Harmonise UserDict.__init__ with dict.__init__ (#6490)

Co-authored-by: Akuli <akuviljanen17@gmail.com>

stdlib/builtins.pyi

@@ -816,6 +816,7 @@ class list(MutableSequence[_T], Generic[_T]):
         def __class_getitem__(cls, __item: Any) -> GenericAlias: ...
 
 class dict(MutableMapping[_KT, _VT], Generic[_KT, _VT]):
+    # __init__ should be kept roughly in line with `collections.UserDict.__init__`, which has similar semantics
     @overload
     def __init__(self: dict[_KT, _VT]) -> None: ...
     @overload

stdlib/collections/__init__.pyi

@@ -1,6 +1,6 @@
 import sys
 from _collections_abc import dict_items, dict_keys, dict_values
-from _typeshed import Self, SupportsLessThan, SupportsLessThanT
+from _typeshed import Self, SupportsKeysAndGetItem, SupportsLessThan, SupportsLessThanT
 from typing import Any, Dict, Generic, NoReturn, Tuple, Type, TypeVar, overload
 from typing_extensions import SupportsIndex, final
 
@@ -35,9 +35,19 @@ else:
         typename: str, field_names: str | Iterable[str], *, verbose: bool = ..., rename: bool = ..., module: str | None = ...
     ) -> Type[Tuple[Any, ...]]: ...
 
-class UserDict(MutableMapping[_KT, _VT]):
+class UserDict(MutableMapping[_KT, _VT], Generic[_KT, _VT]):
     data: dict[_KT, _VT]
-    def __init__(self, __dict: Mapping[_KT, _VT] | None = ..., **kwargs: _VT) -> None: ...
+    # __init__ should be kept roughly in line with `dict.__init__`, which has the same semantics
+    @overload
+    def __init__(self: UserDict[_KT, _VT], __dict: None = ...) -> None: ...
+    @overload
+    def __init__(self: UserDict[str, _VT], __dict: None = ..., **kwargs: _VT) -> None: ...
+    @overload
+    def __init__(self, __dict: SupportsKeysAndGetItem[_KT, _VT], **kwargs: _VT) -> None: ...
+    @overload
+    def __init__(self, __iterable: Iterable[tuple[_KT, _VT]], **kwargs: _VT) -> None: ...
+    @overload
+    def __init__(self: UserDict[str, str], __iterable: Iterable[list[str]]) -> None: ...
     def __len__(self) -> int: ...
     def __getitem__(self, key: _KT) -> _VT: ...
     def __setitem__(self, key: _KT, item: _VT) -> None: ...