Current blockers on runtime deferal of typing imports

Typing
4

@mikeshardmind I would also mention typing.cast(). No runtime effect, just an identity function.

TYPE_CHECKING = False
if TYPE_CHECKING:
    from typing import cast
else:
    cast = lambda typ, v: v

As an aside, several of these seem to (at runtime) reduce to an identity function of some sort, returning the input unchanged. We added a C version of this as _typing._idfunc for use in typing.NewType.__call__, perhaps this could be made public (operator.identity_function?) for these sort of cases?