Document type alias union passthrough support

Author: yzhkali

HISTORY.md

@@ -26,6 +26,8 @@ Our backwards-compatibility policy can be found [here](https://github.com/python
   ([#732](https://github.com/python-attrs/cattrs/pull/732))
 - Support running the test suite without `cbor2` installed.
   ([#748](https://github.com/python-attrs/cattrs/pull/748))
+- The [union passthrough strategy](https://catt.rs/en/stable/strategies.html#union-passthrough) now supports PEP 695 type aliases as union members.
+  ([#753](https://github.com/python-attrs/cattrs/pull/753))
 
 ## 26.1.0 (2026-02-18)
 

docs/defaulthooks.md

@@ -640,6 +640,10 @@ Their hooks can also be overriden using [](customizing.md#predicate-hooks).
 Type aliases using [`typing.TypeAlias`](https://docs.python.org/3/library/typing.html#typing.TypeAlias) aren't supported since there is no way at runtime to distinguish them from their underlying types.
 ```
 
+```{versionchanged} 26.2.0
+The [union passthrough strategy](strategies.md#union-passthrough) also recognizes type aliases used as union members.
+```
+
 ```python
 >>> from datetime import datetime, UTC
 

docs/strategies.md

@@ -367,6 +367,10 @@ The strategy also supports [NewTypes](https://mypy.readthedocs.io/en/stable/more
 12
 ```
 
+```{versionchanged} 26.2.0
+PEP 695 type aliases of supported types are supported when used as union members.
+```
+
 Unions containing unsupported types can be handled if at least one union type is supported by the strategy; the supported union types will be checked before the rest (referred to as the _spillover_) is handed over to the converter again.
 
 For example, if `A` and `B` are arbitrary _attrs_ classes, the union `Literal[10] | A | B` cannot be handled directly by a JSON converter.

src/cattrs/strategies/_unions.py

@@ -173,6 +173,8 @@ def configure_union_passthrough(
     .. versionadded:: 23.2.0
     .. versionchanged:: 25.2.0
         Introduced the `accept_ints_as_floats` parameter.
+    .. versionchanged:: 26.2.0
+        PEP 695 type aliases are now supported as union members.
     """
 
     def get_passthrough_base(type: Any) -> Any: