Account for optional posonly args

Author: sterliakov

mypy/checkexpr.py

@@ -2389,7 +2389,7 @@ def check_argument_count(
         )
 
         # Check for too many or few values for formals.
-        missing_contiguous_pos_block = []
+        missing_contiguous_pos_block: list[int] = []
         seen_kw = False
         for i, kind in enumerate(callee.arg_kinds):
             mapped_args = formal_to_actual[i]
@@ -2445,8 +2445,12 @@ def check_argument_count(
             # missing from the call but doesn't belong to continuous positional prefix,
             # it was already reported as a missing kwarg. All args before first prefix
             # item are guaranteed to have been passed positionally.
-            passed_or_reported_names = callee.arg_names[missing_contiguous_pos_block[-1] + 1 :]
-            assert None not in passed_or_reported_names
+            passed_or_reported_names = [
+                name
+                for name in callee.arg_names[missing_contiguous_pos_block[-1] + 1 :]
+                # None may be there if it was an optional posonly param
+                if name is not None
+            ]
             names_to_use = [None] * missing_contiguous_pos_block[0] + passed_or_reported_names
             self.msg.too_few_arguments(callee, context, names_to_use)
             if object_type and callable_name and "." in callable_name: