Add __set__ to functools.cached_property (#9762)

test_cases/stdlib/check_functools.py

@@ -0,0 +1,28 @@
+from __future__ import annotations
+
+import sys
+
+if sys.version_info >= (3, 8):
+    from functools import cached_property
+    from typing_extensions import assert_type
+
+    class A:
+        def __init__(self, x: int):
+            self.x = x
+
+        @cached_property
+        def x(self) -> int:
+            return 0
+
+    assert_type(A(x=1).x, int)
+
+    class B:
+        @cached_property
+        def x(self) -> int:
+            return 0
+
+    def check_cached_property_settable(x: int) -> None:
+        b = B()
+        assert_type(b.x, int)
+        b.x = x
+        assert_type(b.x, int)