stubgen: Preserve empty tuple annotation (python#16907)

Author: hamdanal

mypy/stubutil.py

@@ -250,6 +250,8 @@ def visit_unbound_type(self, t: UnboundType) -> str:
             self.stubgen.import_tracker.require_name(s)
         if t.args:
             s += f"[{self.args_str(t.args)}]"
+        elif t.empty_tuple_index:
+            s += "[()]"
         return s
 
     def visit_none_type(self, t: NoneType) -> str:

test-data/unit/stubgen.test

@@ -4253,3 +4253,17 @@ def f5(x: int, /, *, y: float): ...
 def f6(x: int = 0, /, *, y: float): ...
 def f7(x: int, /, *, y: float = 1): ...
 def f8(x: int = 0, /, *, y: float = 1): ...
+
+[case testPreserveEmptyTuple]
+ann: tuple[()]
+alias = tuple[()]
+def f(x: tuple[()]): ...
+class C(tuple[()]): ...
+
+[out]
+ann: tuple[()]
+alias = tuple[()]
+
+def f(x: tuple[()]): ...
+
+class C(tuple[()]): ...