[commands] Allow passing of typing.Union into Greedy. Fix #1951

discord/ext/commands/converter.py

@@ -500,7 +500,7 @@ class _Greedy:
             raise TypeError('Greedy[...] only takes a single argument')
         converter = params[0]
 
-        if not inspect.isclass(converter) and not isinstance(converter, Converter):
+        if not inspect.isclass(converter) and not isinstance(converter, Converter) and not hasattr(converter, '__origin__'):
             raise TypeError('Greedy[...] expects a type or a Converter instance.')
 
         if converter is str or converter is type(None) or converter is _Greedy: