[commands] Add fallback behaviour to the default parameter instances

This allows users to explicitly override the default annotation for
CurrentAuthor and CurrentChannel since they're wider than what most
users would expect

discord/ext/commands/parameters.py

@@ -31,6 +31,16 @@ from typing import TYPE_CHECKING, Any, Literal, Optional, OrderedDict, Union
 from discord.utils import MISSING, maybe_coroutine
 
 from .errors import NoPrivateMessage
+from .converter import GuildConverter
+
+from discord import (
+    Member,
+    User,
+    TextChannel,
+    VoiceChannel,
+    DMChannel,
+    Thread,
+)
 
 if TYPE_CHECKING:
     from typing_extensions import Self
@@ -77,7 +87,7 @@ class Parameter(inspect.Parameter):
     .. versionadded:: 2.0
     """
 
-    __slots__ = ('_displayed_default',)
+    __slots__ = ('_displayed_default', '_fallback')
 
     def __init__(
         self,
@@ -93,6 +103,7 @@ class Parameter(inspect.Parameter):
         self._default = default
         self._annotation = annotation
         self._displayed_default = displayed_default
+        self._fallback = False
 
     def replace(
         self,
@@ -218,12 +229,16 @@ An alias for :func:`parameter`.
 Author = parameter(
     default=attrgetter('author'),
     displayed_default='<you>',
+    converter=Union[Member, User],
 )
+Author._fallback = True
 
 CurrentChannel = parameter(
     default=attrgetter('channel'),
     displayed_default='<this channel>',
+    converter=Union[TextChannel, DMChannel, Thread, VoiceChannel],
 )
+CurrentChannel._fallback = True
 
 
 def default_guild(ctx: Context) -> Guild:
@@ -235,6 +250,7 @@ def default_guild(ctx: Context) -> Guild:
 CurrentGuild = parameter(
     default=default_guild,
     displayed_default='<this server>',
+    converter=GuildConverter,
 )