builtins.locals and types.FrameType.f_locals are currently typed with dict[str, Any], which is incorrect:

• typeshed/stdlib/builtins.pyi

  Line 1547 in 85a787b

  def locals() -> dict[str, Any]: ...

  Counterexample:

  from collections import UserDict
  
  class A(type):
      @classmethod
      def __prepare__(cls, name, bases): 
          return UserDict()
  
      def __new__(mcs, name, bases, namespace, /, **kw): 
          return super().__new__(mcs, name, bases, dict(namespace), **kw)
  
  class B(metaclass=A):
      assert isinstance(locals(), dict)  # AssertionError

• typeshed/stdlib/types.pyi

  Lines 585 to 586 in 85a787b

  	@property
  	def f_locals(self) -> dict[str, Any]: ...

  Counterexample as a consequence of PEP 667 (Python >= 3.13):

  from inspect import currentframe
  
  def f():
      print(type(currentframe().f_locals))  # <class 'FrameLocalsProxy'>  (runtime type is `_collections_abc.framelocalsproxy`; not in `.pyi` stubs yet)
      assert isinstance(currentframe().f_locals, dict)  # AssertionError
  
  f()

However, I'm not sure what the actual type should be. From what I can gather, the type should implement a mapping interface, with a few (but not all?) MutableMapping methods (PEP 667 describes what these mutable mapping methods are for FrameType.f_locals, but I'm not sure what's exactly expected of builtins.locals()).

In addition, collections.abc.Mapping and collections.abc.MutableMapping themselves are unsuitable, because they're nominal types, although this could be fixed by defining a Protocol which looks like a Mapping.

So, what should the types of builtins.locals and types.FrameType.f_locals be?