gh-85294: Handle missing arguments to @singledispatchmethod gracefully

Lib/functools.py

@@ -883,6 +883,7 @@ def wrapper(*args, **kw):
     wrapper.dispatch = dispatch
     wrapper.registry = types.MappingProxyType(registry)
     wrapper._clear_cache = dispatch_cache.clear
+    wrapper._funcname = funcname
     update_wrapper(wrapper, func)
     return wrapper
 
@@ -911,6 +912,9 @@ def register(self, cls, method=None):
 
     def __get__(self, obj, cls=None):
         def _method(*args, **kwargs):
+            if not args:
+                raise TypeError(f'{self.dispatcher._funcname} requires at least '
+                                '1 positional argument')
             method = self.dispatcher.dispatch(args[0].__class__)
             return method.__get__(obj, cls)(*args, **kwargs)
 

Lib/test/test_functools.py

@@ -2449,6 +2449,18 @@ def f(*args):
         with self.assertRaisesRegex(TypeError, msg):
             f()
 
+    def test_invalid_positional_argument_singlemethoddispatch(self):
+        class A:
+            @functools.singledispatchmethod
+            def t(self):
+                pass
+            @t.register
+            def _(self, arg: int):
+                return "int"
+
+        msg = 't requires at least 1 positional argument'
+        with self.assertRaisesRegex(TypeError, msg):
+            A().t()
 
 class CachedCostItem:
     _cost = 1

Misc/NEWS.d/next/Library/2020-07-13-23-59-42.bpo-41122.8P_Brh.rst

@@ -0,0 +1,3 @@
+Failing to pass arguments properly to functools.singledispatchmethod
+now throws a TypeError instead of hitting an index out of bounds
+internally.