Fixed *args and **kwargs handling in the functools overlay

pytype is sometimes able to convert args/kwargs to positional/keyword arguments.
The functools overlay did not account for that prior to this change, and ended
up passing the same arguments twice!

See added test case for a minimal repro.

PiperOrigin-RevId: 822295276

Author: superbobry

pytype/abstract/_function_base.py

@@ -192,6 +192,7 @@ def call(
           self.func.__self__  # pytype: disable=attribute-error
       )
     args = args.simplify(node, self.ctx, match_signature=sig)
+    del sig
     posargs = [u.AssignToNewVariable(node) for u in args.posargs]
     namedargs = {
         k: u.AssignToNewVariable(node) for k, u in args.namedargs.items()

pytype/overlays/functools_overlay.py

@@ -115,6 +115,21 @@ def call(
   ) -> tuple[cfg.CFGNode, cfg.Variable]:
     # ``NativeFunction.call`` does not forward *args and **kwargs to the
     # underlying function, so we do it here to avoid changing core pytype APIs.
+    #
+    # The simplification below ensures that the *args/**kwargs cannot in fact
+    # be split into individual arguments. This logic follow the implementation
+    # in the base class.
+    sig = None
+    if isinstance(
+        self.func.__self__,  # pytype: disable=attribute-error
+        abstract.CallableClass,
+    ):
+      sig = function.Signature.from_callable(
+          self.func.__self__  # pytype: disable=attribute-error
+      )
+    args = args.simplify(node, self.ctx, match_signature=sig)
+    del sig
+
     starargs = args.starargs
     starstarargs = args.starstarargs
     if starargs is not None:

pytype/tests/test_functions1.py

@@ -1079,6 +1079,17 @@ def f(a, b=None):
       partial_f(0)
     """)
 
+  def test_functools_partial_with_starstar(self):
+    self.Check("""
+      import functools
+      def f(a: str, b: int, c: list):
+        pass
+      partial_f = functools.partial(f, "foo")
+
+      def test(**kwargs):
+        partial_f(42, **kwargs)
+    """)
+
   def test_functools_partial_overloaded(self):
     self.Check("""
       import functools