Fix structuring of nested generics, add test (#688)

* Fix structuring of nested generics, add test

* Black lint, changelog entry, better test docstring

Author: calebj

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).
 
+## 25.4.0 (UNRELEASED)
+
+- Fix structuring of nested generic classes with stringified annotations.
+  ([#688](https://github.com/python-attrs/cattrs/pull/688))
+
 ## 25.3.0 (2025-10-07)
 
 - **Potentially breaking**: [Abstract sets](https://docs.python.org/3/library/collections.abc.html#collections.abc.Set) are now structured into frozensets.

src/cattrs/converters.py

@@ -1303,10 +1303,11 @@ def gen_structure_typeddict(self, cl: Any) -> Callable[[dict, Any], dict]:
     def gen_structure_attrs_fromdict(
         self, cl: type[T]
     ) -> Callable[[Mapping[str, Any], Any], T]:
-        attribs = fields(get_origin(cl) or cl if is_generic(cl) else cl)
+        origin = get_origin(cl)
+        attribs = fields(origin or cl if is_generic(cl) else cl)
         if attrs_has(cl) and any(isinstance(a.type, str) for a in attribs):
             # PEP 563 annotations - need to be resolved.
-            resolve_types(cl)
+            resolve_types(origin or cl)
         attrib_overrides = {
             a.name: self.type_overrides[a.type]
             for a in attribs

tests/test_generics_nested.py

@@ -0,0 +1,23 @@
+"""Tests un/structure of nested generic classes (stringified only)"""
+
+from __future__ import annotations
+
+from typing import Generic, TypeVar
+
+from attrs import define
+
+T = TypeVar("T")
+
+
+def test_structure_nested_roundtrip(genconverter):
+    @define(auto_attribs=True)
+    class Inner:
+        value: int
+
+    @define(auto_attribs=True)
+    class Container(Generic[T]):
+        data: T
+
+    raw = {"data": {"value": 42}}
+    structured = genconverter.structure(raw, Container[Inner])
+    assert genconverter.unstructure(structured, Container[Inner]) == raw