From 312694fda959357802d6425397accd8cb4b58a73 Mon Sep 17 00:00:00 2001
From: Shantanu <12621235+hauntsaninja@users.noreply.github.com>
Date: Wed, 24 Jul 2024 02:10:16 -0400
Subject: [PATCH] Fix PEP 604 isinstance caching (#17563)

Mentioned by ngnpope
---
 mypy/types.py                         | 11 +++++++++--
 test-data/unit/check-incremental.test | 17 +++++++++++++++++
 2 files changed, 26 insertions(+), 2 deletions(-)

diff --git a/mypy/types.py b/mypy/types.py
index 91b40536f1cf..52b3121f9fb3 100644
--- a/mypy/types.py
+++ b/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):
diff --git a/test-data/unit/check-incremental.test b/test-data/unit/check-incremental.test
index 24292bce3e21..173265e48e6f 100644
--- a/test-data/unit/check-incremental.test
+++ b/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]