Unpack the single type arg for Counter keys when unstructuring

HISTORY.md

@@ -13,6 +13,8 @@ Our backwards-compatibility policy can be found [here](https://github.com/python
 
 ## NEXT (UNRELEASED)
 
+- Fix `Counter` keys not being unstructured with the key type's own hook; the single-type-arg branch passed the whole type-args tuple to the key hook lookup instead of the key type.
+  ([#768](https://github.com/python-attrs/cattrs/pull/768))
 - Fix `create_default_dis_func <cattrs.disambiguators.create_default_dis_func>` (aka `create_uniq_field_dis_func`) failing to disambiguate valid unions depending on the order of the member classes; unique fields are now resolved iteratively to a fixpoint.
   ([#230](https://github.com/python-attrs/cattrs/issues/230) [#765](https://github.com/python-attrs/cattrs/pull/765))
 - Support more recursive types on 3.14+ with specialized factories for [`annotationlib.ForwardRef`](https://docs.python.org/3/library/annotationlib.html#annotationlib.ForwardRef).

src/cattrs/gen/__init__.py

@@ -1029,7 +1029,7 @@ def mapping_unstructure_factory(
             key_arg, val_arg = args
         else:
             # Probably a Counter
-            key_arg, val_arg = args, Any
+            key_arg, val_arg = args[0], Any
         # We can do the dispatch here and now.
         kh = key_handler or converter.get_unstructure_hook(key_arg, cache_result=False)
         if kh == identity:

tests/test_unstructure_collections.py

@@ -148,3 +148,23 @@ def test_collection_unstructure_override_mapping():
     assert c.unstructure({1: 2}) == Map({1: 2})
     assert c.unstructure({1: 2}, unstructure_as=MutableMapping[int, int]) == Map({1: 2})
     assert c.unstructure({1: 2}, unstructure_as=Mapping[int, int]) == Map({1: 2})
+
+
+def test_counter_unstructure_applies_key_hook():
+    """Counter keys must be unstructured with the key type's own hook.
+
+    Regression: the single-type-arg ("Probably a Counter") branch assigned the
+    whole ``args`` tuple to ``key_arg``, so the key hook was resolved for
+    ``(KeyType,)`` (falling back to identity) instead of ``KeyType``, leaving
+    keys un-unstructured.
+    """
+
+    @define(frozen=True)
+    class Key:
+        v: int
+
+    c = Converter()
+    c.register_unstructure_hook(Key, lambda k: k.v)
+
+    result = c.unstructure(Counter({Key(1): 5, Key(2): 3}), unstructure_as=Counter[Key])
+    assert result == {1: 5, 2: 3}