fix(io_models): guard against IndexError for bare iterator return annotations

src/_bentoml_sdk/io_models.py

@@ -432,7 +432,8 @@ def from_output(cls, func: t.Callable[..., t.Any]) -> type[IODescriptor]:
             )
         media_type: str | None = None
         if is_iterator_type(return_annotation):
-            return_annotation = get_args(return_annotation)[0]
+            args = get_args(return_annotation)
+            return_annotation = args[0] if args else t.Any
         elif is_annotated(return_annotation):
             content_type = next(
                 (a for a in get_args(return_annotation) if isinstance(a, ContentType)),

tests/unit/_bentoml_sdk/test_io_models.py

@@ -0,0 +1,15 @@
+from __future__ import annotations
+
+import typing as t
+
+from _bentoml_sdk.io_models import IODescriptor
+
+
+def test_from_output_bare_iterator_does_not_raise():
+    """Bare Iterator/Generator with no type args must not raise IndexError."""
+
+    def fn() -> t.Iterator:
+        yield 1
+
+    spec = IODescriptor.from_output(fn)
+    assert spec is not None