From fbdee000de47ae96fbf53ce8908e8efbb23cfba4 Mon Sep 17 00:00:00 2001
From: Alex Waygood <Alex.Waygood@Gmail.com>
Date: Wed, 7 Jun 2023 18:26:23 +0100
Subject: [PATCH] gh-90015: Document that PEP-604 unions do not support forward
 references (#105366)

---
 Doc/library/stdtypes.rst | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/Doc/library/stdtypes.rst b/Doc/library/stdtypes.rst
index fdef5314b9a4ef..0caa725f75e642 100644
--- a/Doc/library/stdtypes.rst
+++ b/Doc/library/stdtypes.rst
@@ -5162,6 +5162,14 @@ enables cleaner type hinting syntax compared to :data:`typing.Union`.
       def square(number: int | float) -> int | float:
           return number ** 2
 
+   .. note::
+
+      The ``|`` operand cannot be used at runtime to define unions where one or
+      more members is a forward reference. For example, ``int | "Foo"``, where
+      ``"Foo"`` is a reference to a class not yet defined, will fail at
+      runtime. For unions which include forward references, present the
+      whole expression as a string, e.g. ``"int | Foo"``.
+
 .. describe:: union_object == other
 
    Union objects can be tested for equality with other union objects.  Details: