stubgen: Gracefully handle invalid Optional and recognize aliases to PEP 604 unions (#17386)

This Fixes 2 issues with invalid `Optional` (inspired by an error
reported in #17197):
- do not crash on empty `Optional`
- treat `Optional` with more than one index as an unknown type instead
of choosing the first type.

It also fixes PEP 604 unions not being recognized as type aliases.

Author: hamdanal

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 self.stubgen.add_name("_typeshed.Incomplete")
             return f"{node.index.accept(self)} | None"
         base = node.base.accept(self)
         index = node.index.accept(self)
@@ -1060,6 +1062,10 @@ def is_alias_expression(self, expr: Expression, top_level: bool = True) -> bool:
                 else:
                     return False
             return all(self.is_alias_expression(i, top_level=False) for i in indices)
+        elif isinstance(expr, OpExpr) and expr.op == "|":
+            return self.is_alias_expression(
+                expr.left, top_level=False
+            ) and self.is_alias_expression(expr.right, top_level=False)
         else:
             return False
 

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 self.stubgen.add_name("_typeshed.Incomplete")
         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 +4366,33 @@ class Foo(Enum):
 class Bar(Enum):
     A = ...
     B = ...
+
+[case testGracefullyHandleInvalidOptionalUsage]
+from typing import Optional
+
+x: Optional  # invalid
+y: Optional[int]  # valid
+z: Optional[int, str]  # invalid
+w: Optional[int | str]  # valid
+r: Optional[type[int | str]]
+
+X = Optional
+Y = Optional[int]
+Z = Optional[int, str]
+W = Optional[int | str]
+R = Optional[type[int | str]]
+
+[out]
+from _typeshed import Incomplete
+from typing import Optional
+
+x: Incomplete
+y: int | None
+z: Incomplete
+w: int | str | None
+r: type[int | str] | None
+X = Optional
+Y = int | None
+Z = Incomplete
+W = int | str | None
+R = type[int | str] | None