struct smart_holder {
     static smart_holder from_raw_ptr_take_ownership(T *raw_ptr) {
         ensure_pointee_is_destructible<T>("from_raw_ptr_take_ownership");
         smart_holder hld;
-        hld.vptr.reset(raw_ptr, guarded_builtin_delete<T>(true));
-        hld.vptr_del                     = std::get_deleter<guarded_builtin_delete<T>>(hld.vptr);
+        hld.vptr_reset(raw_ptr, guarded_builtin_delete<T>(true));
         hld.vptr_is_using_builtin_delete = true;
         hld.is_populated                 = true;
         return hld;
     }
 
-    void reset_vptr_deleter_armed_flag(bool armed_flag) {
-        if (vptr_del == nullptr) {
-            throw std::runtime_error(
-                "smart_holder::reset_vptr_deleter_armed_flag() called in an invalid context.");
-        }
-        vptr_del->armed_flag = armed_flag;
-    }
-
     // Caller is responsible for ensuring preconditions (SMART_HOLDER_WIP: details).
     void disown() {
         reset_vptr_deleter_armed_flag(false);
@@ -318,11 +310,9 @@ struct smart_holder {
         hld.rtti_uqp_del                 = &typeid(D);
         hld.vptr_is_using_builtin_delete = is_std_default_delete<T>(*hld.rtti_uqp_del);
         if (hld.vptr_is_using_builtin_delete) {
-            hld.vptr.reset(unq_ptr.get(), guarded_builtin_delete<T>(true));
-            hld.vptr_del = std::get_deleter<guarded_builtin_delete<T>>(hld.vptr);
+            hld.vptr_reset(unq_ptr.get(), guarded_builtin_delete<T>(true));
         } else {
-            hld.vptr.reset(unq_ptr.get(), guarded_custom_deleter<T, D>(true));
-            hld.vptr_del = std::get_deleter<guarded_custom_deleter<T, D>>(hld.vptr);
+            hld.vptr_reset(unq_ptr.get(), guarded_custom_deleter<T, D>(true));
         }
         unq_ptr.release();
         hld.is_populated = true;