Implement the fix

Author: matmel

HISTORY.md

@@ -11,6 +11,11 @@ The third number is for emergencies when we need to start branches for older rel
 
 Our backwards-compatibility policy can be found [here](https://github.com/python-attrs/cattrs/blob/main/.github/SECURITY.md).
 
+## NEXT (UNRELEASED)
+
+- Fix an `AttributeError` in `cattrs` internals that could be triggered by using the `include_subclasses` strategy in a `structure_hook_factory`
+  ([#721](https://github.com/python-attrs/cattrs/issues/721), [#722](https://github.com/python-attrs/cattrs/pull/722))
+
 ## 26.1.0 (2026-02-18)
 
 - Add the {mod}`tomllib <cattrs.preconf.tomllib>` preconf converter.

src/cattrs/strategies/_subclasses.py

@@ -191,6 +191,11 @@ def _include_subclasses_with_union_strategy(
 
     original_unstruct_hooks = {}
     original_struct_hooks = {}
+
+    original_working_set = None
+    if hasattr(already_generating, "working_set"):
+        original_working_set = already_generating.working_set.copy()
+
     for cl in union_classes:
         # In the first pass, every class gets its own unstructure function according to
         # the overrides.
@@ -209,6 +214,9 @@ def _include_subclasses_with_union_strategy(
         original_unstruct_hooks[cl] = unstruct_hook
         original_struct_hooks[cl] = struct_hook
 
+    if original_working_set is not None:
+        already_generating.working_set = original_working_set
+
     # Now that's done, we can register all the hooks and generate the
     # union handler. The union handler needs them.
     final_union = Union[union_classes]  # type: ignore

tests/strategies/test_include_subclasses.py

@@ -540,14 +540,30 @@ class Sub(Mid1, Mid2):
 
 
 def test_subclasses_in_struct_factory():
+
+    @frozen
+    class SubA:
+        id: int
+        sub_a: str
+
+    @frozen
+    class SubA1(SubA):
+        pass
+
     @frozen
     class A:
         """Base class"""
 
+        s: SubA
+
     @frozen
     class A1(A):
         a1: int
 
+    @frozen
+    class A2(A):
+        a2: int
+
     @frozen
     class B:
         id: int
@@ -586,11 +602,21 @@ def cls_is_cl(cls, _cl=cl):
 
     unstructured = {
         "id": 0,
-        "c": {"id": 1, "a": {"type": "A1", "a1": 42}, "b": {"id": 2, "b": "hello"}},
+        "c": {
+            "id": 1,
+            "a": {
+                "type": "A1",
+                "s": {"type": "SubA1", "id": 2, "sub_a": "a"},
+                "a1": 42,
+            },
+            "b": {"id": 3, "b": "hello"},
+        },
         "foo": "world",
     }
     res = converter.structure(unstructured, Container2)
 
     assert res == Container2(
-        id=0, c=Container1(id=1, a=A1(a1=42), b=B(id=2, b="Hello"), foo="world")
+        id=0,
+        c=Container1(id=1, a=A1(s=SubA1(id=2, sub_a="a"), a1=42), b=B(id=3, b="hello")),
+        foo="world",
     )