Fix frozendict constructor: drop the argument-less __init__ (#15985) (#15989)

frozendict has __new__ overloads for keyword arguments, a mapping and an
iterable of pairs, but also declared 'def __init__(self) -> None: ...',
so type checkers rejected 'frozendict(a=1)' etc. with 'Expected 0
positional arguments'. Remove the redundant __init__ (like other
__new__-constructed builtins) so the __new__ overloads govern
construction. Added a test case.

Author: apoorvdarshan

stdlib/@tests/test_cases/builtins/check_frozendict-py315.py

@@ -0,0 +1,15 @@
+from __future__ import annotations
+
+import sys
+from typing import Any
+from typing_extensions import assert_type
+
+if sys.version_info >= (3, 15):
+    # Regression test for gh-15985: ``frozendict`` can be constructed from
+    # keyword arguments, a mapping, or an iterable of pairs (not just with no
+    # arguments).
+    assert_type(frozendict(), frozendict[Any, Any])
+    assert_type(frozendict(a=1), frozendict[str, int])
+    assert_type(frozendict({"x": 1}), frozendict[str, int])
+    assert_type(frozendict([("k", 2)]), frozendict[str, int])
+    assert_type(frozendict({"x": 1}, y=2), frozendict[str, int])

stdlib/builtins.pyi

@@ -1389,7 +1389,6 @@ if sys.version_info >= (3, 15):
             cls: type[frozendict[str, _VT]], iterable: Iterable[tuple[str, _VT]], /, **kwargs: _VT
         ) -> frozendict[str, _VT]: ...
 
-        def __init__(self) -> None: ...
         def copy(self) -> frozendict[_KT, _VT]: ...
 
         @overload