[commands] Default construct flags if they're not passed as parameters

This only applies if and only if the flag can be default constructible.
Ergo, all the flags are optional or not required.

discord/ext/commands/core.py

@@ -559,6 +559,8 @@ class Command(_BaseCommand):
             if required:
                 if self._is_typing_optional(param.annotation):
                     return None
+                if hasattr(converter, '__commands_is_flag__') and converter._can_be_constructible():
+                    return await converter._construct_default(ctx)
                 raise MissingRequiredArgument(param)
             return param.default
 

discord/ext/commands/flags.py

@@ -408,6 +408,22 @@ class FlagConverter(metaclass=FlagsMeta):
         """Dict[:class:`str`, :class:`Flag`]: A mapping of flag name to flag object this converter has."""
         return cls.__commands_flags__.copy()
 
+    @classmethod
+    def _can_be_constructible(cls) -> bool:
+        return all(not flag.required for flag in cls.__commands_flags__.values())
+
+    @classmethod
+    async def _construct_default(cls: Type[F], ctx: Context) -> F:
+        self: F = cls.__new__(cls)
+        flags = cls.__commands_flags__
+        for flag in flags.values():
+            if callable(flag.default):
+                default = await maybe_coroutine(flag.default, ctx)
+                setattr(self, flag.attribute, default)
+            else:
+                setattr(self, flag.attribute, flag.default)
+        return self
+
     def __repr__(self) -> str:
         pairs = ' '.join([f'{flag.attribute}={getattr(self, flag.attribute)!r}' for flag in self.get_flags().values()])
         return f'<{self.__class__.__name__} {pairs}>'