Fix crash in vec len() plugin (#20774)

Fixes #20755.

Author: JukkaL

mypy/plugins/default.py

@@ -217,7 +217,7 @@ def get_class_decorator_hook_2(
 
 def len_callback(ctx: FunctionContext) -> Type:
     """Infer a better return type for 'len'."""
-    if len(ctx.arg_types) == 1:
+    if len(ctx.arg_types) == 1 and len(ctx.arg_types[0]) == 1:
         arg_type = ctx.arg_types[0][0]
         arg_type = get_proper_type(arg_type)
         if isinstance(arg_type, Instance) and arg_type.type.fullname == "librt.vecs.vec":

test-data/unit/check-vec.test

@@ -43,3 +43,7 @@ T = TypeVar("T")
 
 def bad_generic_func(v: vec[T]) -> None: ...  # E: Invalid item type for "vec"
 [builtins fixtures/len.pyi]
+
+[case testLenNoArgs]
+len()  # E: Too few arguments for "len"
+[builtins fixtures/len.pyi]