Fix app command copies

changelog/678.bugfix.rst

@@ -0,0 +1 @@
+Make the :func:`.ext.commands.default_member_permissions` decorator always work in cogs.

disnake/ext/commands/base_core.py

@@ -215,6 +215,14 @@ def _ensure_assignment_on_copy(self, other: AppCommandT) -> AppCommandT:
             # _max_concurrency won't be None at this point
             other._max_concurrency = cast(MaxConcurrency, self._max_concurrency).copy()
 
+        if self.body._default_member_permissions != other.body._default_member_permissions:
+            if "default_member_permissions" not in other.__original_kwargs__:
+                other.body._default_member_permissions = self.body._default_member_permissions
+            elif self.body._default_member_permissions is not None:
+                raise ValueError(
+                    "Cannot set `default_member_permissions` in both decorator and slash_command_attrs"
+                )
+
         try:
             other.on_error = self.on_error
         except AttributeError: