Fix stubgen emitting invalid syntax for keyword-only dunder parameters

mypy/stubgen.py

@@ -607,7 +607,10 @@ def _get_func_args(self, o: FuncDef, ctx: FunctionContext) -> list[ArgSig]:
                 if not isinstance(get_proper_type(annotated_type), AnyType):
                     typename = self.print_annotation(annotated_type)
 
-            if actually_pos_only_args and arg_.pos_only:
+            # A keyword-only argument is never positional-only, even when its name
+            # starts with two underscores, so it must not move the "/" marker past
+            # the "*" separator.
+            if actually_pos_only_args and arg_.pos_only and not kind.is_named():
                 pos_only_marker_position += 1
 
             if kind.is_named() and not any(arg.name.startswith("*") for arg in args):

test-data/unit/stubgen.test

@@ -4566,6 +4566,18 @@ def f6(x: int = 0, /, *, y: float): ...
 def f7(x: int, /, *, y: float = 1): ...
 def f8(x: int = 0, /, *, y: float = 1): ...
 
+[case testHistoricalPosOnlyParamsWithKeywordOnly]
+def f1(*, __kw): ...
+def f2(x, /, *, __kw): ...
+def f3(__pos, *, kw): ...
+def f4(__pos, __other, *, __kw): ...
+
+[out]
+def f1(*, __kw) -> None: ...
+def f2(x, /, *, __kw) -> None: ...
+def f3(__pos, /, *, kw) -> None: ...
+def f4(__pos, __other, /, *, __kw) -> None: ...
+
 [case testPreserveEmptyTuple]
 ann: tuple[()]
 alias = tuple[()]