stubgen: Gracefully handle invalid Optional and recognize aliases to PEP 604 unions

mypy/stubgen.py

@@ -314,6 +314,8 @@ def visit_index_expr(self, node: IndexExpr) -> str:
                 return " | ".join([item.accept(self) for item in node.index.items])
             return node.index.accept(self)
         if base_fullname == "typing.Optional":
+            if isinstance(node.index, TupleExpr):
+                return f"{self.stubgen.add_name('_typeshed.Incomplete')} | None"
             return f"{node.index.accept(self)} | None"
         base = node.base.accept(self)
         index = node.index.accept(self)

mypy/stubutil.py

@@ -257,7 +257,9 @@ def visit_unbound_type(self, t: UnboundType) -> str:
         if fullname == "typing.Union":
             return " | ".join([item.accept(self) for item in t.args])
         if fullname == "typing.Optional":
-            return f"{t.args[0].accept(self)} | None"
+            if len(t.args) == 1:
+                return f"{t.args[0].accept(self)} | None"
+            return f"{self.stubgen.add_name('_typeshed.Incomplete')} | None"
         if fullname in TYPING_BUILTIN_REPLACEMENTS:
             s = self.stubgen.add_name(TYPING_BUILTIN_REPLACEMENTS[fullname], require=True)
         if self.known_modules is not None and "." in s:

test-data/unit/stubgen.test

@@ -4366,3 +4366,25 @@ class Foo(Enum):
 class Bar(Enum):
     A = ...
     B = ...
+
+[case testGracefullyHandleInvalidOptionalUsage]
+from typing import Optional
+
+x: Optional
+y: Optional[int]
+z: Optional[int, str]
+
+X = Optional
+Y = Optional[int]
+Z = Optional[int, str]
+
+[out]
+from _typeshed import Incomplete
+from typing import Optional
+
+x: Incomplete | None
+y: int | None
+z: Incomplete | None
+X = Optional
+Y = int | None
+Z = Incomplete | None