Make Counter generic over the value

Closes: python#3438

stdlib/collections/__init__.pyi

@@ -34,6 +34,7 @@ _KT = TypeVar("_KT")
 _VT = TypeVar("_VT")
 _KT_co = TypeVar("_KT_co", covariant=True)
 _VT_co = TypeVar("_VT_co", covariant=True)
+_C = TypeVar("_C", default=int)
 
 # namedtuple is special-cased in the type checker; the initializer is ignored.
 def namedtuple(
@@ -261,13 +262,13 @@ class deque(MutableSequence[_T]):
     if sys.version_info >= (3, 9):
         def __class_getitem__(cls, item: Any, /) -> GenericAlias: ...
 
-class Counter(dict[_T, int], Generic[_T]):
+class Counter(dict[_T, _C]):
     @overload
     def __init__(self, iterable: None = None, /) -> None: ...
     @overload
-    def __init__(self: Counter[str], iterable: None = None, /, **kwargs: int) -> None: ...
+    def __init__(self: Counter[str, _C], iterable: None = None, /, **kwargs: _C) -> None: ...
     @overload
-    def __init__(self, mapping: SupportsKeysAndGetItem[_T, int], /) -> None: ...
+    def __init__(self: Counter[_T, _C], mapping: SupportsKeysAndGetItem[_T, _C], /) -> None: ...
     @overload
     def __init__(self, iterable: Iterable[_T], /) -> None: ...
     def copy(self) -> Self: ...

test_cases/stdlib/collections/check_counter.py

@@ -0,0 +1,31 @@
+from collections import Counter
+from typing_extensions import assert_type, Never
+
+
+class Foo: ...
+
+
+# Test the constructor
+assert_type(Counter(), Counter[Never, int])
+assert_type(Counter(foo=42.2), Counter[str, float])
+assert_type(Counter({42: "bar"}), Counter[int, str])
+assert_type(Counter([1, 2, 3]), Counter[int, int])
+
+int_c: Counter[str] = Counter()
+assert_type(int_c, Counter[str, int])
+int_c["a"] = 1
+int_c["a"] += 3
+int_c["a"] += 3.5  # type: ignore
+
+float_c = Counter(foo=42.2)
+assert_type(float_c, Counter[str, float])
+float_c["a"] = 1.0
+float_c["a"] += 3.0
+float_c["a"] += 42
+float_c["a"] += "42"  # type: ignore
+
+custom_c: Counter[str, Foo] = Counter()
+assert_type(custom_c, Counter[str, Foo])
+custom_c["a"] = Foo()
+custom_c["a"] += Foo()  # type: ignore
+custom_c["a"] += 42  # type: ignore