Avoid removing existing starred arg annotations in infer

Summary:
Noticed during inference codemods that pre-existing annotations on starred args were being deleted for no apparent reason.

It turns out I think there is a bug in the annotation application transform, where rather than taking the existing node and making changes to the pieces where we want to set annotations, we're taking the inferred annotations as the base object and returning that.

cc stroxler are we tracking delta of this module vs. the open sourced version of LibCST? What's the best way to keep changes here maintainable (I think I've made one or two over the course of bug-correcting during the codemods)?

Reviewed By: stroxler

Differential Revision: D34277896

fbshipit-source-id: 3d7719711bc0edfd68bb0e9e98f5094f062bdb7a

client/commands/tests/infer_test.py

@@ -1406,6 +1406,48 @@ def _assert_in_place(
         )
         self.assertEqual(expected_code, annotated_code)
 
+    def test_apply_functions(self) -> None:
+        self._assert_in_place(
+            """
+            def foo(x: int) -> None: ...
+            """,
+            """
+            def foo(x):
+                pass
+            """,
+            """
+            def foo(x: int) -> None:
+                pass
+            """,
+        )
+        self._assert_in_place(
+            """
+            def incomplete_stubs(x: int, y) -> None: ...
+            """,
+            """
+            def incomplete_stubs(x, y: int):
+                pass
+            """,
+            """
+            def incomplete_stubs(x: int, y: int) -> None:
+                pass
+            """,
+        )
+        self._assert_in_place(
+            """
+            def incomplete_stubs_with_stars(x: int, *args, **kwargs) -> None: ...
+            """,
+            """
+            def incomplete_stubs_with_stars(x, *args: P.args, **kwargs: P.kwargs):
+                pass
+            """,
+            """
+            def incomplete_stubs_with_stars(x: int, *args: P.args, **kwargs: P.kwargs) -> None:
+                pass
+            """,
+        )
+
+
     def test_apply_globals(self) -> None:
         self._assert_in_place(
             """

client/libcst_vendored_visitors/_apply_type_annotations.py

@@ -545,7 +545,7 @@ def update_annotation(
                 annotated_parameters.append(parameter)
             return annotated_parameters
 
-        return annotations.parameters.with_changes(
+        return updated_node.params.with_changes(
             params=update_annotation(
                 updated_node.params.params, annotations.parameters.params
             ),