[commands] Use typing.get_type_hints to resolve ForwardRefs

discord/ext/commands/core.py

@@ -27,6 +27,7 @@ import functools
 import inspect
 import typing
 import datetime
+import sys
 
 import discord
 
@@ -299,17 +300,35 @@ class Command(_BaseCommand):
         signature = inspect.signature(function)
         self.params = signature.parameters.copy()
 
-        # PEP-563 allows postponing evaluation of annotations with a __future__
-        # import. When postponed, Parameter.annotation will be a string and must
-        # be replaced with the real value for the converters to work later on
-        for key, value in self.params.items():
-            if isinstance(value.annotation, str):
-                self.params[key] = value = value.replace(annotation=eval(value.annotation, function.__globals__))
+        # see: https://bugs.python.org/issue41341
+        resolve = self._recursive_resolve if sys.version_info < (3, 9) else self._return_resolved
 
+        try:
+            type_hints = {k: resolve(v) for k, v in typing.get_type_hints(function).items()}
+        except NameError as e:
+            raise NameError(f'unresolved forward reference: {e.args[0]}') from None
+
+        for key, value in self.params.items():
+            # coalesce the forward references
+            self.params[key] = value = value.replace(annotation=type_hints.get(key))
             # fail early for when someone passes an unparameterized Greedy type
             if value.annotation is converters.Greedy:
                 raise TypeError('Unparameterized Greedy[...] is disallowed in signature.')
 
+    def _return_resolved(self, type, **kwargs):
+        return type
+
+    def _recursive_resolve(self, type, *, globals=None):
+        if not isinstance(type, typing.ForwardRef):
+            return type
+
+        resolved = eval(type.__forward_arg__, globals)
+        args = typing.get_args(resolved)
+        for index, arg in enumerate(args):
+            inner_resolve_result = self._recursive_resolve(arg, globals=globals)
+            resolved[index] = inner_resolve_result
+        return resolved
+
     def add_check(self, func):
         """Adds a check to the command.