Fix unstructuring underspecified generics

HISTORY.md

@@ -4,6 +4,8 @@
 
 - Fix a regression when unstructuring dictionary values typed as `Any`.
   ([#453](https://github.com/python-attrs/cattrs/issues/453) [#462](https://github.com/python-attrs/cattrs/pull/462))
+- Fix a regression when unstructuring unspecialized generic classes.
+  ([#465](https://github.com/python-attrs/cattrs/issues/465))
 - Optimize function source code caching.
   ([#445](https://github.com/python-attrs/cattrs/issues/445))
 - Generate unique files only in case of linecache enabled.

src/cattrs/converters.py

@@ -956,7 +956,7 @@ def gen_unstructure_optional(self, cl: Type[T]) -> Callable[[T], Any]:
         other = union_params[0] if union_params[1] is NoneType else union_params[1]
 
         # TODO: Remove this special case when we make unstructuring Any consistent.
-        if other is Any:
+        if other is Any or isinstance(other, TypeVar):
             handler = self.unstructure
         else:
             handler = self._unstructure_func.dispatch(other)

tests/conftest.py

@@ -1,3 +1,4 @@
+import sys
 from os import environ
 
 import pytest
@@ -27,3 +28,7 @@ def converter_cls(request):
 settings.register_profile("fast", settings.get_profile("tests"), max_examples=10)
 
 settings.load_profile("fast" if "FAST" in environ else "tests")
+
+collect_ignore = []
+if sys.version_info < (3, 10):
+    collect_ignore.append("test_generics_604.py")

tests/test_generics.py

@@ -184,6 +184,9 @@ def test_unstructure_generic_attrs(genconverter):
     class Inner(Generic[T]):
         a: T
 
+    inner = Inner(Inner(1))
+    assert genconverter.unstructure(inner) == {"a": {"a": 1}}
+
     @define
     class Outer:
         inner: Inner[int]
@@ -203,6 +206,16 @@ class OuterStr:
     assert genconverter.structure(raw, OuterStr) == OuterStr(Inner("1"))
 
 
+def test_unstructure_optional(genconverter):
+    """Generics with optional fields work."""
+
+    @define
+    class C(Generic[T]):
+        a: Union[T, None]
+
+    assert genconverter.unstructure(C(C(1))) == {"a": {"a": 1}}
+
+
 def test_unstructure_deeply_nested_generics(genconverter):
     @define
     class Inner:

tests/test_generics_604.py

@@ -0,0 +1,16 @@
+"""Tests for generics under PEP 604 (unions as pipes)."""
+from typing import Generic, TypeVar
+
+from attrs import define
+
+T = TypeVar("T")
+
+
+def test_unstructure_optional(genconverter):
+    """Generics with optional fields work."""
+
+    @define
+    class C(Generic[T]):
+        a: T | None
+
+    assert genconverter.unstructure(C(C(1))) == {"a": {"a": 1}}