Fix PEP 604 isinstance caching (#17563)

Mentioned by ngnpope

Author: hauntsaninja

mypy/types.py

@@ -2900,12 +2900,19 @@ def relevant_items(self) -> list[Type]:
             return [i for i in self.items if not isinstance(get_proper_type(i), NoneType)]
 
     def serialize(self) -> JsonDict:
-        return {".class": "UnionType", "items": [t.serialize() for t in self.items]}
+        return {
+            ".class": "UnionType",
+            "items": [t.serialize() for t in self.items],
+            "uses_pep604_syntax": self.uses_pep604_syntax,
+        }
 
     @classmethod
     def deserialize(cls, data: JsonDict) -> UnionType:
         assert data[".class"] == "UnionType"
-        return UnionType([deserialize_type(t) for t in data["items"]])
+        return UnionType(
+            [deserialize_type(t) for t in data["items"]],
+            uses_pep604_syntax=data["uses_pep604_syntax"],
+        )
 
 
 class PartialType(ProperType):

test-data/unit/check-incremental.test

@@ -6726,3 +6726,20 @@ from typing_extensions import TypeIs
 def guard(x: object) -> TypeIs[int]:
     pass
 [builtins fixtures/tuple.pyi]
+
+[case testStartUsingPEP604Union]
+# flags: --python-version 3.10
+import a
+[file a.py]
+import lib
+
+[file a.py.2]
+from lib import IntOrStr
+assert isinstance(1, IntOrStr)
+
+[file lib.py]
+from typing_extensions import TypeAlias
+
+IntOrStr: TypeAlias = int | str
+assert isinstance(1, IntOrStr)
+[builtins fixtures/type.pyi]