Fix crash on a callable attribute with single unpack (#17641)

Fixes #17518

mypy/typeops.py

@@ -310,12 +310,14 @@ class B(A): pass
     if not func.arg_types:
         # Invalid method, return something.
         return cast(F, func)
-    if func.arg_kinds[0] == ARG_STAR:
+    if func.arg_kinds[0] in (ARG_STAR, ARG_STAR2):
         # The signature is of the form 'def foo(*args, ...)'.
         # In this case we shouldn't drop the first arg,
         # since func will be absorbed by the *args.
-
         # TODO: infer bounds on the type of *args?
+
+        # In the case of **kwargs we should probably emit an error, but
+        # for now we simply skip it, to avoid crashes down the line.
         return cast(F, func)
     self_param_type = get_proper_type(func.arg_types[0])
 

test-data/unit/check-typeddict.test

@@ -3579,6 +3579,24 @@ class Test:
         run(test2, other=0, **params)  # E: Argument "other" to "run" has incompatible type "int"; expected "str"
 [builtins fixtures/tuple.pyi]
 
+[case testTypedDictUnpackSingleWithSubtypingNoCrash]
+from typing import Callable
+from typing_extensions import TypedDict, Unpack
+
+class Kwargs(TypedDict):
+    name: str
+
+def f(**kwargs: Unpack[Kwargs]) -> None:
+    pass
+
+class C:
+    d: Callable[[Unpack[Kwargs]], None]
+
+# TODO: it is an old question whether we should allow this, for now simply don't crash.
+class D(C):
+    d = f
+[builtins fixtures/tuple.pyi]
+
 [case testTypedDictInlineNoOldStyleAlias]
 # flags: --enable-incomplete-feature=InlineTypedDict
 X = {"int": int, "str": str}