Account for user installations in Channel.permissions_for

Co-authored-by: owocado <24418520+owocado@users.noreply.github.com>

Author: Kile

discord/abc.py

@@ -699,6 +699,7 @@ def permissions_for(self, obj: Union[Member, Role], /) -> Permissions:
         - Member overrides
         - Implicit permissions
         - Member timeout
+        - User installed app
 
         If a :class:`~discord.Role` is passed, then it checks the permissions
         someone with that role would have, which is essentially:
@@ -714,6 +715,12 @@ def permissions_for(self, obj: Union[Member, Role], /) -> Permissions:
         .. versionchanged:: 2.0
             ``obj`` parameter is now positional-only.
 
+        .. versionchanged:: 2.4
+            User installed apps are now taken into account.
+            The permissions returned for a user installed app mirrors the
+            permissions Discord returns in :attr:`~discord.Interaction.app_permissions`,
+            though it is recommended to use that attribute instead.
+
         Parameters
         ----------
         obj: Union[:class:`~discord.Member`, :class:`~discord.Role`]
@@ -745,6 +752,13 @@ def permissions_for(self, obj: Union[Member, Role], /) -> Permissions:
             return Permissions.all()
 
         default = self.guild.default_role
+        if default is None:
+
+            if self._state.self_id == obj.id:
+                return Permissions._user_installed_permissions(in_guild=True)
+            else:
+                return Permissions.none()
+
         base = Permissions(default.permissions.value)
 
         # Handle the role case first

discord/permissions.py

@@ -208,6 +208,22 @@ def _dm_permissions(cls) -> Self:
         base.send_messages_in_threads = False
         return base
 
+    @classmethod
+    def _user_installed_permissions(cls, *, in_guild: bool) -> Self:
+        base = cls.none()
+        base.send_messages = True
+        base.attach_files = True
+        base.embed_links = True
+        base.external_emojis = True
+        base.send_voice_messages = True
+        if in_guild:
+            # Logically this is False but if not set to True,
+            # permissions just become 0.
+            base.read_messages = True
+            base.send_tts_messages = True
+            base.send_messages_in_threads = True
+        return base
+
     @classmethod
     def all_channel(cls) -> Self:
         """A :class:`Permissions` with all channel-specific permissions set to