Mark varargs as pos-only (#19022)

Fixes #19019. This PR marks `*args`
as positional only, as you cannot pass the argument by its name.

---------

Co-authored-by: Anthony Sottile <[email protected]>

Author: A5rocks

mypy/argmap.py

@@ -78,7 +78,7 @@ def map_actuals_to_formals(
         elif actual_kind.is_named():
             assert actual_names is not None, "Internal error: named kinds without names given"
             name = actual_names[ai]
-            if name in formal_names:
+            if name in formal_names and formal_kinds[formal_names.index(name)] != nodes.ARG_STAR:
                 formal_to_actual[formal_names.index(name)].append(ai)
             elif nodes.ARG_STAR2 in formal_kinds:
                 formal_to_actual[formal_kinds.index(nodes.ARG_STAR2)].append(ai)

test-data/unit/check-varargs.test

@@ -145,6 +145,14 @@ f(*it1, 1, *it2, 2)  # E: Argument 3 to "f" has incompatible type "*Tuple[str]";
 f(*it1, '') # E: Argument 2 to "f" has incompatible type "str"; expected "int"
 [builtins fixtures/for.pyi]
 
+[case testCallVarArgsWithMatchingNamedArgument]
+def foo(*args: int) -> None: ...  # N: "foo" defined here
+foo(args=1)  # E: Unexpected keyword argument "args" for "foo"
+
+def bar(*args: int, **kwargs: str) -> None: ...
+bar(args=1)  # E: Argument "args" to "bar" has incompatible type "int"; expected "str"
+[builtins fixtures/for.pyi]
+
 
 -- Calling varargs function + type inference
 -- -----------------------------------------