Fix IndexOutOfRangeException on empty **kwargs call to overloaded method (#132)

Calling an overloaded method with an empty kwargs mapping (e.g.
obj.Method(arg, **{}), common when forwarding *args/**kwargs from a
wrapper) crashed with an unhandled IndexOutOfRangeException in
MethodBinder.CheckMethodArgumentsMatch. Since 10e721b (PR #83), the
parameter names array is only populated when there are named arguments,
but the kwargs code paths only checked the kwargs dictionary for null,
so a non-null empty dict indexed into an empty names array.

Treat an empty kwargs dict as no keyword arguments, matching Python
semantics where f(x, **{}) is equivalent to f(x).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: jhonabreul

src/embed_tests/TestMethodBinder.cs

@@ -814,6 +814,23 @@ public string ImplicitConversionSameArgumentCount2(string symbol, decimal quanti
 
             // ----
 
+            public string GetValue(string name)
+            {
+                return "GetValue(name)";
+            }
+
+            public string GetValue(string name, string defaultValue)
+            {
+                return "GetValue(name, defaultValue)";
+            }
+
+            public int GetValue(string name, int defaultValue)
+            {
+                return defaultValue;
+            }
+
+            // ----
+
             public string VariableArgumentsMethod(params CSharpModel[] paramsParams)
             {
                 return "VariableArgumentsMethod(CSharpModel[])";
@@ -895,6 +912,35 @@ def call_method(instance):
             Assert.AreEqual(expectedResult, result);
         }
 
+        [TestCase("GetValue('name', **{})", "GetValue(name)")]
+        [TestCase("GetValue('name', 'default-value', **{})", "GetValue(name, defaultValue)")]
+        [TestCase("GetValue('name', defaultValue='default-value', **{})", "GetValue(name, defaultValue)")]
+        [TestCase("GetValue('name', **{'defaultValue': 'default-value'})", "GetValue(name, defaultValue)")]
+        [TestCase("Method1('abc', **{})", "Method1 Overload 1")]
+        public void BindsOverloadedMethodCalledWithEmptyOrUnpackedKwargs(string methodCallCode, string expectedResult)
+        {
+            using var _ = Py.GIL();
+
+            dynamic module = PyModule.FromString("BindsOverloadedMethodCalledWithEmptyOrUnpackedKwargs", @$"
+def call_method(instance):
+    return instance.{methodCallCode}
+
+def call_method_forwarding_args_and_kwargs(instance):
+    # Common decorator/monkeypatch idiom: forward *args and **kwargs,
+    # with kwargs being an empty dict when no keyword arguments are passed
+    def wrapper(name, *args, **kwargs):
+        return instance.GetValue(name, *args, **kwargs)
+    return wrapper('name')
+");
+
+            var instance = new OverloadsTestClass();
+            var result = module.call_method(instance).As<string>();
+            Assert.AreEqual(expectedResult, result);
+
+            var forwardedResult = module.call_method_forwarding_args_and_kwargs(instance).As<string>();
+            Assert.AreEqual("GetValue(name)", forwardedResult);
+        }
+
         public class CSharpClass
         {
             public string CalledMethodMessage { get; private set; }

src/runtime/MethodBinder.cs

@@ -475,11 +475,14 @@ internal Binding Bind(BorrowedReference inst, BorrowedReference args, BorrowedRe
 
         internal Binding Bind(BorrowedReference inst, BorrowedReference args, BorrowedReference kw, MethodBase info)
         {
-            // If we have KWArgs create dictionary and collect them
+            // If we have KWArgs create dictionary and collect them.
+            // An empty kwargs dict (e.g. calling with **{}) is equivalent to no kwargs at all,
+            // so we only create the dictionary if there are actual keyword arguments,
+            // else the binding code below would try to index into empty parameter name arrays.
             Dictionary<string, PyObject> kwArgDict = null;
-            if (kw != null)
+            var pyKwArgsCount = kw == null ? 0 : (int)Runtime.PyDict_Size(kw);
+            if (pyKwArgsCount > 0)
             {
-                var pyKwArgsCount = (int)Runtime.PyDict_Size(kw);
                 kwArgDict = new Dictionary<string, PyObject>(pyKwArgsCount);
                 using var keylist = Runtime.PyDict_Keys(kw);
                 using var valueList = Runtime.PyDict_Values(kw);
@@ -490,7 +493,7 @@ internal Binding Bind(BorrowedReference inst, BorrowedReference args, BorrowedRe
                     kwArgDict[keyStr!] = new PyObject(value);
                 }
             }
-            var hasNamedArgs = kwArgDict != null && kwArgDict.Count > 0;
+            var hasNamedArgs = kwArgDict != null;
 
             // Fetch our methods we are going to attempt to match and bind too.
             var methods = info == null ? GetMethods()