Another fix to the argument handling in the functools overlay

Turns out

* positional arguments aren't necessarily present in `Signature.param_names`;
* *args/**kwargs need special handling as well.

PiperOrigin-RevId: 823630645

Author: superbobry

pytype/overlays/functools_overlay.py

@@ -179,7 +179,12 @@ def get_signatures(self) -> Sequence[function.Signature]:
         for name, value, _ in sig.iter_args(args):
           if value is None:
             continue
-          if sig.param_names.index(name) < sig.posonly_count:
+          if name == sig.varargs_name or name == sig.kwargs_name:
+            continue  # Nothing to do for packed parameters.
+          if (
+              name not in sig.param_names or
+              sig.param_names.index(name) < sig.posonly_count
+          ):
             # The parameter is positional-only, meaning that it cannot be
             # overwritten via a keyword argument. Remove it.
             bound_param_names.add(name)

pytype/tests/test_attr2.py

@@ -244,6 +244,33 @@ class Foo:
       assert_type(foo.x, int)
     """)
 
+  def test_partial_with_star_args_as_converter(self):
+    self.Check("""
+      import attr
+      import functools
+      def f(*args: str) -> str:
+        return "".join(args)
+      @attr.s
+      class Foo:
+        x = attr.ib(converter=functools.partial(f, "foo", "bar"))
+      foo = Foo(x=0)
+      assert_type(foo.x, str)
+    """)
+
+  def test_partial_as_converter_with_factory(self):
+    # This is a smoke test for signature construction in the functools overlay.
+    self.Check("""
+      import collections
+      import functools
+      import attr
+      @attr.s(auto_attribs=True)
+      class Foo(object):
+        x = attr.ib(
+            factory=dict,
+            converter=functools.partial(collections.defaultdict, lambda: 0),
+        )
+    """)
+
   def test_partial_overloaded_as_converter(self):
     self.Check("""
       import attr