chore(internal): update test helper function (#1371)

Author: stainless-bot

tests/utils.py

@@ -97,7 +97,22 @@ def assert_matches_type(
             assert_matches_type(key_type, key, path=[*path, "<dict key>"])
             assert_matches_type(items_type, item, path=[*path, "<dict item>"])
     elif is_union_type(type_):
-        for i, variant in enumerate(get_args(type_)):
+        variants = get_args(type_)
+
+        try:
+            none_index = variants.index(type(None))
+        except ValueError:
+            pass
+        else:
+            # special case Optional[T] for better error messages
+            if len(variants) == 2:
+                if value is None:
+                    # valid
+                    return
+
+                return assert_matches_type(type_=variants[not none_index], value=value, path=path)
+
+        for i, variant in enumerate(variants):
             try:
                 assert_matches_type(variant, value, path=[*path, f"variant {i}"])
                 return